The history of the discovery and study of cells. Cell theory. History of the discovery of cells and stages of development of cytology Facts from the history of the study of cells table
The vast majority of cells are microscopically small and cannot be seen with the naked eye. It became possible to see a cell and begin to study it only when the microscope was invented. The first microscopes appeared at the beginning of the 17th century. The microscope was first used for scientific research by the English scientist Robert Hooke (1665). Examining thin sections of cork under a microscope, he saw numerous small cells on them. Hooke called these cells, separated from each other by dense walls, cells, using the term “cell” for the first time.
In the subsequent period, which covered the second half of the 17th century, the entire 18th century. and the beginning of the 19th century. The microscope was being improved and data on animal and plant cells was accumulating. By the middle of the 19th century, the microscope had been significantly improved and much had become known about the cellular structure of plants and animals. The main materials about the cellular structure of plants at this time were collected and summarized by the German botanist M. Schleiden.
All the data obtained about the cell served as the basis for the creation of the cellular theory of the structure of organisms, which was formulated in 1838 by the German zoologist T. Schwann. Studying the cells of animals and plants, Schwann discovered that they were similar in structure, and established that the cell is a common elementary structural unit of animal and plant organisms. Schwann outlined the theory of the cellular structure of organisms in his classic work “Microscopic studies on the correspondence in the structure and growth of animals and plants.”
At the beginning of the last century, the famous scientist, academician of the Russian Academy of Sciences Karl Baer discovered the mammalian egg and showed that all organisms begin their development from one cell. This cell is a fertilized egg, which splits, forms new cells, and from them the tissues and organs of the future organism are formed.
Baer's discovery complemented the cell theory and showed that The cell is not only a unit of structure, but also a unit of development of all living organisms.
An extremely significant addition to cell theory was the discovery of cell division. After the discovery of the process of cell division, it became quite obvious that new cells are formed by dividing existing ones, and do not arise anew from non-cellular matter.
The theory of the cellular structure of organisms also includes the most important materials for proving the unity of the origin, structure and development of the entire organic world. F. Engels highly appreciated the creation of the cellular theory, placing it in importance next to the law of conservation of energy and the theory of natural selection of Charles Darwin.
By the end of the 19th century. The microscope was improved to such an extent that it became possible to study the details of the cell structure and its main structural components were discovered. At the same time, knowledge began to accumulate about their functions in the life of the cell. The emergence of cytology, which currently represents one of the most intensively developing biological disciplines, dates back to this time.
Methods for studying cells. Modern cytology has numerous and often quite complex research methods that have made it possible to establish subtle structural details and identify the functions of a wide variety of cells and their structural components. The light microscope continues to play an exceptionally important role in cytological studies, which today is a complex, sophisticated device that provides magnification up to 2500 times. But even such a high magnification is far from sufficient to see the fine details of the cell structure, even if we consider sections 5–10 mm thick. µm 1, painted with special dyes.
A completely new era in the study of cell structure began with the invention of the electron microscope, which provides magnification of tens and hundreds of thousands of times. Instead of light, an electron microscope uses a fast flow of electrons, and the glass lenses of a light-optical microscope are replaced by electromagnetic fields. Electrons flying at high speed are first concentrated on the object under study, and then fall on a screen, similar to a television screen, on which you can either observe an enlarged image of the object or photograph it. The electron microscope was designed in 1933, and has become especially widely used for the study of biological objects in the last 10–15 years.
To be examined under an electron microscope, cells undergo very complex processing. The thinnest sections of cells are prepared, the thickness of which is 100–500 A. Only such thin sections are suitable for electron microscopic examination due to their low permeability to electrons.
Recently, chemical methods for studying cells have been used more and more. A special branch of chemistry - biochemistry - today has numerous subtle methods that make it possible to accurately establish not only the presence, but also the role of chemical substances in the life of a cell and the whole organism. Complex devices called centrifuges have been created, which develop enormous rotation speeds (several tens of thousands of revolutions per minute). Using such centrifuges, you can easily separate the structural components of a cell from each other, since they have different specific gravity. This very important method makes it possible to study separately the properties of each part of the cell.
Studying a living cell, its finest structures and functions is a very difficult task, and only a combination of efforts and colossal work of cytologists, biochemists, physiologists, geneticists and biophysicists made it possible to study its structural elements in detail and determine their role.
The prerequisites for the creation of the cell theory were the invention and improvement of the microscope and the discovery of cells (1665, R. Hooke - when studying a section of the bark of a cork tree, elderberry, etc.). The works of famous microscopists: M. Malpighi, N. Grew, A. van Leeuwenhoek - made it possible to see the cells of plant organisms. A. van Leeuwenhoek discovered single-celled organisms in water. First, the cell nucleus was studied. R. Brown described the nucleus of a plant cell. Ya. E. Purkine introduced the concept of protoplasm - liquid gelatinous cellular contents.
The German botanist M. Schleiden was the first to come to the conclusion that every cell has a nucleus. The founder of CT is considered to be the German biologist T. Schwann (together with M. Schleiden), who in 1839 published the work “Microscopic studies on the correspondence in the structure and growth of animals and plants.” Its provisions:
1) the cell is the main structural unit of all living organisms (both animals and plants);
2) if any formation visible under a microscope has a nucleus, then it can be considered a cell;
3) the process of formation of new cells determines the growth, development, differentiation of plant and animal cells.
Additions to the cell theory were made by the German scientist R. Virchow, who in 1858 published his work “Cellular Pathology”. He proved that daughter cells are formed by dividing mother cells: each cell from a cell. At the end of the 19th century. mitochondria, the Golgi complex, and plastids were discovered in plant cells. After staining dividing cells with special dyes, chromosomes were discovered. Modern CT provisions
1. The cell is the basic unit of structure and development of all living organisms, and is the smallest structural unit of a living thing.
2. The cells of all organisms (both unicellular and multicellular) are similar in chemical composition, structure, basic manifestations of metabolism and vital activity.
3. Cells reproduce by dividing them (each new cell is formed by dividing the mother cell); In complex multicellular organisms, cells have different shapes and are specialized according to the functions they perform. Similar cells form tissues; tissues consist of organs that form organ systems; they are closely interconnected and subject to nervous and humoral regulatory mechanisms (in higher organisms).
The importance of cell theory
It has become clear that the cell is the most important component of living organisms, their main morphophysiological component. A cell is the basis of a multicellular organism, the place where biochemical and physiological processes occur in the body. All biological processes ultimately occur at the cellular level. The cell theory made it possible to conclude that the chemical composition of all cells and the general plan of their structure are similar, which confirms the phylogenetic unity of the entire living world.
2. Life. Properties of living matter
Life is a macromolecular open system, which is characterized by a hierarchical organization, the ability to reproduce itself, self-preservation and self-regulation, metabolism, and a finely regulated flow of energy.
Properties of living structures:
1) self-renewal. The basis of metabolism is made up of balanced and clearly interconnected processes of assimilation (anabolism, synthesis, formation of new substances) and dissimilation (catabolism, decay);
2) self-reproduction. In this regard, living structures are constantly reproduced and updated, without losing their similarities with previous generations. Nucleic acids are capable of storing, transmitting and reproducing hereditary information, as well as implementing it through protein synthesis. The information stored on DNA is transferred to the protein molecule using RNA molecules;
3) self-regulation. Based on the totality of flows of matter, energy and information through a living organism;
4) irritability. Associated with the transfer of information from the outside to any biological system and reflects the reaction of this system to an external stimulus. Thanks to irritability, living organisms are able to selectively react to environmental conditions and extract from it only what is necessary for their existence;
5) maintaining homeostasis - the relative dynamic constancy of the internal environment of the body, the physical and chemical parameters of the existence of the system;
6) structural organization - orderliness, of a living system, discovered during the study - biogeocenoses;
7) adaptation – the ability of a living organism to constantly adapt to changing conditions of existence in the environment;
8) reproduction (reproduction). Since life exists in the form of individual living systems, and the existence of each such system is strictly limited in time, the maintenance of life on Earth is associated with the reproduction of living systems;
9) heredity. Ensures continuity between generations of organisms (based on information flows). Thanks to heredity, traits that ensure adaptation to the environment are passed on from generation to generation;
10) variability - due to variability, a living system acquires characteristics that were previously unusual for it. First of all, variability is associated with errors during reproduction: changes in the structure of nucleic acids lead to the emergence of new hereditary information;
11) individual development (the process of ontogenesis) – the embodiment of the initial genetic information embedded in the structure of DNA molecules into the working structures of the body. During this process, such a property as the ability to grow appears, which is expressed in an increase in body weight and its size;
12) phylogenetic development. Based on progressive reproduction, heredity, struggle for existence and selection. As a result of evolution, a huge number of species appeared;
13) discreteness (discontinuity) and at the same time integrity. Life is represented by a collection of individual organisms, or individuals. Each organism, in turn, is also discrete, since it consists of a collection of organs, tissues and cells.
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History of the study of cells. Cell theory.
Fill out the table: “Main stages in the development of cell theory” Year Scientist Contribution to the development of the theory
History of the study of cells The history of the study of cells is inextricably linked with the development of microscopic technology and research methods. Man was able to penetrate the secret of cellular structure only thanks to the invention of the microscope at the end of the 16th century.
Zachary Jansen 1590 By combining two lenses together, he first invented a primitive microscope
Robert Hooke 1665 First described the structure of the bark of a cork oak tree and the stem of a plant, and introduced the term “cell” into science.
Antoni van Leeuwenhoek improved the microscope. Observed and sketched a number of protozoa, sperm, bacteria, red blood cells and their movement in capillaries. Discovered bacteria. Second half of the 17th century
Karl Baer 1827 Discovered the mammalian egg Conclusion: every organism develops from a single cell
Robert Brown 1831-1833 He discovered the nucleus in plant cells - the most important component of the cell.
Cell theory In 1839, Theodor Schwann published the book “Microscopic Studies on the Correspondence in the Structure and Growth of Animals and Plants” in Berlin, in which he formulated the cell theory.
When creating the cell theory, T. Schwann proceeded from the discovery by M. Schleiden in 1838 of the cellular structure of plants and the homology of the origin of cells.
The first version of the cell theory All organisms, both plant and animal, consist of the simplest parts - cells. A cell is an individual independent whole. In one organism, all cells act together, forming a harmonious unity.
Rudolf Virchow 1858 Proved that cells arise from cells through reproduction, which complemented the cell theory.
