We all know that a person’s appearance, some habits and even diseases are inherited. All this information about a living being is encoded in genes. So what do these notorious genes look like, how do they function and where are they located?
So, the carrier of all genes of any person or animal is DNA. This compound was discovered in 1869 by Johann Friedrich Miescher. Chemically, DNA is deoxyribonucleic acid. What does this mean? How does this acid carry the genetic code of all life on our planet?
Let's start by looking at where DNA is located. A human cell contains many organelles that perform various functions. DNA is located in the nucleus. The nucleus is a small organelle, which is surrounded by a special membrane, and in which all the genetic material - DNA - is stored.
What is the structure of a DNA molecule?
First of all, let's look at what DNA is. DNA is a very long molecule consisting of structural elements - nucleotides. There are 4 types of nucleotides - adenine (A), thymine (T), guanine (G) and cytosine (C). The chain of nucleotides schematically looks like this: GGAATTCTAAG... This sequence of nucleotides is the DNA chain.The structure of DNA was first deciphered in 1953 by James Watson and Francis Crick.
In one DNA molecule there are two chains of nucleotides that are helically twisted around each other. How do these nucleotide chains stay together and twist into a spiral? This phenomenon is due to the property of complementarity. Complementarity means that only certain nucleotides (complementary) can be found opposite each other in two chains. Thus, opposite adenine there is always thymine, and opposite guanine there is always only cytosine. Thus, guanine is complementary to cytosine, and adenine is complementary to thymine. Such pairs of nucleotides opposite each other in different chains are also called complementary.
It can be shown schematically as follows:
G - C
T - A
T - A
C - G
These complementary pairs A - T and G - C form a chemical bond between the nucleotides of the pair, and the bond between G and C is stronger than between A and T. The bond is formed strictly between complementary bases, that is, the formation of a bond between non-complementary G and A is impossible.
"Packaging" of DNA, how does a DNA strand become a chromosome?
Why do these DNA nucleotide chains also twist around each other? Why is this necessary? The fact is that the number of nucleotides is huge and a lot of space is needed to accommodate such long chains. For this reason, two strands of DNA twist around each other in a helical manner. This phenomenon is called spiralization. As a result of spiralization, DNA chains are shortened by 5-6 times.Some DNA molecules are actively used by the body, while others are rarely used. In addition to spiralization, such rarely used DNA molecules undergo even more compact “packaging.” This compact packaging is called supercoiling and shortens the DNA strand by 25-30 times!
How do DNA helices pack?
Supercoiling uses histone proteins, which have the appearance and structure of a rod or spool of thread. Spiralized strands of DNA are wound onto these “coils” - histone proteins. Thus, the long thread becomes very compactly packaged and takes up very little space. If it is necessary to use one or another DNA molecule, the process of “unwinding” occurs, that is, the DNA strand is “unwound” from the “spool” - the histone protein (if it was wound onto it) and unwinds from the spiral into two parallel chains. And when the DNA molecule is in such an untwisted state, then the necessary genetic information can be read from it. Moreover, genetic information is read only from untwisted DNA strands!
A set of supercoiled chromosomes is called heterochromatin, and the chromosomes available for reading information are euchromatin.
What are genes, what is their connection with DNA?
Now let's look at what genes are. It is known that there are genes that determine blood type, eye color, hair, skin and many other properties of our body. A gene is a strictly defined section of DNA, consisting of a certain number of nucleotides arranged in a strictly defined combination. Location in a strictly defined DNA section means that a specific gene is assigned its place, and it is impossible to change this place. It is appropriate to make the following comparison: a person lives on a certain street, in a certain house and apartment, and a person cannot voluntarily move to another house, apartment or to another street. A certain number of nucleotides in a gene means that each gene has a specific number of nucleotides and they cannot become more or less. For example, the gene encoding insulin production consists of 60 nucleotide pairs; the gene encoding the production of the hormone oxytocin - of 370 nucleotide pairs. The strict nucleotide sequence is unique for each gene and strictly defined. For example, the sequence AATTAATA is a fragment of a gene that codes for insulin production. In order to obtain insulin, exactly this sequence is used; to obtain, for example, adrenaline, a different combination of nucleotides is used. It is important to understand that only a certain combination of nucleotides encodes a certain “product” (adrenaline, insulin, etc.). Such a unique combination of a certain number of nucleotides, standing in “its place” - this is gene.
In addition to genes, the DNA chain contains so-called “non-coding sequences”. Such non-coding nucleotide sequences regulate the functioning of genes, help in the spiralization of chromosomes, and mark the starting and ending point of a gene. However, to date, the role of most non-coding sequences remains unclear.
What is a chromosome? Sex chromosomes
The collection of genes of an individual is called the genome. Naturally, the entire genome cannot be contained in one DNA. The genome is divided into 46 pairs of DNA molecules. One pair of DNA molecules is called a chromosome. So, humans have 46 of these chromosomes. Each chromosome carries a strictly defined set of genes, for example, chromosome 18 contains genes encoding eye color, etc. Chromosomes differ from each other in length and shape. The most common shapes are X or Y, but there are others as well. Humans have two chromosomes of the same shape, which are called pairs. Due to such differences, all paired chromosomes are numbered - there are 23 pairs. This means that there is chromosome pair No. 1, pair No. 2, No. 3, etc. Each gene responsible for a specific trait is located on the same chromosome. Modern guidelines for specialists may indicate the location of the gene, for example, as follows: chromosome 22, long arm.What are the differences between chromosomes?