19th century The basic structures of cells were discovered. The process of cell division has been studied. A. Weisman established: the storage and transmission of hereditary characteristics in a cell is carried out using the nucleus.
Basic provisions of cell theory at the present stage of development of biology
A cell is the elementary unit of living things. The cell is the smallest structural and functional unit of living things and is an open, self-regulating, self-reproducing system. There is no life outside the cell.
All cells are similar in their chemical composition and have a general structural plan. Cells also have specific features associated with the performance of special functions and resulting from cellular differentiation.
A cell comes only from a cell.
Multicellular organisms are complexly organized integrated systems consisting of interacting cells.
The similar cellular structure of organisms is evidence that all living things have a single origin.
Homework § 2.1 pp. 24 – 28.
On the topic: methodological developments, presentations and notes
The presentation lesson was developed using computer technology, the main theoretical material is reflected in the presentation. Conducting a lesson in such a non-standard form helps to increase motivation...
Lesson topic: Cage. Cellular theory of the structure of organisms. (10th grade chemistry-bio group)Type of lesson: two-purpose lesson (lesson of systematization and generalization of knowledge, application of knowledge, skills and abilities)Teaching methods...
Cell theory- one of the generally accepted biological generalizations that affirm the unity of the principle of the structure and development of the world of plants, animals and other living organisms with the cellular structure, in which the cell is considered as a single structural element of living organisms.
Cell theory is a fundamental theory for biology, formulated in the middle of the 19th century, which provided the basis for understanding the laws of the living world and for the development of evolutionary teaching. Matthias Schleiden and Theodor Schwann formulated the cell theory based on many studies about the cell (1838). Rudolf Virchow later (1858) supplemented it with the most important position (every cell comes from another cell).
Schleiden and Schwann, summarizing the existing knowledge about the cell, proved that the cell is the basic unit of any organism. Animal, plant and bacterial cells have a similar structure. Later, these conclusions became the basis for proving the unity of organisms. T. Schwann and M. Schleiden introduced into science the fundamental concept of the cell: there is no life outside cells. The cell theory was supplemented and edited every time.
Provisions of the Schleiden-Schwann cell theory
∙ All animals and plants are made up of cells.
∙ Plants and animals grow and develop through the emergence of new cells.
∙ A cell is the smallest unit of living things, and a whole organism is a collection of cells.
∙ Basic provisions of modern cell theory[edit | edit source text]
∙ A cell is an elementary, functional unit of the structure of all living things. (Except for viruses that do not have a cellular structure)
∙ A cell is a single system; it includes many naturally interconnected elements, representing an integral formation consisting of conjugated functional units - organelles.
∙ The cells of all organisms are homologous.
∙ A cell comes into being only by dividing the mother cell.
∙ A multicellular organism is a complex system of many cells united
And integrated into systems of tissues and organs connected to each other.
∙ The cells of multicellular organisms are totipotent.
Methods for studying cells.
1. Light microscopy method.
The resolution of a light microscope is ~0.1 - 0.2 micrometers.
Types of light microscopy: phase contrast, fluorescence and polarization microscopy.
2. Electron microscopy method. Resolution ~0.10 nanometer.Methods for studying fixed cells.
3. Histological methods.
Methods of fixation, preparation of preparations with their subsequent staining.
4. Cytochemical methods are selective staining of various chemical elements (components) of the cell (DNA, protein...).
5. Morphological methods are a quantitative method that studies the parameters of basic cellular structures.
6. Tagged isotope method.
Heavy carbon or hydrogen atoms are used. These labeled atoms are included in the precursors for the synthesis of certain molecules. For example: during DNA synthesis, labeled thymidine H3, a precursor of thymine, is used.
7. To detect the mark in cytology, the autoradiography method is used. Histological preparations are made and coated with photoemulsion in the dark, kept for a certain time at a certain temperature, then the preparations are developed using photoreagents, and the mark is revealed in the form of silver grains. This method was used to determine the parameters of the mitodic cycle.
8. The cell fractionation method allows the study of intracellular components. The cells are destroyed, placed in special centrifuges, and different cellular components are precipitated at different centrifugation speeds.
9. The X-ray diffraction method is used to study the crystal lattice of the nucleus of an atom.
Methods for studying living cells.
10. The cell structure method allows you to study a living cell.
11. Microsurgery method. For example: implantation of a microelectrode.
12. Cloning methods.
11. Cell nucleus, its organization, purpose. Nuclear chromatin.
The nucleus (Latin nucleus) is one of the structural components of a eukaryotic cell, containing genetic information (DNA molecules) and performing the following functions:
1) storage and reproduction of genetic information 2) regulation of metabolic processes occurring in the cell
The shape of the nucleus depends largely on the shape of the cell; it can be completely irregular. There are spherical and multi-lobed kernels. Invaginations and outgrowths of the nuclear membrane significantly increase the surface of the nucleus and thereby strengthen the connection of nuclear and cytoplasmic structures and substances.
Structure of the nucleus The nucleus is surrounded by a shell, which consists of two membranes with a typical structure.
The outer nuclear membrane on the surface facing the cytoplasm is covered with ribosomes, the inner membrane is smooth.
The nuclear envelope is part of the cell membrane system. The outgrowths of the outer nuclear membrane connect with the channels of the endoplasmic reticulum, forming a single system of communicating channels. Metabolism between the nucleus and the cytoplasm occurs in two main ways. Firstly, the nuclear envelope is penetrated by numerous pores through which molecules are exchanged between the nucleus and the cytoplasm. Secondly, substances from the nucleus into the cytoplasm and back can enter due to the release of invaginations and outgrowths of the nuclear membrane. Despite the active exchange of substances between the nucleus and the cytoplasm, the nuclear envelope limits the nuclear contents from the cytoplasm, thereby ensuring differences in the chemical composition of the nuclear juice and the cytoplasm. This is necessary for the normal functioning of nuclear structures.
The contents of the nucleus are divided into nuclear juice, chromatin and nucleolus.
In a living cell, nuclear sap appears as a structureless mass that fills the gaps between the structures of the nucleus. Nuclear juice contains various proteins, including most nuclear enzymes, chromatin proteins and ribosomal proteins. Nuclear juice also contains free nucleotides necessary for the construction of DNA and RNA molecules, amino acids, all types of RNA, as well as products of the activity of the nucleolus and chromatin , then transported from the nucleus to the cytoplasm.
Chromatin (Greek chroma - color, color) is the name given to clumps, granules and network-like structures of the nucleus, which are intensely stained with some dyes and differ in shape from the nucleolus. Chromatin contains DNA and proteins and represents spiralized and compacted sections of chromosomes. Spiraled sections of chromosomes are genetically inactive.
Their specific role—transfer of genetic information—can only be performed by despiralized-untwisted sections of chromosomes, which, due to their small thickness, are not visible in a light microscope.
The third structure characteristic of a cell is the nucleolus. It is a dense round body immersed in nuclear juice. In the nuclei of different cells, as well as in the nucleus of the same cell, depending on its functional state, the number of nucleoli can vary from 1 to 5-7 or more. The number of nucleoli may exceed the number of chromosomes in the set; this occurs due to the selective reduplication of genes responsible for rRNA synthesis. Nucleoli are present only in non-dividing nuclei; during mitosis they disappear due to the spiralization of chromosomes and the release of all previously formed ribosomes into the cytoplasm, and after completion of division they appear again.
The nucleolus is not an independent structure of the nucleus. It is formed around the region of the chromosome in which the rRNA structure is encoded. This part of the chromosome - the gene - is called the nucleolar organizer (NO), and r-RNA synthesis occurs on it.
In addition to the accumulation of r-RNA, ribosomal subunits are formed in the nucleolus, which then move into the cytoplasm and, combining with the participation of Ca2+ cations, form integral ribosomes capable of participating in protein biosynthesis.
Thus, the nucleolus is an accumulation of r-RNA and ribosomes at different stages of formation, which is based on a section of the chromosome that carries the gene - the nucleolar organizer, which contains hereditary information about the structure of r-RNA.
12.Structure and functions of cell membranes.
The cell membrane (or cytolemma, or plasmalemma, or plasma membrane) separates the contents of any cell from the external environment, ensuring its integrity; regulates the exchange between the cell and the environment; intracellular membranes divide the cell into specialized closed compartments, compartments or organelles, in which certain environmental conditions are maintained.
All biological membranes have common structural features and properties. Currently, the liquid-mosaic model of membrane structure is generally accepted. The basis of the membrane is a lipid bilayer formed mainly by phospholipids. Phospholipids are triglycerides in which one fatty acid residue is replaced by a phosphoric acid residue; the section of the molecule containing the phosphoric acid residue is called the hydrophilic head, the sections containing the fatty acid residues are called the hydrophobic tails. In the membrane, phospholipids are arranged in a strictly ordered manner: the hydrophobic tails of the molecules face each other, and the hydrophilic heads face outward, towards the water.
In addition to lipids, the membrane contains proteins (on average ≈ 60%). They determine most of the specific functions of the membrane (transport of certain molecules, catalysis of reactions, receiving and converting signals from the environment, etc.). There are: 1) peripheral proteins (located on the outer or inner surface of the lipid bilayer), 2) semi-integral proteins (immersed in the lipid bilayer to varying depths), 3) integral, or transmembrane proteins (penetrate the membrane through, contacting the outer , and with the internal environment of the cell). Integral proteins are in some cases called channel-forming or channel proteins, since they can be considered as hydrophilic channels through which polar molecules pass into the cell (the lipid component of the membrane would not let them through).
The membrane may contain carbohydrates (up to 10%). The carbohydrate component of membranes is represented by oligosaccharide or polysaccharide chains associated with protein molecules (glycoproteins) or lipids (glycolipids). Carbohydrates are mainly located on the outer surface of the membrane. Carbohydrates provide receptor functions of the membrane. In animal cells, glycoproteins form a supra-membrane complex, the glycocalyx, which is several tens of nanometers thick. It contains many cell receptors, and with its help cell adhesion occurs.
Molecules of proteins, carbohydrates and lipids are mobile, capable of moving in the plane of the membrane. The thickness of the plasma membrane is approximately 7.5 nm.
Functions of membranes Membranes perform the following functions:
1. separation of cellular contents from the external environment,
2. regulation of metabolism between the cell and the environment,
3. division of the cell into compartments (“compartments”),
4. place of localization of “enzymatic conveyors”,
5. ensuring communication between cells in the tissues of multicellular organisms (adhesion),
6. signal recognition.
The most important property of membranes is selective permeability, i.e. membranes are highly permeable to some substances or molecules and poorly permeable (or completely impermeable) to others. This property underlies the regulatory function of membranes, ensuring the exchange of substances between the cell and the external environment. The process of substances passing through the cell membrane is called substance transport. There are: 1) passive transport - the process of passage of substances that occurs without energy consumption; 2) active transport - the process of passage of substances that occurs with the expenditure of energy.
13. Nucleic acids. DNA, its structure and role in the cell.
Nucleic acids are phosphorus-containing biopolymers of living organisms that ensure the storage and transmission of hereditary information. They were discovered in 1869 by the Swiss biochemist F. Miescher in the nuclei of leukocytes and salmon sperm. Subsequently, nucleic acids were found in all plant and animal cells, viruses, bacteria and fungi.
There are two types of nucleic acids in nature - deoxyribonucleic acids (DNA) and ribonucleic acids (RNA). The difference in names is explained by the fact that the DNA molecule contains the five-carbon sugar deoxyribose, and the RNA molecule contains ribose. Currently, a large number of varieties of DNA and RNA are known, differing from each other in structure and significance in metabolism.
DNA is found primarily in the chromosomes of the cell nucleus (99% of all cell DNA), as well as in mitochondria and chloroplasts. RNA is part of ribosomes; RNA molecules are also contained in the cytoplasm, matrix of plastids and mitochondria.
Nucleotides are structural components of nucleic acids. Nucleic acids are biopolymers whose monomers are nucleotides.
Nucleotides are complex substances. Each nucleotide contains a nitrogenous base, a five-carbon sugar (ribose or deoxyribose) and a phosphoric acid residue.
There are five main nitrogenous bases: adenine, guanine, uracil, thymine and cytosine. The first two are purines; their molecules consist of two rings, the first contains five members, the second
Six. The next three are pyrimidines and have one five-membered ring. The names of nucleotides are derived from the name of the corresponding nitrogenous bases; both are designated by capital letters: adenine - adenylate (A), guanine - guanylate (G), cytosine - cytidylate (C), thymine - thymidylate (T), uracil - uridylate (U).
The number of nucleotides in a nucleic acid molecule varies - from 80 in transfer RNA molecules to several hundred million in DNA.
DNA. A DNA molecule consists of two polynucleotide chains, spirally twisted relative to each other.
IN The nucleotide composition of a DNA molecule includes four types of nitrogenous bases: adenine, guanine, thymine and cytocin. IN In a polynucleotide chain, neighboring nucleotides are interconnected by covalent bonds that are formed between the phosphate group of one nucleotide and the 3"-hydroxyl group of the pentose of another. Such bonds are called phosphodiester. The phosphate group forms a bridge between the 3"-carbon of one pentose ring and the 5- carbon next. The backbone of the DNA chains is thus formed by sugar phosphate residues (Fig. 1.2).
Although DNA contains four types of nucleotides, their different sequences along the long chain result in a huge variety of molecules. The polynucleotide chain of DNA is twisted in the form of a spiral like a spiral staircase and is connected to another, complementary chain, using hydrogen bonds formed between adenine and thymine (two bonds), as well as guanine and cytosine (three bonds). Nucleotides A and T, G and C are called complementary.
IN As a result, in every organism the number of adenyl nucleotides is equal to the number of thymidyl nucleotides, and the number of guanyl nucleotides is equal to the number of cytidyl nucleotides. This pattern is called the “Chargaff rule”. Thanks to this property, the sequence of nucleotides in one chain determines their sequence in the other. This ability to selectively combine nucleotides is called complementarity, and this property underlies the formation of new DNA molecules based on the original molecule (replication, i.e. doubling).
The chains in a DNA molecule are in opposite directions (antiparallel). So, if for one chain we choose the direction from the 3" end to the 5" end, then the second chain with this direction will be oriented opposite to the first - from the 5 end to the 3" end, in other words, the "head" of one chain is connected with a “tail” the other and vice versa.
The model of the DNA molecule was first proposed in 1953 by the American scientist J. Watson and the Englishman F. Crick based on E. Chargaff’s data on the ratio of purine and pyrimidine bases of DNA molecules and the results of X-ray structural analysis obtained
M. Wilkins and R. Franklin. For the development of the double-stranded model of the DNA molecule, Watson, Crick and Wilkins were awarded the Nobel Prize in 1962.
DNA is the largest biological molecules. Their length ranges from 0.25 (in some bacteria) to 40 mm (in humans). This is significantly larger than the largest protein molecule, which, when unfolded, reaches a length of no more than 100-200 nm. The mass of a DNA molecule is 6x10-12 g.
The diameter of the DNA molecule is 2 nm, the helix pitch is 3.4 nm; Each turn of the helix contains 10 pairs of nucleotides. The helical structure is maintained by numerous hydrogen bonds occurring between complementary nitrogenous bases and hydrophobic interactions. The DNA molecules of eukaryotic organisms are linear. In prokaryotes, DNA, on the contrary, is closed in a ring and has neither 3- nor 5-ends.
When conditions change, DNA, like proteins, can change. undergo denaturation, which is called melting. With a gradual return to normal conditions, the DNA renatures. The function of DNA is the storage, transmission and reproduction of genetic information over generations. The DNA of any cell encodes information about all the proteins of a given organism, about which proteins, in what sequence and in what quantities will be synthesized. The sequence of amino acids in proteins is written in DNA by the so-called genetic (triplet) code.
The main property of DNA is its ability to replicate.
Replication is the process of self-duplication of DNA molecules, which occurs under the control of enzymes. Replication occurs before each nuclear division. It begins with the DNA helix temporarily unwinding under the action of the enzyme DNA polymerase. On each of the chains formed after the rupture of hydrogen bonds, a daughter DNA strand is synthesized according to the principle of complementarity. The material for synthesis is free nucleotides, which are present in the nucleus (Fig. 1.3).
Thus, each polynucleotide chain acts as a template for a new complementary chain (therefore, the process of doubling DNA molecules belongs to template synthesis reactions). The result is two DNA molecules, each of which has one chain remaining from the parent molecule (half), and the other newly synthesized. Moreover, one new chain is synthesized as a whole, and the second - first in the form of short fragments, which are then stitched into a long chain a special enzyme called DNA ligase. As a result of replication, two new DNA molecules are an exact copy of the original molecule.
The biological meaning of replication lies in the accurate transfer of hereditary information from the mother cell to the daughter cells, which occurs during the division of somatic cells.
14. Ribonucleic acids, their types, structure, purpose.
RNA. The structure of RNA molecules is in many ways similar to the structure of DNA molecules. However, there are a number of significant differences. In the RNA molecule, instead of deoxyribose, the nucleotides contain ribose, and instead of thymidyl nucleotide (T), there is uridyl nucleotide (U). The main difference from DNA is that the RNA molecule is a single strand. However, its nucleotides are capable of forming hydrogen bonds with each other (for example, in tRNA, rRNA molecules), but in this case we are talking about an intra-chain connection of complementary nucleotides. RNA chains are much shorter than DNA.
There are several types of RNA in a cell, which differ in molecular size, structure, location in the cell and functions:
1. Messenger RNA (mRNA). This species is the most heterogeneous in size and structure. mRNA is an open polynucleotide chain. It is synthesized in the nucleus with the participation of the enzyme RNA polymerase, complementary to the region of DNA where its synthesis occurs. Despite its relatively low content (3-5% of cell RNA), it performs an important function in the cell: it serves as a matrix for the synthesis of proteins, transmitting information about their structure from DNA molecules. Each cell protein is encoded by a specific mRNA, so the number of their types in the cell corresponds to the number of protein types.
2. Ribosomal RNA (rRNA). These are single-stranded nucleic acids that form ribosomes in complex with proteins - organelles on which protein synthesis occurs. Ribosomal RNAs are synthesized in the nucleus. Information about their structure is encoded in sections of DNA that are located in the region of the secondary constriction of chromosomes. Ribosomal RNAs make up 80% of the total RNA in a cell because there are a huge number of ribosomes in the cell. Ribosomal RNAs have a complex secondary and tertiary structure, forming loops at complementary sites, which leads to the self-organization of these molecules into a body of complex shape. Ribosomes contain three types of rRNA in prokaryotes and four types of rRNA in eukaryotes.
3. Transport (transfer) RNA (tRNA). A tRNA molecule consists of an average of 80 nucleotides. The tRNA content in the cell is about 15% of all RNA. The function of tRNA is to transport amino acids to the site of protein synthesis. The number of different types of tRNA in a cell is small(20-60). They all have a similar spatial organization. Thanks to intrastrand hydrogen bonds, the tRNA molecule acquires a characteristic secondary structure called cloverleaf. The three-dimensional model of tRNA looks somewhat different. There are four loops in tRNA: an acceptor loop (serves as a site for amino acid attachment), an anticodon loop (recognizes a codon in mRNA during translation), and two side loops.
15.Organic substances in cells, their purpose.
IN The cell contains a wide variety of organic compounds, varied in structure and function. Organic substances can be low molecular weight (amino acids, sugars, organic acids, nucleotides, lipids, etc.) and high molecular weight. Most high-molecular organic compounds in cells are biopolymers. Polymers are molecules consisting of a large number of repeating units - monomers, connected to each other by covalent bonds. To biopolymers, i.e. The polymers that make up the cell include proteins, polysaccharides and nucleic acids.
A special group of organic cell compounds are lipids (fats and fat-like substances). All of them are hydrophobic compounds, i.e. insoluble in water, but soluble in non-polar organic solvents (chloroform, benzene, ether). Lipids include neutral fats, phospholipids, waxes, steroids and some other compounds. The functions of lipids in living organisms are diverse. Phospholipids are present in all cells, performing a structural function as the basis of biological membranes. The steroid cholesterol is an important component of membranes in animals. Neutral fats and some other lipids provide energy function. They accumulate in living organisms as reserve nutrients. The oxidation of 1 g of fat releases 38 kJ of energy, which is twice as much as the oxidation of the same amount of glucose. The energy function of fats is related to their storage function. A significant portion of the body's energy reserves are stored in the form of fat. In addition, fats serve as a source of water, which is released during its oxidation. This is especially important for desert animals experiencing water shortages. For example, it is fat deposits that are located in the hump of a camel. A number of lipids have a protective function. In mammals, subcutaneous fat acts as a thermal insulator. Wax protects feathers and animal hair from getting wet. A number of lipids perform a regulatory function in the body. For example, the hormones of the adrenal cortex are steroids by their chemical nature. Some lipids take an active part in metabolism, for example fat-soluble vitamins A, D, E and K.