How else do chromosomes differ from each other? What does the term long shoulder mean? Let's take chromosomes of the form X. The intersection of DNA strands can occur strictly in the middle (X), or it can occur not centrally. When such an intersection of DNA strands does not occur centrally, then relative to the point of intersection, some ends are longer, others, respectively, shorter. Such long ends are usually called the long arm of the chromosome, and short ends are called the short arm. In chromosomes of the Y shape, most of the arms are occupied by long arms, and the short ones are very small (they are not even indicated in the schematic image). The size of the chromosomes varies: the largest are chromosomes of pairs No. 1 and No. 3, the smallest chromosomes are pairs No. 17, No. 19.
In addition to their shape and size, chromosomes differ in the functions they perform. Of the 23 pairs, 22 pairs are somatic and 1 pair is sexual. What does it mean? Somatic chromosomes determine all the external characteristics of an individual, the characteristics of his behavioral reactions, hereditary psychotype, that is, all the traits and characteristics of each individual person. A pair of sex chromosomes determines a person’s gender: male or female. There are two types of human sex chromosomes: X (X) and Y (Y). If they are combined as XX (x - x) - this is a woman, and if XY (x - y) - we have a man.
Hereditary diseases and chromosome damage
However, “breakdowns” of the genome occur, and then genetic diseases are detected in people. For example, when there are three chromosomes in the 21st pair of chromosomes instead of two, a person is born with Down syndrome.There are many smaller “breakdowns” of genetic material that do not lead to disease, but on the contrary, impart good properties. All “breakdowns” of genetic material are called mutations. Mutations leading to diseases or deterioration of the body's properties are considered negative, and mutations leading to the formation of new beneficial properties are considered positive.
However, with most of the diseases that people suffer from today, it is not the disease that is inherited, but only a predisposition. For example, the father of a child absorbs sugar slowly. This does not mean that the child will be born with diabetes, but the child will have a predisposition. This means that if a child abuses sweets and flour products, he will develop diabetes.
Today, the so-called predicative medicine. As part of this medical practice, a person’s predispositions are identified (based on the identification of the corresponding genes), and then he is given recommendations - what diet to follow, how to properly alternate between work and rest so as not to get sick.
How to read the information encoded in DNA?
How can you read the information contained in DNA? How does its own body use it? DNA itself is a kind of matrix, but not simple, but encoded. To read information from the DNA matrix, it is first transferred to a special carrier - RNA. RNA is chemically ribonucleic acid. It differs from DNA in that it can pass through the nuclear membrane into the cell, while DNA lacks this ability (it can only be found in the nucleus). The encoded information is used in the cell itself. So, RNA is a carrier of encoded information from the nucleus to the cell.How does RNA synthesis occur, how is protein synthesized using RNA?
The DNA strands from which information needs to be “read” unwind, a special “builder” enzyme approaches them and synthesizes a complementary RNA chain parallel to the DNA strand. The RNA molecule also consists of 4 types of nucleotides - adenine (A), uracil (U), guanine (G) and cytosine (C). In this case, the following pairs are complementary: adenine - uracil, guanine - cytosine. As you can see, unlike DNA, RNA uses uracil instead of thymine. That is, the “builder” enzyme works as follows: if it sees A in the DNA strand, then it attaches Y to the RNA strand, if G, then it attaches C, etc. Thus, a template is formed from each active gene during transcription - a copy of RNA that can pass through the nuclear membrane. How does the synthesis of a protein encoded by a specific gene occur?
After leaving the nucleus, RNA enters the cytoplasm. Already in the cytoplasm, RNA can be embedded as a matrix into special enzyme systems (ribosomes), which can synthesize, guided by RNA information, the corresponding sequence of protein amino acids. As you know, a protein molecule consists of amino acids. How does the ribosome know which amino acid to add to the growing protein chain? This is done based on the triplet code. The triplet code means that the sequence of three nucleotides of the RNA chain ( triplet, for example, GGU) code for a single amino acid (in this case glycine). Each amino acid is encoded by a specific triplet. And so, the ribosome “reads” the triplet, determines which amino acid should be added next as it reads the information in the RNA. When a chain of amino acids is formed, it takes on a certain spatial shape and becomes a protein capable of performing the enzymatic, construction, hormonal and other functions assigned to it.Protein for any living organism is the product of a gene. It is proteins that determine all the various properties, qualities and external manifestations of genes.
How is it possible that DNA is outside the cell?
Bacterial cells can release DNA into the environment - this is due to the processes of their reproduction and information exchange. For example, antibiotic resistance spreads: one bacterium acquires the corresponding gene, copies it, and shares it with the rest of the population. In eukaryotic (nuclear) organisms, such processes were unknown for a long time: it was believed that they use DNA only to store, read and transmit information.