Carbohydrates (sugars, saccharides) are compounds with the general chemical formula Cn(H2O)n. Based on the number of links in the polymer chain, there are three main classes of carbohydrates: monosaccharides (simple sugars), oligosaccharides (consist of 2-10 molecules of simple sugars) and polysaccharides (consist of more than 10 molecules of simple sugars). Depending on the number of carbon atoms included in the monosaccharide, trioses, tetroses, pentoses, hexoses and heptoses are distinguished.
IN In nature, the most common are hexoses (glucose and fructose) and pentoses (ribose and deoxyribose). Glucose is the main source of energy for the cell; with complete oxidation of 1 g of glucose, 17.6 kJ of energy is released. Ribose and deoxyribose are part of nucleic acids. Of the oligosaccharides, the most common disaccharides are maltose (malt sugar), lactose (milk sugar), and sucrose (beet sugar). Monosaccharides and disaccharides are highly soluble in water and have a sweet taste. Polysaccharides have a high molecular weight, do not have a sweet taste, and are insoluble in water. They are biopolymers. The most common polysaccharides in nature include glucose polymers starch, glycogen and cellulose, as well as chitin, consisting of glucosamine residues. Starch is the main storage substance in plants, glycogen in animals. Cellulose and chitin perform a protective function, ensuring the strength of the integument of plants, animals and fungi. Thus, the main functions of carbohydrates in nature are energy, storage and structural.
Proteins are biopolymers whose monomers are amino acids. 20 different amino acids are involved in the formation of proteins. Amino acids in protein molecules are connected by covalent peptide bonds. A protein molecule can contain up to several thousand amino acids. There are 4 levels of spatial organization of protein molecules. The sequence of amino acids in a polypeptide chain is called the primary structure of a protein. The primary structure of the molecule of any protein is unique and determines its spatial organization, properties and functions in the cell. The secondary structure of a protein is determined by the folding of a chain of amino acids into specific structures called an a-helix and a b-sheet. The secondary structure of a protein is formed by hydrogen bonds. The tertiary structure is formed by folding the polypeptide chain with elements of the secondary structure into a coil (globule) and is maintained by ionic, hydrophilic and covalent (disulfide) bonds between various amino acid residues.
Quaternary structure is characteristic of proteins consisting of several polypeptide chains. The loss of a protein molecule's structural organization, for example due to heating, is called denaturation. Denaturation can be reversible or irreversible. With reversible denaturation, the quaternary, tertiary and secondary structures of the protein may be disrupted, but the primary structure is not disrupted, and when normal conditions return, due to this, renaturation is possible - restoration of the normal configuration. When the primary structure is damaged, denaturation is irreversible.
The most important function of proteins is catalytic. All enzymes and biological catalysts are proteins. Thanks to enzymes, the rate of chemical reactions in a cell increases millions of times. Enzymes are highly specific: each enzyme catalyzes a specific type of chemical reaction in the cell. It is thanks to enzymes that all metabolic reactions occurring in living organisms are possible.
Nucleic acids! (see question 13 above)
16. Minerals in cells, their role, purpose. Osmotic processes in plant and animal cells.
Depending on their content in the body, minerals are divided into 3 groups: macroelements, microelements and ultramicroelements.
Macronutrients are a group of inorganic chemicals present in the body from a few tens of grams to more than a kilogram. The recommended daily intake is more than 200 mg. These include calcium, magnesium, phosphorus, potassium, sodium, chlorine and sulfur. Macroelements ensure the normal functioning of all systems and organs; the cells of the body are “built” from them. Without them, metabolism in the human body is impossible.
Microelements include mineral substances, the content of which in the body ranges from several grams to tenths of a gram. The need for them is calculated in milligrams, but they participate in biochemical processes and are necessary for the body. These include: iron, copper, manganese, zinc, cobalt, iodine, fluorine, chromium, molybdenum, vanadium, nickel, strontium, silicon and selenium. Recently, the term micronutrient, borrowed from European languages, has begun to be used.
Ultramicroelements are contained in the body in negligible quantities, but have high biological activity. The main representatives are gold, lead, mercury, silver, radium, rubidium, uranium. Some of them are distinguished not only by their low content in ordinary foods, but also by their toxicity if consumed in relatively large doses. MINERAL SUBSTANCES - ROLE IN THE BODY Minerals play a large and diverse role in the human body. They are part of its structure and perform a large number of important functions.
1. Regulate water-salt metabolism.
2. Maintain osmotic pressure in cells and intercellular fluids.
3. Maintain acid-base balance.
4. Ensure normal functioning of the nervous and cardiac-vascular, digestive and other systems.
5. Provide hematopoiesis and blood clotting processes.
6. They are part of or activate the action of enzymes, hormones, vitamins and thus participate in all types of metabolism.
7. They regulate the transmembrane potential necessary for the normal functioning of cells, the conduction of nerve impulses and the contraction of muscle fibers.
8. Maintains the structural integrity of the body.
9. They participate in the construction of body tissues, especially bones, where phosphorus and calcium are the main structural components.
10. They maintain the normal salt composition of the blood and participate in the structure of the elements that form it.
11. Affect the protective functions of the body, its immunity.
12. They are an essential part of food, and their prolonged deficiency or excess in the diet leads to metabolic disorders and even diseases.
Osmotic refers to phenomena occurring in a system consisting of two solutions separated by a semi-permeable membrane. In a plant cell, the role of semi-permeable films is performed by the boundary layers of the cytoplasm: plasmalemma and tonoplast.
Plasmolemma is the outer membrane of the cytoplasm adjacent to the cell membrane. Tonoplast is the inner membrane of the cytoplasm surrounding the vacuole. Vacuoles are cavities in the cytoplasm filled with cell sap - an aqueous solution of carbohydrates, organic acids, salts, low molecular weight proteins, and pigments.
The concentration of substances in cell sap and in the external environment (soil, water bodies) are usually not the same. If the intracellular concentration of substances is higher than in the external environment, water from the environment will diffuse into the cell, more precisely into the vacuole, at a higher speed than in the opposite direction, i.e. from the cell into the environment. The greater the concentration of substances contained in the cell sap, the stronger the suction force - the force with which the cell<всасывает воду>. With an increase in the volume of cell sap, due to the entry of water into the cell, its pressure on the cytoplasm, which fits tightly to the membrane, increases. When a cell is completely saturated with water, it has its maximum volume. The state of internal cell tension caused by high
water content and the developing pressure of the cell contents on its membrane is called turgor. Turgor ensures that organs retain their shape (for example, leaves, non-lignified stems) and position in space, as well as their resistance to the action of mechanical factors. If a cell is in a hypertonic solution, the concentration of which is greater than the concentration of cell sap, then the rate of diffusion of water from the cell sap will exceed the rate of diffusion of water into the cell from the surrounding solution. Due to the release of water from the cell, the volume of cell sap is reduced and turgor decreases. A decrease in the volume of the cell vacuole is accompanied by the separation of the cytoplasm from the membrane - plasmolysis occurs.
17. Biosynthesis of proteins in cells.
Protein biosynthesis occurs in every living cell. It is most active in young growing cells, where proteins are synthesized to build their organelles, as well as in secretory cells, where enzyme proteins and hormone proteins are synthesized.
The main role in determining the structure of proteins belongs to DNA. A piece of DNA containing information about the structure of one protein is called a gene. A DNA molecule contains several hundred genes. The DNA molecule contains a code for the sequence of amino acids in a protein in the form of specifically combined nucleotides. The DNA code was almost completely deciphered. Its essence is as follows. Each amino acid corresponds to a section of a DNA chain consisting of three adjacent nucleotides.
For example, the T-T-T section corresponds to the amino acid lysine, the A-C-A section corresponds to cystine, C-A-A to valine, etc. There are 20 different amino acids, the number of possible combinations of 4 nucleotides of 3 is 64. Therefore , triplets are abundantly sufficient to encode all amino acids.
Protein synthesis is a complex multi-stage process, representing a chain of synthetic reactions proceeding according to the principle of matrix synthesis.
Since DNA is located in the cell nucleus, and protein synthesis occurs in the cytoplasm, there is an intermediary that transfers information from DNA to ribosomes. This messenger is mRNA. : In protein biosynthesis, the following stages are determined, occurring in different parts of the cell:
1. The first stage is synthesis i-RNA occurs in the nucleus, during which the information contained in the DNA gene is transcribed into i-RNA. This process is called transcription (from the Latin “transcript” - rewriting).
2. At the second stage, amino acids are combined with molecules tRNAs, which sequentially consist of three nucleotides - anticodons, with the help of which their triplet codon is determined.
3. The third stage is the process of direct synthesis of polypeptide bonds, called translation. It occurs in ribosomes.
4. At the fourth stage, the formation of the secondary and tertiary structure of the protein occurs, that is, the formation of the final structure of the protein.
Thus, in the process of protein biosynthesis, new protein molecules are formed in accordance with the exact information contained in the DNA. This process ensures the renewal of proteins, metabolic processes, cell growth and development, that is, all the life processes of the cell.
18.Energy metabolism in cells.
The body needs energy to function. Plants accumulate solar energy in organic matter during photosynthesis. In the process of energy metabolism, organic substances are broken down and the energy of chemical bonds is released. Partially it is dissipated in the form of heat, and partially stored in ATP molecules. In animals, energy metabolism occurs in three stages.
The first stage is preparatory. Food enters the body of animals and humans in the form of complex high-molecular compounds. Before entering cells and tissues, these substances must be broken down into low-molecular substances that are more accessible for cellular absorption. At the first stage, hydrolytic breakdown of organic substances occurs, which occurs with the participation of water. It occurs under the action of enzymes in the digestive tract of multicellular animals, in the digestive vacuoles of unicellular animals, and at the cellular level in lysosomes. Preparatory stage reactions:
proteins + H20 -> amino acids + Q;
fats + H20 -> glycerol + higher fatty acids + Q; polysaccharides -> glucose + Q.