But in 1948, extracellular DNA was discovered in blood plasma - a fraction of DNA that is not associated with cells and exists separately from them. Over the ensuing years, scientists have found such DNA in all organisms studied, from plants to animals and humans. It was found in the intercellular substance, in circulating fluids, and even in individual cell cultures. It appears that DNA occurs regularly outside cells and may therefore play a role in the life of the organism.
Extracellular DNA is not like normal DNA.
Genomic DNA consists of long strands called chromosomes, while cfDNA is a collection of small sequences, sometimes a million times shorter than a chromosome.
Whether the choice of these sequences is random or not is still a controversial issue.
Extracellular DNA is not always in solution on its own. Sometimes it is associated with histones, proteins that the cell uses to compactly package DNA strands in the nucleus. In other cases, cfDNA may occur inside exosomes, membrane-enclosed vesicles that bud from cells, travel throughout the body, and can fuse with other cells. Moreover, one group of scientists isolated from the blood of animals a whole complex of DNA, fats and proteins responsible for its copying. That is, there are probably entire molecular machines floating around the body, copying and distributing information on the go. However, what part of the total cfDNA consists of such structures is still unknown.
Where does extracellular DNA come from?
It seems logical that cfDNA does not form itself, but is secreted by cells. What can cause cells to release molecules that carry their hereditary information?
The cell death hypothesis suggests that DNA is released when cells are destroyed. This theory helps explain why cfDNA comes in small pieces: for example, in apoptosis (programmed cell death), the DNA inside a cell is cut into small pieces before the entire cell disintegrates. This is also consistent with the fact that in conditions accompanied by cell death (myocardial infarction, burns), the amount of cfDNA in the blood increases.
But not everything is so simple: extracellular DNA was found in any tissue culture, even where there was no mass cell death. The “metabolic DNA” hypothesis tries to explain this: probably, cells constantly synthesize new DNA during their life, increasing the number of copies of information to make it more convenient to read. Over time, DNA molecules wear out and cells release them into the environment along with metabolic products.
There is also an opinion that the release of cfDNA is a way for cells to exchange signals. For example, membrane vesicles containing small amounts of DNA were isolated from various intraorganismal fluids. Such vesicles can fuse with cells, transferring DNA molecules to them.
They found DNA in my blood. This is bad?
Cell-free DNA is a natural component of blood plasma and can be found in any person. Normally, its concentration is quite low, although it can vary, but in the case of pathological and stressful conditions, the amount of cfDNA increases sharply. For example, for burns or diseases associated with massive cell death - myocardial infarction or rheumatoid arthritis. Even a healthy person can experience large fluctuations in cfDNA levels if they are exposed to stress, such as intense physical activity. However, after stopping the load, concentrations return to normal values.
The situation is more complicated with cancer. It is not entirely clear where cfDNA comes from in this case - as a result of the death of healthy tissues or as a product of the targeted isolation of tumor cells. However, its quantity is also very different from the norm.
Medicine has not yet learned to diagnose specific diseases based on the concentration of cfDNA in the blood, but it is already possible to assess the severity of the condition and predict the development of the disease.
So, if we compare the amounts of cfDNA in people who have suffered a myocardial infarction, it turns out that the more cfDNA, the greater the complications and the risk of recurrent heart attack or cardiac arrest.
Perhaps careful study of cfDNA sequences will help make more accurate diagnoses. They also rely on this in the field of prenatal diagnostics: fetal cfDNA is present in the mother’s blood, which means that it is possible to obtain the child’s genetic material without surgical intervention. This opens up a wide field for genetic tests - to identify fetal diseases (for example, Rh conflict) or determine sex.
What do cells “think” about extracellular DNA?
Extracellular DNA is constantly present around cells, and it can be assumed that a change in its concentration or properties will serve as a signal to which other cells will respond.
In some cells, increasing the concentration of cfDNA has an activating effect. Cells of the immune system are capable of triggering an immune response upon recognition of cfDNA. This occurs due to a mechanism that is normally responsible for the reaction to foreign DNA molecules, such as viral DNA, in the blood. The same receptors that recognize viral DNA also react to the body’s own cfDNA, activating cells of the immune system.
For other cells, cfDNA can act as an alarm signal—they develop a “bystander effect.” Let's say we have a cell culture that is under stress: low oxygen levels, radiation, or other abnormal conditions. In these cells, DNA is damaged and oxidative stress develops - aggressive substances accumulate that destroy cellular contents. If cfDNA isolated from damaged cells is transferred to a culture of healthy cells, DNA damage and oxidative stress also begin to be detected in them. Such cells are called “witnesses” because they experience stress without being affected by the original factors.
However, the effects of cfDNA do not end there: it can stimulate or slow down cell division, influence gene activity and protein synthesis. Apparently, cfDNA has a systemic effect on many cells, changing their physiology, but its specific nature remains mysterious.
What else do we (not) know about extracellular DNA?
Recently, evidence has emerged that animal and human cells can absorb cfDNA from the blood. In some cases, these molecules reach the cell nucleus, penetrate inside and integrate into the cell's own genome. Often the integration of such a wandering cfDNA molecule into the genome ends with damage to the DNA of the recipient cell and its death. But if the integration is successful, the processes of reading information change: fragments of the former cfDNA block the work of genes in the cell nucleus or trigger the reading of their own information. Thus, we have before us a special mechanism of intercellular interaction - the exchange of genetic information.