In mammals and humans, proteins are broken down into amino acids in the stomach and duodenum under the action of enzymes - peptide hydrolases (pepsin, trypsin, chemotrypsin). The breakdown of polysaccharides begins in the oral cavity under the action of the enzyme ptyalin, and then continues in the duodenum under the action of amylase. Fats are also broken down there by the action of lipase. All energy released in this case is dissipated in the form of heat.
The resulting low molecular weight substances enter the blood and are delivered to all organs and cells. In cells they enter the lysosome or directly into the cytoplasm. If cleavage occurs at the cellular level in lysosomes, the substance immediately enters the cytoplasm. At this stage, substances are prepared for intracellular breakdown.
The second stage is oxygen-free oxidation. The second stage is carried out at the cellular level in the absence of oxygen. It occurs in the cytoplasm of the cell. Let's consider the breakdown of glucose as one of the key metabolic substances in the cell. All other organic substances (fatty acids, glycerol, amino acids) are drawn into the processes of its transformation at different stages. The oxygen-free breakdown of glucose is called glycolysis. Glucose undergoes a series of successive transformations (Fig. 16). First, it is converted to fructose and phosphorylated
Activated by two ATP molecules and converted into fructose diphosphate. Next, the six-carbon carbohydrate molecule breaks down into two three-carbon compounds - two molecules of glycerophosphate (triose). After a series of reactions, they are oxidized, losing two hydrogen atoms each, and are converted into two molecules of pyruvic acid (PVA). As a result of these reactions, four ATP molecules are synthesized. Since two ATP molecules were initially spent on activating glucose, the total result is 2 ATP. Thus, the energy released during the breakdown of glucose is partially stored in two ATP molecules, and partially consumed in the form of heat. The four hydrogen atoms that were removed during the oxidation of glycerophosphate combine with the hydrogen carrier NAD+ (nicotinamide dinucleotide phosphate). It is the same hydrogen carrier as NADP+, but is involved in energy metabolism reactions.
The third stage is biological oxidation, or respiration. This stage occurs only in the presence of oxygen and is otherwise called oxygen. It occurs in mitochondria.
Pyruvic acid from the cytoplasm enters the mitochondria, where it loses a molecule of carbon dioxide and is converted into acetic acid, combining with the activator and carrier coenzyme-A (Fig. 17). The resulting acetyl-CoA then enters into a series of cyclic reactions. The products of oxygen-free decomposition - lactic acid, ethyl alcohol - also further undergo changes and undergo oxidation with oxygen. Lactic acid is converted into pyruvic acid if it is formed due to a lack of oxygen in animal tissues. Ethyl alcohol is oxidized to acetic acid and binds to CoA.
Cyclic reactions in which acetic acid is converted are called the cycle of di- and tricarboxylic acids, or the Krebs cycle, named after the scientist who first described these reactions. As a result of a series of sequential reactions, decarboxylation occurs - the removal of carbon dioxide and oxidation - the removal of hydrogen from the resulting substances. Carbonic
the gas formed during decarboxylation of PVC and in the Krebs cycle is released from the mitochondria, and then from the cell and body during respiration. Thus, carbon dioxide is formed directly during the decarboxylation of organic substances. All the hydrogen that is removed from the intermediate substances combines with the NAD+ transporter, and NAD 2H is formed. During photosynthesis, carbon dioxide combines with intermediate substances and is reduced with hydrogen. Here the process is reversed.
Let us now trace the path of the NAD 2H molecules. They arrive at the cristae of mitochondria, where the respiratory chain of enzymes is located. On this chain, hydrogen is abstracted from the carrier with the simultaneous removal of electrons. Each molecule of reduced NAD 2H donates two hydrogens and two electrons. The energy of the removed electrons is very high. They enter the respiratory chain of enzymes, which consists of proteins - cytochromes. Moving through this system in cascade, the electron loses energy. Due to this energy, ATP molecules are synthesized in the presence of the enzyme ATPase. Simultaneously with these processes, hydrogen ions are pumped through the membrane to its outer side. In the process of oxidation of 12 molecules of NAD-2H, which were formed during glycolysis (2 molecules) and as a result of reactions in the Krebs cycle (10 molecules), 36 ATP molecules are synthesized. The synthesis of ATP molecules coupled with the process of hydrogen oxidation is called oxidative phosphorylation. This process was first described by the Russian scientist V.A. Engelhardt in 1931. The final electron acceptor is an oxygen molecule that enters the mitochondria during respiration. Oxygen atoms on the outside of the membrane accept electrons and become negatively charged. Positive hydrogen ions combine with negatively charged oxygen to form water molecules. Let us remember that atmospheric oxygen is formed as a result of photosynthesis during the photolysis of water molecules, and hydrogen is used to reduce carbon dioxide. In the process of energy exchange, hydrogen and oxygen are recombined and converted into water.
19.Organization of the hereditary apparatus in eukaryotic cells. Somatic cell genome. The genetic apparatus of a eukaryotic cell is located in the nucleus and is protected by a membrane. Eukaryotic DNA is linear, in a 50/50 ratio connected to proteins. They form a chromosome. Unlike eukaryotes, DNA in prokaryotes is circular, naked (almost not connected to proteins), lies in a special region of the cytoplasm - the nucleoid and is separated from the rest of the cytoplasm using a membrane. A eukaryotic cell divides by mitosis, meiosis, or a combination of these methods. The life cycle of eukaryotes consists of two nuclear phases. The first (haplophase) is distinguished by a single set of chromosomes. In the second phase (diplophase), two haploid cells fuse to form a diploid cell, which contains a double set of chromosomes. After a few divisions, the cell becomes haploid again.
There are 24 different chromosomes in the genome: 22 of them do not affect sex, and two chromosomes (X and Y) determine sex. Chromosomes 1 to 22 are numbered in order of decreasing size. Somatic cells usually have 23 chromosome pairs: one copy of chromosomes 1 to 22 from each parent, respectively, as well as an X chromosome from the mother and a Y or X chromosome from the father. In total, it turns out that a somatic cell contains 46 chromosomes.
20.Gene, genotype, homo and heterozygosity. Genetic determination of the phenotype. Gene is a structural and functional unit of heredity of living organisms. Gene
is a section of DNA that specifies the sequence of a specific polypeptide or functional RNA. Genes (more precisely, gene alleles) determine the hereditary characteristics of organisms that are transmitted from parents to offspring during reproduction. At the same time, some organelles (mitochondria, plastids) have their own DNA that determines their characteristics, which is not part of the genome of the organism.
Among some organisms, mostly unicellular, horizontal gene transfer is found that is not associated with reproduction.
The term "gene" was coined in 1909 by the Danish botanist Vilhelm Johansen, three years after the term "genetics" was coined by William Bateson.
Gene properties:
1. stability - the ability to maintain structure;
2. lability - the ability to mutate repeatedly;
3. multiple allelism - many genes exist in a population in multiple molecular forms;
4. allelicity - in the genotype of diploid organisms there are only two forms of the gene;
5. specificity - each gene encodes its own trait;
6. pleiotropy - multiple effect of a gene;
7. expressivity - the degree of expression of a gene in a trait;
8. penetrance - frequency of manifestation of a gene in a phenotype;
9. amplification - increasing the number of copies of a gene.
GENOTYPE, all the genes of an organism, which together determine all the characteristics of the organism - its phenotype. If the genome is the genetic characteristic of a species, then the genotype is the genetic characteristic (constitution) of a particular organism. When studying the inheritance of certain traits, not all genes are called a genotype, but only those that determine these traits.
The genotype is not a mechanical sum of autonomous, independently acting genes, but a complex and integral system - a genotypic environment in which the work and implementation of each gene depends on the influence of other genes. Thus, with the interaction of allelic genes, in addition to simple cases of dominance and recessiveness, incomplete dominance, codominance (the manifestation of two allelic genes at once) and overdominance (a stronger manifestation of the trait in heterozygotes compared to homozygotes) are possible.
Individuals with the same genotype, developing under different environmental conditions, can have different phenotypes. In this regard, genetics developed the idea of a reaction norm, i.e., the boundaries within which the phenotype of a given genotype can change under the influence of different environmental conditions. Thus, the scope of phenotypic variability is also determined by the genotype, or, in other words, the phenotype is the result of the interaction of the genotype and the external environment. Obtaining cells and individuals with the same genotype through vegetative propagation and cloning is important both for solving scientific problems and practical problems in agriculture, medicine, and biotechnology.
Homozygosity is a state of the hereditary apparatus of an organism in which homologous chromosomes have the same form of a given gene. The transition of a gene to a homozygous state leads to the manifestation of recessive alleles in the structure and function of the body (phenotype), the effect of which, in heterozygosity, is suppressed by dominant alleles. The test for homozygosity is the absence of segregation during certain types of crossing. A homozygous organism produces only one type of gamete for a given gene.
Heterozygosity is a condition inherent in any hybrid organism, in which its homologous chromosomes carry different forms (alleles) of a particular gene or differ in the relative position of genes. The term “Heterozygosity” was first introduced by the English geneticist W. Bateson in 1902. Heterozygosity occurs when gametes of different genetic or structural composition merge into a heterozygote. Structural heterozygosity occurs when a chromosomal rearrangement of one of the homologous chromosomes occurs; it can be detected in meiosis or mitosis. Heterozygosity is revealed using test crossing. Heterozygosity, as a rule, is a consequence of the sexual process, but can arise as a result of mutation. At
heterozygosity, the effect of harmful and lethal recessive alleles is suppressed by the presence of the corresponding dominant allele and manifests itself only when this gene transitions to a homozygous state. Therefore, heterozygosity is widespread in natural populations and is, apparently, one of the causes of heterosis. The masking effect of dominant alleles in heterozygosity is the reason for the persistence and spread of harmful recessive alleles in the population (the so-called heterozygous carriage).
Phenotype (from the Greek word phainotip - I reveal, I reveal) is a set of characteristics inherent in an individual at a certain stage of development. The phenotype is formed on the basis of the genotype, mediated by a number of environmental factors. In diploid organisms, dominant genes appear in the phenotype.
Phenotype is a set of external and internal characteristics of an organism acquired as a result of ontogenesis (individual development).