This mechanism likely plays an important role in the development of diseases. Several years ago, the concept of “gene metastasis” was formulated: it is assumed that tumor cells can secrete multiple copies of their mutant genes. Healthy cells absorb them, integrate them into their genome and begin to produce mutant tumor proteins.
Similar processes can theoretically occur during the interaction of cells of different organisms. Although fetal cfDNA is found in the mother's blood, there is no evidence yet of its uptake into cells. But it was discovered that during blood transfusion, not only the donor’s cells settle in the recipient’s body, but the donor’s cfDNA can also integrate into the recipient’s cells.
Research into the functions of extracellular DNA began relatively recently, but we can already talk about the discovery of a fundamentally new mechanism of communication both between cells within an organism and, probably, between the organisms themselves.
Task 1 Structure of chromosomes
When are chromosomes visible in the cell nucleus?Task 2 Cell life cycle
Look at the picture and answer the questions:
What periods of interphase are indicated by numbers 1 - 3? What is the set of chromosomes and DNA at different periods of interphase? What periods of mitosis are indicated by numbers 4 - 7? What is the set of chromosomes and DNA at different periods of mitosis?
Task 3. Mitotic cycle
Fill the table:
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Periods of interphase and mitosis |
Ongoing processes |
Number of chromosomes (n) and amount of DNA (s) |
|
Presynthetic (G1) Synthetic (S) Postsynthetic (G2) | ||
|
Metaphase Telophase |
Task 4. Mitotic cycle
Test 1. During what period of the mitotic cycle does the amount of DNA double?
1. During the presynthetic period.
2. During the synthetic period.
4. Into metaphase.
Test 2. During what period does active cell growth occur?
1. During the presynthetic period.
2. During the synthetic period.
3. During the post-synthetic period.
4. Into metaphase.
Test 3. During what period of the life cycle does a cell have a set of chromosomes and DNA 2n4c and prepare to divide?
1. During the presynthetic period.
2. During the synthetic period.
3. During the post-synthetic period.
4. Into metaphase.
Test 4. At what period of the mitotic cycle does chromosome spiralization begin and the nuclear membrane dissolves?
1. During the presynthetic period.
2. During the synthetic period.
3. During the post-synthetic period.
4. Into metaphase.
Test 5. During what period of the mitotic cycle do chromosomes line up along the equator of the cell?
1. During the presynthetic period.
2. During the synthetic period.
3. During the post-synthetic period.
4. Into metaphase.
Test 6. During what period of the mitotic cycle do chromatids move away from each other and become independent chromosomes?
1. During the presynthetic period.
2. During the synthetic period.
3. During the post-synthetic period.
4. Into metaphase.
Test 7. During what periods of mitosis is the number of chromosomes and DNA equal to 2n4c?
1. In prophase.
2. In metaphase.
3. Into anaphase.
4. In telophase.
Test 8. At what stage of mitosis is the number of chromosomes and DNA equal to 4n4c?
1. In prophase.
2. In metaphase.
3. Into anaphase.
4. In telophase.
Test 9. What is the inactive part of DNA in a cell called?
1. Chromatin.
2. Euchromatin.
3. Heterochromatin.
4. All DNA in the cell is active.
Test 10. What are chromosomes called during the interphase period?
1. Chromatin.
2. Euchromatin.
3. Heterochromatin.
4. Chromosomes.
Task 5. Mitosis
Give answers to the questions:
1. What is a diploid set of chromosomes?
2. What is a haploid set of chromosomes?
3. What is the set of chromosomes and DNA in the presynthetic period of interphase?
4. What is the set of chromosomes and DNA during the post-synthetic period of interphase?
5. What is the set of chromosomes and DNA in prophase and metaphase of mitosis?
6. What is the set of chromosomes and DNA in anaphase of mitosis?
7. What is the set of chromosomes and DNA in the telophase of mitosis?
8. How many DNA molecules are in the nucleus of a human somatic cell before mitosis?
9. How many DNA molecules are in the nucleus of a human somatic cell after mitosis?
10. What are chromosomes called during the interphase period?
Task 6. Give definitions or expand on the concepts:
1. Interphase. 2. Chromatin. 3. Chromosome. 4. Chromatids. 5. Centromere. 6. Prophase. 7. Metaphase. 8.Anaphase. 9. Telophase. 10. Diploid set of chromosomes.
Topic: Cell division. Meiosis
Task 7. First and second divisions of meiosis
Look at the picture and answer the questions:
|
What is the set of chromosomes and DNA in cells before the first meiotic division? What is the set of chromosomes and DNA in cells during different periods of the first meiotic division? What is the set of chromosomes and DNA in cells before the second meiotic division? What is the set of chromosomes and DNA in cells during different periods of the second meiotic division? At what stage of meiosis does conjugation and crossing of chromosomes occur? ***In meiosis, recombination of genetic material occurs three times. When? What is the biological meaning of meiosis?