First, most of the molecules and structures encoded by genetic material are not noticeable in the external appearance of the organism, although they are part of the phenotype. For example, this is exactly the case with human blood groups. Therefore, the expanded definition of phenotype should include characteristics that can be detected by technical, medical or diagnostic procedures. A further, more radical expansion may include acquired behavior or even the influence of an organism on the environment and other organisms. Phenotype can be defined as the "carrying out" of genetic information towards environmental factors. To a first approximation, we can talk about two characteristics of the phenotype: a) the number of directions of removal characterizes the number of environmental factors to which the phenotype is sensitive - the dimension of the phenotype; b) the “distance” of removal characterizes the degree of sensitivity of the phenotype to a given environmental factor. Together, these characteristics determine the richness and development of the phenotype. The more multidimensional the phenotype and the more sensitive it is, the further the phenotype is from the genotype, the richer it is.
21.Genetic code, its properties:
The genetic code is a system for the arrangement of nucleotides in a DNA molecule that controls the sequence of amino acids in a protein molecule.
In the variety of proteins that exist in nature, about 20 different amino acids have been discovered. To encrypt such a number of them, a sufficient number of combinations of nucleotides can only be provided by a triplet code, in which each amino acid is encrypted by three adjacent nucleotides. In this case, = 64 triplets are formed from four nucleotides. A code consisting of two nucleotides would make it possible to encrypt only = 16 different amino acids.
1) the same amino acids can be encoded by different triplets (codon synonyms). This code is called degenerate or redundant. Duplicate triplets differ in the third nucleotide.
2) In a DNA molecule, each nucleotide is included only in any one codon. Therefore the DNA code non-overlapping. Continuity– the nucleotide sequence is read triplet by triplet without gaps. Doc-vom non-overlapping gene. The code serves to replace only one amino acid in the peptide when replacing one nucleotide in DNA.
3) Specificity - Each triplet is capable of encoding only one specific amino acid.
4) Versatility ( complete correspondence of the code in different species of living organisms.) genetic code indicates the unity of origin of the entire diversity of living forms on Earth in the process of biological evolution.
The sequence of triplets determines the order of amino acids in the protein molecule, i.e., collinearity occurs. In other words, collinearity is a property that produces the sequence of amino acids in a protein in which the corresponding codons are located in the gene. This means that the position of each amino acid in the polypeptide chain depends on a specific region of the gene. The genetic code is considered collinear if the codons of nucleic acids and their corresponding amino acids in the protein are located in the same linear order.
22. The structure of chromosomes, their types, classification in the human karyotype.
The term chromosome was proposed in 1888 by the German morphologist W. Waldeyer, who used it to designate the intranuclear structures of a eukaryotic cell that are well stained with basic dyes (from the Greek chromium - color, paint, and soma - body).
Chem. chromosome composition:
They consist mainly of DNA and proteins, which form a nucleoprotein complex called chromatin, which received its name for its ability to be stained with basic dyes. Chromatin consists of two types of proteins: histones and non-histone proteins.
Histones are presented in five fractions: HI, H2A, H2B, NZ, H4. Being positively charged basic proteins, they bind quite firmly to DNA molecules, which prevents the reading of the biological information contained in it. This is their regulatory role. In addition, these proteins perform a structural function, ensuring the spatial organization of DNA in chromosomes.
The number of non-histone protein fractions exceeds 100. Among them are enzymes for RNA synthesis and processing, DNA reduplication and repair. Acidic proteins of chromosomes also perform structural and regulatory roles. In addition to DNA and proteins, chromosomes also contain RNA, lipids, polysaccharides, and metal ions.
Chromosome RNA is represented partly by transcription products that have not yet left the site of synthesis. Some fractions have a regulatory function.
The regulatory role of chromosome components is to prohibit or allow copying of information from the DNA molecule.
The mass ratios of DNA: histones: non-histone proteins: RNA: lipids are 1:1:(0.2-0.5): (0.1-0.15):(0.01--0.03). Other components are found in small quantities.
Chromosome morphology
Light microscopy. In the first half of mitosis, they consist of two chromatids connected to each other in the region of the primary constriction (centromere or kinetochore), a specially organized region of the chromosome common to both sister chromatids. In the second half of mitosis, the chromatids separate from each other. They form single-filamentous daughter chromosomes distributed between daughter cells.
∙ equal-armed, or metacentric (with a centromere in the middle),
∙ unequal arms, or submetacentric (with the centromere shifted to one end),
∙ rod-shaped, or acrocentric (with a centromere located almost at the end of the chromosome),
∙ point - very small, the shape of which is difficult to determine
The set of all structural and quantitative features of the complete set of chromosomes characteristic of cells of a particular type of living organism is called a karyotype.
The karyotype of the future organism is formed during the fusion of two germ cells (sperm and egg). In this case, their chromosome sets are combined. The nucleus of a mature germ cell contains half the set of chromosomes (for humans - 23). Such a single set of chromosomes, similar to that in germ cells, is called haploid and is designated - n. When an egg is fertilized by a sperm, a species-specific karyotype is recreated in a new organism, which includes 46 chromosomes in humans. The complete chromosome composition of an ordinary somatic cell is diploid (2n). In a diploid set, each chromosome has another paired chromosome similar in size and centromere location. Such chromosomes are called homologous. Homologous chromosomes not only look alike, but also contain genes responsible for the same traits.
A woman’s karyotype normally contains two X chromosomes, and can be written as 46, XX. The karyotype of a man includes X and Y chromosomes (46, XY). All the remaining 22 pairs of chromosomes are called
autosomes. Autosome groups:
∙ Group A includes 3 pairs of the longest chromosomes (1, 2, 3rd);
∙ group B combines 2 pairs of large submetacentric chromosomes (4 and 5th).
∙ group C, including 7 pairs of medium-sized submetacentric autosomes (with 6th to 12th). Based on morphological features, chromosome X is difficult to distinguish from this group.
∙ Medium acrocentric chromosomes 13, 14 and The 15th pairs are in Group D.
∙ Three pairs of small submetacentric chromosomes make up group E (16, 17 and 18th).
∙ The smallest metacentric chromosomes (19 and 20) make up group F.
The 21st and 22nd pairs of short acrocentric chromosomes are included in group G. The Y chromosome is morphologically very similar to the autosomes of this group.
23. Chromosomal theory of T. Morgan.
Chromosomal theory of heredity - a theory according to which the transmission of hereditary information over a number of generations is associated with the transmission of chromosomes, in which genes are located in a certain and linear sequence.
1. Material carriers of heredity - genes are located in chromosomes and are located in them linearly at a certain distance from each other.
2. Genes located on the same chromosome belong to the same linkage group. The number of linkage groups corresponds to the haploid number of chromosomes.
3. Traits whose genes are located on the same chromosome are inherited linked.
4. In the offspring of heterozygous parents, new combinations of genes located in the bottom pair of chromosomes can arise as a result of crossing over during the process of meiosis.
5. The frequency of crossing over, determined by the percentage of crossover individuals, depends on the distance between genes.
6. Based on the linear arrangement of genes on a chromosome and the frequency of crossing over as an indicator of the distance between genes, chromosome maps can be constructed.
The work of T. Morgan and his colleagues not only confirmed the importance of chromosomes as the main carriers of hereditary material represented by individual genes, but also established the linearity of their location along the length of the chromosome.
Proof of the connection between the material substrate of heredity and variability with chromosomes was, on the one hand, the strict correspondence of the patterns of inheritance of characters discovered by G. Mendel to the behavior of chromosomes during mitosis, meiosis and fertilization. On the other hand, in the laboratory of T. Morgan, a special type of inheritance of traits was discovered, which was well explained by the connection of the corresponding genes with the X chromosome. We are talking about sex-linked inheritance of eye color in Drosophila.
The idea of chromosomes as carriers of gene complexes was expressed on the basis of the observation of linked inheritance of a number of parental characteristics with each other during their transmission over a series of generations. This linkage of non-alternative traits was explained by the placement of the corresponding genes on one chromosome, which is a fairly stable structure that preserves the composition of genes over generations of cells and organisms.
According to the chromosomal theory of heredity, the totality of genes that make up one chromosome forms clutch group. Each chromosome is unique
the set of genes contained in it. The number of linkage groups in the hereditary material of organisms of a given species is thus determined by the number of chromosomes in the haploid set of their germ cells. During fertilization, a diploid set is formed, in which each linkage group is represented by two variants - paternal and maternal chromosomes, carrying original sets of alleles of the corresponding gene complex.
The idea of the linear arrangement of genes on each chromosome arose on the basis of the observation of often occurring recombination (interchange) between maternal and paternal gene complexes contained in homologous chromosomes. It was found that the frequency of recombination is characterized by a certain constancy for each pair of genes in a given linkage group and is different for different pairs. This observation made it possible to suggest a connection between the frequency of recombination and the sequence of genes on the chromosome and the process of crossing over that occurs between homologues in prophase I of meiosis (see section 3.6.2.3).
The idea of a linear distribution of genes explained well the dependence of the frequency of recombination on the distance between them in the chromosome.
The discovery of linked inheritance of non-alternative traits formed the basis for the development of a technique for constructing genetic maps of chromosomes using the hybridological method of genetic analysis.
Thus, at the beginning of the 20th century. The role of chromosomes as the main carriers of hereditary material in a eukaryotic cell was irrefutably proven. Confirmation of this was obtained by studying the chemical composition of chromosomes.
24. Division of somatic cells. Characteristics of the phases of mitosis.
The division of a somatic cell and its nucleus (mitosis) is accompanied by complex multiphase transformations of chromosomes: 1) in the process of mitosis, the doubling of each chromosome occurs based on the complementary replication of a DNA molecule with the formation of two sister filamentous copies (chromatids) connected at the centromere; 2) subsequently, sister chromatids are separated and equivalently distributed over the nuclei of daughter cells.
As a result, the identity of the chromosome set and genetic material is maintained in dividing somatic cells.
Special mention should be made about neurons - highly differentiated postmitotic cells that do not undergo cell division throughout life. The compensatory capabilities of neurons in response to the action of damaging factors are limited to intracellular regeneration and DNA repair in the non-dividing nucleus, which largely determines the specificity of neuropathological processes of hereditary and non-hereditary nature.
Mitosis is a complex division of the cell nucleus, the biological significance of which lies in the exact identical distribution of daughter chromosomes with the genetic information they contain between the nuclei of daughter cells; as a result of this division, the nuclei of daughter cells have a set of chromosomes identical in quantity and quality to that of the mother cell.
Chromosomes are the main substrate of heredity; they are the only structure for which the independent ability of reduplication has been proven. All other cell organelles capable of reduplication carry it out under the control of the nucleus. In this regard, it is important to maintain a constant number of chromosomes and distribute them evenly between daughter cells, which is achieved by the entire mechanism of mitosis. This method of division in plant cells was discovered in 1874 by the Russian botanist I. D. Chistyakov, and in animal cells - in 1878 by the Russian histologist P. I. Peremezhko (1833-1894).