Task 8. Meiosis
Fill the table:
|
Meiotic divisions |
Ongoing processes |
Number of chromosomes (n) and amount of DNA (s) |
|
Prophase-1 Metaphase-1 Anaphase-1 Telophase-1 | ||
|
Interphase | ||
|
Prophase-2 Metaphase-2 Anaphase-2 Telophase-2 |
Task 9. Meiosis
Please indicate the correct answers:
Test 1. When does conjugation of homologous chromosomes occur during meiosis?
1. ProphaseProphase 2.
2. MetaphaseMetaphase 2.
3. AnaphaseAnaphase 2.
4. Telophase Telophase 2.
Test 2. What is the set of chromosomes and DNA at the end of the 1st meiotic division?
1. 1n1c. 5. 2n4c.
2. 1n2c. 6. 4n4c.
Test 3. What is the set of chromosomes and DNA at the end of the 2nd meiotic division?
1. 1n1c. 5. 2n4c.
2. 1n2c. 6. 4n4c.
Test 4. At what stages of meiosis is the set of chromosomes and 1n4c DNA?
1. ProphaseProphase 2.
2. MetaphaseMetaphase 2.
3. AnaphaseAnaphase 2.
4. Telophase Telophase 2.
Test 5. At what stages of meiosis is the set of chromosomes and DNA 2n4c?
1. ProphaseProphase 2.
2. MetaphaseMetaphase 2.
3. AnaphaseAnaphase 2.
4. Telophase Telophase 2.
Test 6. At what stages of meiosis is the set of chromosomes and DNA 1n2c?
1. ProphaseProphase 2.
2. MetaphaseMetaphase 2.
3. AnaphaseAnaphase 2.
4. Telophase Telophase 2.
Test 7. At what stages of meiosis is the set of chromosomes and DNA 2n2c?
1. ProphaseProphase 2.
2. MetaphaseMetaphase 2.
3. AnaphaseAnaphase 2.
4. Telophase Telophase 2.
Test 8. At what stages of meiosis is the set of chromosomes and DNA 1n1c?
1. ProphaseProphase 2.
2. MetaphaseMetaphase 2.
3. AnaphaseAnaphase 2.
4. Telophase Telophase 2.
***Test 9. At what stages of meiosis does recombination of genetic material occur?
1. ProphaseProphase 2.
2. MetaphaseMetaphase 2.
3. AnaphaseAnaphase 2.
4. Telophase Telophase 2.
Test 10. At what stages of meiosis does crossing over occur?
1. ProphaseProphase 2.
2. MetaphaseMetaphase 2.
3. AnaphaseAnaphase 2.
4. Telophase Telophase 2.
Task 10. Meiosis
Give answers to the questions:
1. What is the set of chromosomes and DNA before the first meiotic division?
2. What is the set of chromosomes and DNA before the second meiotic division?
3. Which chromosomes are called homologous?
4. What processes occur in prophase 1 of meiosis?
5. During what phases of the first meiotic division does recombination of genetic material occur?
6. What is characteristic of the interphase between the first and second divisions of meiosis?
7. What is the set of chromosomes and DNA in prophase-2 and metaphase-2?
8. In what phase of the second meiotic division does the recombination of genetic material occur?
9. What is the set of chromosomes and DNA at the end of the second meiotic division?
10. How many cells are formed as a result of meiosis from one mother cell?
Task 11. Give definitions or expand on the concepts:
1. Homologous chromosomes. 2. Conjugation. 3. Crossing over. 4. Diploid set of chromosomes. 5. Haploid set of chromosomes. 6. Reduction division of meiosis. 7. Recombination into anaphaseRecombination into anaphaseBiological meaning of meiosis.
Topic: Asexual and sexual reproduction
Task 12. Various forms of asexual reproduction
Look at the picture and answer the questions:
|
What forms of asexual reproduction are indicated in the figure by numbers 1 - 6? What genetic material do daughters have during asexual reproduction?
Task 13. Characteristics of various forms of asexual reproduction
Fill the table:
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Forms of asexual reproduction |
Characteristics |
|
1. Asexual reproduction of bacteria 2. Binary fission 3. Schizogony 4. Sporulation 5. Budding 6. Fragmentation 7. Vegetative propagation 8. Polyembryony 9. Cloning |
Task 14. Comparison of asexual and sexual reproduction
Fill the table:
|
Comparable characteristics |
Asexual reproduction |
Sexual reproduction |
|
1. Number of individuals participating in reproduction 2. Genetic material of the offspring 3. Recombination of genetic material 4. Implications for selection |
Task 15. Asexual and sexual reproduction
Please indicate the correct answers:
Test 1. What form of asexual reproduction is most typical for mosses and ferns?
Test 2. What form of asexual reproduction is most characteristic of hydra and yeast?
1. Binary fission. 5. Cloning.
2. Schizogony. 6. Vegetative propagation.
3. Fragmentation. 7. Polyembryony.
4. Budding. 8. Sporulation.
Test 3. What form of asexual reproduction is used for the propagation of fruit and berry crops?
1. Binary fission. 5. Cloning.
2. Schizogony. 6. Vegetative propagation.
3. Fragmentation. 7. Polyembryony.
4. Budding. 8. Sporulation.
Test 4. What natural form of asexual reproduction is known in humans?
1. Binary fission. 5. Cloning.
2. Schizogony. 6. Vegetative propagation.
3. Fragmentation. 7. Polyembryony.
4. Budding. 8. Sporulation.
Test 5. What form of asexual reproduction is characteristic of planaria and some annelids?
1. Binary fission. 5. Cloning.
2. Schizogony. 6. Vegetative propagation.
3. Fragmentation. 7. Polyembryony.
4. Budding. 8. Sporulation.
Test 6. What is characteristic of asexual reproduction?
1. The offspring has the genes of only one, the maternal organism.
2. Offspring are genetically different from the parent organisms.
3. One individual participates in the formation of offspring.
4. Two individuals usually participate in the formation of offspring.
Test 7. What form of reproduction allows you to adapt to changing environmental conditions?
1. Asexual reproduction.
2. Sexual reproduction.
3. Both asexual and sexual reproduction are equally.
4. The form of reproduction does not matter.
**Test 8. Please indicate the correct statements:
1. Parthenogenesis is a special form of asexual reproduction.
2. Parthenogenesis is a special form of sexual reproduction.
3. Parthenogenetic development is known in aphids, bees, and daphnia.
4. Parthenogenetic development is known in humans.
**Test 9. Please indicate the correct statements:
1. Hermaphrodites are organisms that can produce both male and female gametes.
2. Gametes have a haploid set of chromosomes, the zygote has a diploid set.
3. developed methods for purposefully obtaining 100% of individuals of the same sex.
4. Bacteria divide by mitosis.
**Test 10. Please indicate the correct statements:
1. Asexual reproduction has no advantages over sexual reproduction.
2. Gametes and the zygote have a haploid set of chromosomes.
3. Two individuals always take part in sexual reproduction.
4. Sexual reproduction sharply increases the hereditary variability of offspring.
Topic: Formation of germ cells and fertilization
Task 16. Gametogenesis
Look at the picture and answer the questions:
1. ***What is indicated in the figure by numbers 1 - 12?
2. What is the size of a human egg?
3. What is in the cytoplasm of the egg?
4. Where are the nucleus and mitochondria located in the sperm?
Task 18. Gametogenesis. Fertilization
Please indicate the correct answers:
Test 1. What set of chromosomes do the precursors of gametes in the reproduction zone have?
1. Diploid.
2. Haploid.
3. Spermatogonia are diploid, oogonia are haploid.
4. Spermatogonia are haploid, oogonia are diploid.
Test 2. What set of chromosomes do cells have in the maturation zone after the first meiotic division?
Test 3. What set of chromosomes do gametes have?
Test 4. How many normal eggs are produced from one oocyte after two meiotic divisions?
Test 5. How many normal sperm are produced from one spermatocyte after two meiotic divisions?
Test 6. Where is the Golgi complex located in the sperm?
1. In the head.
2. In the cervix.
3. In the intermediate department.
4. In a ponytail.
Test 7. Where are the mitochondria located in the sperm?
1. In the head.
2. In the cervix.
3. In the intermediate department.
4. In a ponytail.
Test 8. Where are the centrioles located in the sperm?
1. In the head.
2. In the cervix.
3. In the intermediate department.
4. In a ponytail.
**Test 9. Please indicate the correct statements:
1. In the growth zone, the chromosome set is 2n.
2. In the maturation zone, two divisions of meiosis occur - reduction and equationation.
3. During oogenesis, four normal eggs are formed from one oocyte.
4. During oogenesis, one normal egg and four guiding (polar) bodies are formed from one oocyte.
***Test 10. Please indicate the correct statements:
1. The human egg measures about 0.1 mm.
2. Human eggs are formed at the embryonic stage.
3. The human egg has two shells - shiny and radiant.
4. There are no ribosomes or mitochondria in the human egg.
Topic: Individual development of organisms
Task 19. Main stages of embryogenesis
Look at the picture and answer the questions:
|
***What is indicated in the figure by numbers 1 - 10? What is formed as a result of the fragmentation of a zygote? ***What is subsequently formed from the blastocoel? What is the name of the opening in the gastrula? From which germ layer does the neural tube form? What is the name of an embryo with a formed axial complex? What happens if a section of the ectoderm from which the nervous system is formed is taken from one gastrula and transplanted under the abdominal ectoderm of another gastrula?
Task 20. Derivatives of germ layers
Fill the table:
|
Germ layers |
Germ layer derivatives |
|
Ectoderm | |
|
Endoderm | |
|
Mesoderm |
Task 21. Ontogenesis
Please indicate the correct answers:
Test 1.What is formed as a result of complete fragmentation of the zygote?
1. Neirula.
2. Blastula.
3. Gastrula.
4. Morula.
Test 2. What is the name of the cavity inside the blastula?
1. Blastocoel.
2. Gastrocele.
3. Secondary body cavity.
Test 3. What is the name of a two-layer embryo with germ layers: ectoderm and endoderm?
1. Gastrula.
2. Blastula.
3. Neirula.
4. Morula.
Test 4. What is the name of the cavity into which the primary mouth leads?