IN In the process of mitosis (Fig. 2.15), five phases occur sequentially: prophase, prometaphase, metaphase, anaphase and telophase. These phases, immediately following each other, are connected by imperceptible transitions. Each previous one determines the transition to the next one.
IN As a cell begins to divide, the chromosomes take on the appearance of a ball of many thin, weakly spiraled threads. At this time, each chromosome consists of two sister chromatids. The formation of chromatids occurs according to the matrix principle in S-period of the mitotic cycle as a consequence of DNA replication.
At the very beginning of prophase, and sometimes even before its onset, the centriole is divided into two, and they diverge towards
poles of the nucleus. At the same time, the chromosomes undergo a process of twisting (spiralization), as a result of which they are significantly shortened and thickened. The chromatids move somewhat away from each other, remaining connected only by centromeres. A gap appears between the chromatids. Towards the end of prophase in animal cells, a radiate figure forms around the centrioles. Most plant cells do not have centrioles.
By the end of prophase, the nucleoli disappear, the nuclear membrane dissolves under the action of enzymes from lysosomes, and the chromosomes are immersed in the cytoplasm. At the same time, an achromatin figure appears, which consists of threads stretching from the poles of the cell (if there are centrioles, then from them). Achromatic filaments are attached to the centromeres of chromosomes. A characteristic figure resembling a spindle is formed. Electron microscopic studies have shown that the spindle threads are tubes, tubules.
In prometaphase, in the center of the cell there is cytoplasm, which has insignificant viscosity. The chromosomes immersed in it are directed to the equator of the cell.
In metaphase, chromosomes are in an ordered state at the equator. All chromosomes are clearly visible, due to which the study of karyotypes (counting the number, studying the shapes of chromosomes) is carried out precisely at this stage. At this time, each chromosome consists of two chromatids, the ends of which have diverged. Therefore, on metaphase plates (and idiograms from metaphase chromosomes) the chromosomes are A-shaped. The study of chromosomes is carried out precisely at this stage.
In anaphase, each chromosome is longitudinally split along its entire length, including in the region
centromeres, or more precisely, divergence of chromatids occurs, which then become sister, or daughter, chromosomes. They have a rod-shaped shape, curved in the area of the primary constriction. The spindle threads contract, move towards the poles, and behind them the daughter chromosomes begin to diverge towards the poles. Their divergence is carried out quickly and
everyone at the same time, as if on command. This is clearly shown by film footage of dividing cells. Violent processes also occur in the cytoplasm, which on film resembles a boiling liquid.
During telophase, the daughter chromosomes reach the poles. After this, the chromosomes despiral, lose their clear outlines, and nuclear membranes form around them. The nucleus acquires a structure similar to that of the interphase mother cell. The nucleolus is restored.
25. Human germ cells, their structure. Types of egg structure.
To participate in sexual reproduction, gametes are produced in parent organisms - cells specialized to ensure the generative function.
The fusion of maternal and paternal gametes leads to the emergence of a zygote - a cell that is a daughter individual at the first, earliest stage of individual development.
U In some organisms, a zygote is formed as a result of the union of gametes that are indistinguishable in structure. In such cases we talk about isogamy.
U In most species, according to structural and functional characteristics, germ cells are divided into
maternal (eggs) and paternal (sperm). As a rule, eggs and sperm are produced by different organisms - female (females) and male (males). The phenomenon lies in the division of gametes into eggs and sperm, and individuals into females and males sexual dimorphism (Fig. 5.1; 5.2). Its presence in nature reflects differences in the tasks solved in the process of sexual reproduction by male or female gametes, male or female.
Human male reproductive cells - sperm , or sperm, about 70 microns long, have a head, neck and tail.
The sperm is covered with a cytolemma, which in the anterior section contains a receptor that ensures recognition of the receptors of the egg.
The sperm head includes a small dense nucleus with a haploid set of chromosomes. The anterior half of the nucleus is covered with a flat sac, which makes up the sperm cap. It contains the acrosome (from the Greek asgo - top, soma - body),
consisting of a modified Golgi complex. The acrosome contains a set of enzymes. In the nucleus of a human spermatozoon, occupying
the bulk of the head contains 23 chromosomes, one of which is the sex chromosome (X or Y), the rest are autosomes. The tail section of the sperm consists of intermediate, main and terminal parts.
When examining spermatozoons under an electron microscope, it was discovered that the protoplasm of its head is not in a colloidal, but in a liquid crystalline state. This achieves spermatozoon resistance to adverse environmental influences. For example, they are less damaged by ionizing radiation compared to immature germ cells.
All spermatozoa carry the same (negative) electrical charge, which prevents them from sticking together.
A person produces about 200 million sperm
Eggs or oocytes(from Latin ovum - egg), mature in immeasurably smaller quantities than sperm. During a woman's sexual cycle (24-28 days), as a rule, one egg matures. Thus, during the childbearing period, about 400 mature eggs are formed.
The release of an oocyte from the ovary is called ovulation. The oocyte released from the ovary is surrounded by a crown of follicular cells, the number of which reaches 3-4 thousand. It is picked up by the fimbriae of the fallopian tube (oviduct) and moves along it. Here the maturation of the germ cell ends. Egg has a spherical shape, a larger volume of cytoplasm than a sperm, and does not have the ability to move independently.
Structure. The human egg has a diameter of about 130 microns. Adjacent to the cytolemma is the shiny, or transparent, zone and then a layer of follicular cells. The nucleus of the female germ cell has a haploid set of chromosomes with an X-sex chromosome, a well-defined nucleolus, and many pore complexes in the karyolemma.
IN During the growth period of the oocyte, intensive processes of mRNA and rRNA synthesis occur in the nucleus.
IN The protein synthesis apparatus (endoplasmic reticulum, ribosomes) and the Golgi apparatus are developed in the cytoplasm. The number of mitochondria is moderate; they are located near the yolk nucleus, where intensive synthesis occurs
yolk, the cell center is absent. In the early stages of development, the Golgi apparatus is located near the nucleus, and during the maturation of the egg it moves to the periphery of the cytoplasm.
The eggs are covered with layers that perform a protective function, provide the necessary type of metabolism, in placental mammals they serve to introduce the embryo into the wall of the uterus, and also perform other functions.
The cytolemma of the egg has microvilli located between the processes of the follicular cells. Follicular cells perform trophic and protective functions.
Oocytes are much larger than somatic cells. The intracellular structure of the cytoplasm in them is specific for each animal species, which ensures species-specific (and often individual) developmental characteristics. The eggs contain a number of substances necessary for the development of the embryo. These include the nutrient material (yolk).
Classification of eggs is based on the presence, quantity and distribution of yolk (lecithos), which is a protein-lipid inclusion in the cytoplasm used to nourish the embryo.
There are no-yolk (alecital), low-yolk (oligolecithal), medium-yolk (mesolecithal), multi-yolk (polylecithal) eggs.
In humans, the presence of a small amount of yolk in the egg is due to the development of the embryo in the mother’s body.
Ovule polarity. When there is a small amount of yolk in the egg, it is usually distributed evenly in the cytoplasm and the nucleus is located approximately in the center. Such eggs are called isolecithal(from Greek isos - equal). In most vertebrates, there is a lot of yolk, and it is distributed unevenly in the cytoplasm of the egg. This anisolecithal cells. The bulk of the yolk accumulates at one of the poles of the cell - vegetative pole. Such eggs are called telolecytal(from Greek telos - end). The opposite pole, to which the active cytoplasm free from yolk is pushed, is called animal. If the yolk is nevertheless immersed in the cytoplasm and is not isolated from it in the form of a separate fraction, as in sturgeons and amphibians, eggs are called moderately telolecithal. If the yolk is completely separated from the cytoplasm, as in amniotes, then this sharply telolecithal eggs.
26. Reproduction of the living. Classification of reproduction methods.
Reproduction, or reproduction, is one of the main properties that characterize life. Reproduction refers to the ability of organisms to produce others like themselves. The phenomenon of reproduction is closely related to one of the features that characterize life - discreteness. As you know, a whole organism consists of discrete units - cells. The life of almost all cells is shorter than the life of an individual, so the existence of each individual is supported by cell reproduction. Each type of organism is also discrete, that is, it consists of individual individuals. Each of them is mortal. The existence of a species is supported by the reproduction (reproduction) of individuals. Consequently, reproduction is a necessary condition for the existence of a species and the continuity of successive generations within a species. The classification of forms of reproduction is based on the type of cell division: mitotic (asexual) and meiotic (sexual). Forms of reproduction can be represented as the following diagram
Asexual reproduction. In unicellular eukaryotes this is a division based on mitosis, in prokaryotes it is the division of the nucleoid, and in multicellular organisms it is vegetative (Latin vegetatio
Grow) reproduction, i.e., by parts of the body or a group of somatic cells.
Asexual reproduction of unicellular organisms. In unicellular plants and animals, the following forms of asexual reproduction are distinguished: fission, endogony, multiple fission (schizogony) and budding.
Division is typical for unicellular organisms (amoebas, flagellates, ciliates). First, mitotic division of the nucleus occurs, and then an ever-deepening constriction appears in the cytoplasm. In this case, daughter cells receive an equal amount of information. Organelles are usually evenly distributed. In a number of cases, it was found that division is preceded by their doubling. After division, the daughter individuals grow and, having reached the size of the mother’s body, move on to a new division.
Endogony is internal budding. When two daughter individuals are formed - endodiogony - the mother gives only two offspring (this is how Toxoplasma reproduces), but there may be multiple internal budding, which will lead to schizogony.
Schizogony, or multiple fission, is a form of reproduction that developed from the previous one. It is also found in unicellular organisms, for example in the causative agent of malaria - Plasmodium falciparum. With schizogony, multiple divisions of the nucleus occur without cytokinesis, and then the entire cytoplasm is divided into particles that separate around the nuclei. One cell produces many daughter cells. This form of reproduction usually alternates with sexual reproduction.
Budding consists of the initial formation of a small tubercle containing a daughter nucleus, or nucleoid, on the mother cell. The bud grows, reaches the size of the mother and then separates from it. This form of reproduction is observed in bacteria, yeast fungi, and among unicellular animals - in sucking ciliates.