1. Blastocoel.
2. Gastrocele.
3. Secondary body cavity.
4. Mixed body cavity (mixocel).
Test 5. What organisms are deuterostomes?
1. Coelenterates and sponges.
2. Flat and roundworms.
3. Molluscs and arthropods.
4. Echinoderms and chordates.
Test 6. What is the name of an embryo with an axial complex of organs?
1. Gastrula.
2. Blastula.
3. Neirula.
4. Morula.
Test 7. Specify the derivatives of ectoderm:
Test 8. Specify endoderm derivatives:
1. Epidermis of the skin. 6. Digestive system.
2. Epithelium of the digestive system. 7. Digestive glands.
3. Circulatory system. 8. Respiratory system.
4. Excretory system. 9. Reproductive system.
5. Nervous system. 10. Sense organs.
Test 9. Specify derivatives of mesoderm:
1. Epidermis of the skin. 6. Digestive system.
2. Epithelium of the digestive system. 7. Digestive glands.
3. Circulatory system. 8. Respiratory system.
4. Excretory system. 9. Reproductive system.
5. Nervous system. 10. Sense organs.
Test 10. Specify animals with indirect postembryonic development:
1. Mammals. 5. Butterflies.
2. Birds. 6. Locust.
3. Reptiles. 7. Spiders.
4. Amphibians. 8. Cockroaches.
Task 22. Ontogenesis
Biological dictation:
1. What is the name of the individual development of an organism from the formation of a zygote to the end of life?
2. What is the development of an organism from the zygote to birth or to emergence from the egg shells called?
3. What is the period from birth to the end of life called?
4. How does the fragmentation period end?
5. What is the name of an embryo with three germ layers: ectoderm, endoderm and mesoderm?
6. What organisms are deuterostomes?
7. What is the name of the embryo in which the axial complex of organs has formed?
8. What organ systems are formed from the ectoderm?
9. Specify the derivatives of endoderm.
10. Write down two species of animals with direct and indirect types of postembryonic development.
Task 23. Give definitions or expand on the concepts:
1. Fertilization. 2. Zygote. 3. Blastomeres. 4. Blastula. 5. Blastocoel (primary cavity). 6. Gastrula. 7. Mesoderm. 8. Secondary mouth. 9. Neirula. 10. Indirect postembryonic development.
Used materials from the Honored School Teacher of the Russian Federation; , Ph.D.
Class time- 90 min.
Location- classroom
Type of activity- seminar lesson
Lesson objectives:
- Educational:
Summarize students’ knowledge of the studied material, abilities, skills; assess the level of knowledge; carry out control of knowledge, skills and abilities; systematize knowledge.
- Developmental:
Teach to analyze, highlight the main thing, develop professional skills
- Educational:
Cultivating perseverance and determination in achieving goals, confidence in knowledge, and developing the ability to think; nurturing a culture of communication, curiosity, objectivity.
- Methodical
To intensify the cognitive activity of students by solving the tasks assigned to them.
Tasks:
1. Development of the student’s speech, logical thinking and attention, the ability to analyze, compare, and highlight the main thing.
2. fostering a value-based attitude to life, the value of practical knowledge.
3. deepening students’ knowledge of this material, enhancing cognitive activity.
Form of work: individual, group.
Qualification requirements
To knowledge:
Students should know material on the topics: “Properties of living organisms”, “Cell”, “Cell division”, “Mitosis”, “Meiosis”.
To skills:
Students must be able to freely navigate the material of the topics studied.
Compare knowledge and find solutions.
Draw conclusions, conclusions, justify your point of view.
Interdisciplinary connections:Anatomy, psychology, medicine.
Intrasubject connections: Topics: “Properties of living organisms”, “Cell”, “Cell division”, “Mitosis”, “Meiosis”, “Fertilization”, “Forms of reproduction of organisms”
Equipment: illustrative material, video program, multimedia complex, light microscopes, magnetic board, micropreparations “Mitosis in the root of an onion”, “Division of the egg”.
Equipment:
- Multimedia complex
- Didactic material: cards
- Literature:
Main literature
Internet resources:
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1. Russian State Library [Electronic resource] / Information Center. RSL technologies; ed. Vlasenko T.V. ; Webmaster Kozlova N.V. — Electron. Dan. - M.:Ros. state b-ka, 1997—Access mode: http://www.rsl.ru, free. — Cap. from the screen.—Lang. Russian, English 2. A selection of online materials for biology teachers on various biological disciplines [Electronic resource] / NPB im. K.D. Ushinsky RAO - Access mode: http://www.gnpbu.ru 3. Unified collection of digital educational resources [Electronic resource] / 2006-2012 FGAU State Research Institute of Information Technology "Informika" 4. Website for student teachers [Electronic resource] / Publishing House “First of September” - Access mode: http://1september.ru, free. - Cap. From the screen. - Yaz. rus. 5.Personal website of biology teacher Kapshuchenko A.N. [Electronic resource] free. - Cap. From the screen. - Yaz. rus. |
Rationale for the topic
The topic "Mitosis" is one of the key topics in biology. It connects most branches of biology into a single whole. It is key for studying such topics as “Fertilization”, “Embryonic development”, “Ontogenesis”, “Patterns of inheritance of characters”, “Variability” and others. The topic is directly related to the study of a number of medical sciences: obstetrics, gynecology, anatomy, physiology, medical genetics, psychology.