Sporulation found in animals belonging to the phylum of protozoa, the class of Sporozoans. A spore is one of the stages of the life cycle that serves for reproduction; it consists of a cell covered with a membrane that protects it from unfavorable environmental conditions. Some bacteria can form spores after sexual intercourse. Bacterial spores serve not for reproduction, but for surviving unfavorable conditions and in their biological significance differ from the spores of protozoa and multicellular plants.
Vegetative propagation of multicellular organisms w-nyh During vegetative reproduction in multicellular animals, a new organism is formed from a group of cells that separates from the mother organism. Vegetative reproduction occurs only in the most primitive of multicellular animals: sponges, some coelenterates, flatworms and annelids.
In sponges and hydra, due to multiplication, groups of cells on the body are formed protrusions (kidneys). The kidney contains ecto- and endoderm cells. In the hydra, the bud gradually enlarges, tentacles form on it, and finally it separates from the mother. Ciliated and annelid worms are divided by constrictions into several parts; in each of them the missing organs are restored. This can form a chain of individuals. In some coelenterates, reproduction occurs by strobilation, which consists in the fact that a polyploid organism grows quite intensively and, upon reaching a certain size, begins to divide into daughter individuals by transverse constrictions. At this time, the polyp resembles a stack of plates. The resulting individuals
Jellyfish break away and begin an independent life. In many species (for example, coelenterates), the vegetative form of reproduction alternates with sexual reproduction.
Sexual reproduction
Sexual process. Sexual reproduction is distinguished by the presence of a sexual process, which ensures the exchange of hereditary information and creates conditions for the emergence of hereditary variability. As a rule, two individuals participate in it - a female and a male, which form haploid female and male reproductive cells - gametes. As a result of fertilization, i.e., the fusion of female and male gametes, a diploid zygote is formed with a new combination of hereditary characteristics, which becomes the ancestor of a new organism.
Sexual reproduction, compared to asexual reproduction, ensures the appearance of hereditarily more diverse offspring. The forms of the sexual process are conjugation and copulation.
Conjugation is a peculiar form of the sexual process in which fertilization occurs through the mutual exchange of migrating nuclei moving from one cell to another along a cytoplasmic bridge formed by two individuals. During conjugation, there is usually no increase in the number of individuals, but an exchange of genetic material between cells occurs, which ensures the recombination of hereditary properties. Conjugation is typical for ciliated protozoa (for example, ciliates), some algae (Spirogyra).
Copulation (gametogamy)- a form of the sexual process in which two cells that differ in sex - gametes - merge and form a zygote. In this case, the gamete nuclei form one zygote nucleus.
The following main forms of gametogamy are distinguished: isogamy, anisogamy and oogamy.
With isogamy, mobile, morphologically identical gametes are formed, but physiologically they differ into “male” and “female”. Isogamy occurs in many algae.
At anisogamy (heterogamy) mobile gametes differing morphologically and physiologically are formed. This type of sexual process is characteristic of many algae.
In the case of oogamy, the gametes are very different from each other. The female gamete is a large, immobile egg containing a large supply of nutrients. Male gametes - sperm
Small, most often motile cells that move using one or more flagella. In seed plants, male gametes - sperm - do not have flagella and are delivered to the egg using a pollen tube. Oogamy is characteristic of animals, higher plants and many fungi.
27. Oogenesis and spermatogenesis.
Spermatogenesis. The testis consists of numerous tubules. A cross section through the tubule shows that it contains several layers of cells. They represent successive stages of spermatozoon development.
The outer layer (reproduction zone) is made up of spermatogonia - round-shaped cells; they have a relatively large nucleus and a significant amount of cytoplasm. During embryonic development and after birth until puberty, spermatogonia divide by mitosis, due to which the number of these cells and the testis itself increase. The period of intensive division is called the breeding period
After the onset of puberty, some spermatogonia also continue to divide mitotically and form the same cells, but some of them move to the next growth zone, located closer to the lumen of the tubule. Here there is a significant increase in cell size due to an increase in the amount of cytoplasm. At this stage they are called primary spermatocytes.
The third period of development of male gametes is called maturation period. During this period, two divisions occur quickly, one after the other. Each primary spermatocyte first produces two secondary spermatocyte, and then four spermatids, oval in shape and significantly smaller in size. Cell division during the maturation period is accompanied by rearrangement of the chromosomal apparatus (meiosis occurs; see below). Spermatids move to the zone closest to the lumen of the tubules, where spermatozoons are formed from them.
In most wild animals, spermatogenesis occurs only during certain periods of the year. In the spaces between them, the testicular tubules contain only spermatogonia. But in humans and most domestic animals, spermatogenesis occurs throughout the year.
Oogenesis. The phases of oogenesis are comparable to those of spermatogenesis. This process also has breeding season, when oogonia are intensively dividing - small cells with a relatively large nucleus and a small amount of cytoplasm. In mammals and humans, this period ends before birth. Formed by this time primary oocytes remain unchanged for many years. With the onset of puberty, individual oocytes periodically enter a period of cell growth, enlarge, and accumulate yolk, fat, and pigments.
Complex morphological and biochemical transformations occur in the cytoplasm of the cell, its organelles and membranes. Each oocyte is surrounded by small follicular cells that provide its nutrition.
Next comes maturation period. during which two successive divisions occur, associated with the transformation of the chromosomal apparatus (meiosis). In addition, these divisions are accompanied by an uneven division of the cytoplasm between daughter cells. When a primary oocyte divides, one large cell is formed - secondary oocyte, containing almost all the cytoplasm, and a small cell called primary polocyte. During the second maturation division, the cytoplasm is again distributed unevenly. One large secondary oocyte and a secondary polocyte are formed. At this time, the primary polocyte can also divide into two cells. Thus, from one primary oocyte, one secondary oocyte and three polocytes (reduction bodies) are formed. Next, an egg is formed from the secondary oocyte, and the polocytes are resorbed or stored on the surface of the egg, but do not take part in further development. The uneven distribution of cytoplasm provides the egg cell with a significant amount of cytoplasm and nutrients that will be required in the future for the development of the embryo.
U In mammals and humans, periods of reproduction and growth of eggs take place in follicles (Fig. 3.5). A mature follicle is filled with fluid and contains an egg cell inside it. During ovulation, the wall of the follicle bursts, the egg enters the abdominal cavity, and then, as a rule, into the fallopian tubes. The period of egg maturation takes place in the tubes, and fertilization occurs here.
U In many animals, oogenesis and egg maturation occur only during certain seasons of the year. In women, usually one egg matures every month, and during the entire period of puberty
About 400. For humans, the fact that primary oocytes form
are formed even before birth and then remain throughout life, and only gradually some of them begin to mature and give rise to cells for the egg. This means that various unfavorable factors to which the female body is exposed during life can affect their further development; toxic substances (including nicotine and alcohol) that enter the body can enter the oocytes and subsequently cause disturbances in the normal development of future offspring.
You already know that all living organisms are made up of cells. Some are from just one cell (many bacteria and protists), others are multicellular.
A cell is an elementary structural and functional unit of an organism, possessing all the basic characteristics of a living thing.
Cells are able to reproduce, grow, exchange matter and energy with the environment, and respond to changes occurring in this environment. Each cell contains hereditary material, which contains information about all the characteristics and properties of a given organism. In order to understand how a living organism exists and works, you need to know how cells are organized and function. Many processes inherent to the body as a whole occur in each of its cells (for example, the synthesis of organic substances, respiration, etc.). Studying the structure of the cell and the principles of its life activity cytology (from Greek kitos - cell, cage and logos –
teaching, science). The history of the discovery of the cell.
Most cells are small and therefore cannot be seen with the naked eye. Today it is known that the diameter of most cells is in the range of 20 - 100 microns, and in spherical bacteria it does not exceed 0.5 microns. Therefore, the discovery of the cell became possible only after the invention of a magnifying device - a microscope. This happened at the end of the 16th - beginning of the 17th century. However, only half a century later, in 1665, the Englishman R. Hooke used a microscope to study living organisms and saw cells. R. Hooke cut off a thin layer of cork and saw its cellular structure, similar to a honeycomb. R. Hooke called these cells cells. Soon the cellular structure of plants was confirmed by the Italian doctor and microscopist M. Malpighi and the English botanist N. Grew. Their attention was attracted by the shape of the cells and the structure of their membranes. As a result, an idea was given of cells as “bags” or “bubbles” filled with “nutritional juice”.
A significant contribution to the study of cells was made by the Dutch microscopist A. van Leeuwenhoek, who discovered single-celled organisms - ciliates, amoebas, bacteria. He also observed animal cells for the first time - red blood cells and sperm. At the beginning of the 19th century. Attempts are being made to study the internal contents of the cell. In 1825, the Czech scientist J. Purkinė discovered the nucleus in the egg of birds. He also introduced the concept of “protoplasm” (from the Greek. protos – first and decorated), which corresponds to today's concept of cytoplasm. In 1831, the English botanist R. Brown first described the nucleus in plant cells, and in 1833 he came to the conclusion that the nucleus is an essential part of the plant cell. Thus, at this time, the idea of the structure of cells changed: the main thing in the organization of a cell began to be considered not the cell wall, but its internal contents.*
Cell theory. In 1838, the work of the German botanist Matthias Schleiden was published, in which he expressed the idea that the cell is the basic structural unit of plants. Based on the works of M. Schleiden, German zoologist and physiologist T. Schwann just a year later he published the book “Microscopic Studies on the Correspondence in the Structure and Growth of Animals and Plants,” in which he considered the cell as a universal structural component of animals and plants. T. Schwann made a number of generalizations, which were later called cell theory:
All living things are made of cells;
Plant and animal cells have a similar structure;
Each cell is capable of independent existence;
The activity of an organism is the sum of the vital processes of its constituent cells.
T. Schwann, like M. Schleiden, mistakenly believed that cells in the body arise from non-cellular matter. Therefore, a very important addition to cell theory was the principle of Rudolf Virchow: “Every cell is from a cell” (1859).
In 1874, the young Russian botanist I.D. Chistyakov first observed cell division. Later, the German scientist Walter Fleming described in detail the stages of cell division, and Oscar Hertwig and Eduard Strassburger independently came to the conclusion that information about the hereditary characteristics of a cell is contained in the nucleus. Thus, the work of many researchers confirmed and expanded the cellular theory, the foundation of which was laid by T. Schwann.
Currently, cell theory includes the following main provisions.