Allows you to consider a number of social aspects, prospects and achievements of modern science. Guide students to study subsequent topics in biology. Determine interdisciplinary connections.
Lesson plan
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№ |
Lesson stage |
Time |
Activity |
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teacher |
student |
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Organizational |
Announcement of the topic, purpose of the lesson |
Greets students, organizes attention, and communicates the topic and purpose of the lesson. |
Greet the teacher |
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Assessing the readiness of the audience and students |
Checks those present |
Participate in roll call |
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Characteristics of the procedure for conducting a seminar lesson. |
Explains the procedure for conducting a seminar lesson, evaluation criteria for a practical lesson. Clarifies questions that caused difficulties and provides explanations |
Listen carefully and ask questions |
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Systematization of knowledge |
Frontal survey |
Asking questions |
Answer questions |
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Control of knowledge and skills. |
Characteristics of the procedure for conducting practical work |
Explains the procedure for completing tasks, monitors implementation, gives explanations, individual consultations |
Doing the job |
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The final stage |
Generalization, conclusions |
Analysis of goal achievements. Evaluation of student work. |
Listen, analyze, evaluate their work |
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Answers to student questions |
Answers student questions and provides necessary explanations |
Ask questions, listen to answers |
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Total |
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Annex 1.
Questions for frontal survey
- What types of cell division are there?
- How does amitosis differ from other types of cell division?
- What is mitosis? What is its biological meaning?
- What processes occur in the nucleus during interphase?
- Why do chromosomes consist of two chromatins at the beginning of mitosis?
- What changes occur in the nucleus during prophase of mitosis?
- To which part of the chromosome is the spindle thread attached?
- What is characteristic of metaphase of mitosis?
- Why is telophase called “prophase in reverse”?
- Which chromosomes move to the poles of the cell during anaphase?
- What are chromosomes at the beginning of interphase?
- How many cells and with what set of chromosomes are formed as a result of mitosis?
- What cells are characterized by mitosis?
- Which chromosomes are called homologous?
- What is characteristic of prophase?
- How many cells are produced by mitosis?
- What is the difference between mitosis and meiosis?
Appendix 2
- Repetition of covered material. The following terms are written on the board:
- Centriole
- Replication
- Cell cycle
- Chromatin
- Chromatids
- Chromosomes
- Centromere
- Interphase
Students are asked to answer the following questions and choose the correct answer by writing it down digitally:
- What is the name of the complex consisting of DNA and proteins - histones?
- What is the name of the structure formed before nuclear division?
- What is the name of the period preceding nuclear fission?
- What is the name of the area where the spindle filaments are attached?
- What is the name of the cell center structure?
- What is the process of doubling a DNA molecule called?
- What is the period in the life of a cell from its formation to division into daughters called?
- What is the name of one of the two nucleoprotein strands formed during chromosome duplication?
Appendix 3
- Determination of the time and place of mitosis in the cell cycle.
On the magnetic board there is an image of the cell cycle, the “mitosis” section is highlighted, and the average time is determined: interphase lasts 10 - 20 hours, mitosis 1 - 2 hours. It is also possible to determine the genetic material before division. After mitosis, cytokinesis occurs.
- Definition of mitosis
“mitosis (from the gr. - mitosis - thread) is an indirect division of the cell nucleus and its body, during which each of the two emerging cells receives genetic material identical to the original cell.” Synonyms for nuclear division are - karyokinesis (translated from the gr. karyon - nut, kernel of a nut, kinesis - movement)
- Phases of mitosis: prophase, metaphase, anaphase, telophase, and then cytokinesis follows (work in notebook)
Appendix 4
- Students' laboratory work. Assignment: each group receives an envelope containing information about each phase of mitosis, in addition to illustrated material. Examining micropreparations, find a certain phase according to the description, put together a certain text from the fragments, and stick it on a sheet of paper.
1 group. Prophase.
The chromatids shorten and thicken. The chromatids are clearly visible. Centromeres are not detected. Centrioles diverge to the poles. A star of microtubules begins to form. The nucleoli become smaller. Towards the end of prophase, the nuclear membrane disintegrates and a fission spindle is formed.
2nd group. Metaphase.
Pairs of chromatids are attached by their centromeres to the spindle filaments and move up and down the spindle until their centromeres line up at the equator of the cell.
3rd group. Anaphase.
Short stage. Each centromere splits into two, and the spindle filaments pull off the daughter centromeres with the opposite pole. Centromeres pull along the separated chromatids, which are now called chromosomes.
4th group. Telophase.
Chromosomes reach the cell poles, despiral, elongate, indistinguishable spindle strands are destroyed, and centrioles are replicated. A nuclear membrane forms around the chromosomes. The nucleolus appears.
5 group. Cytokinesis.
Following telophase and leading to the first period of interphase, organelles are distributed between daughter cells. As a result, two cells are formed with a set of chromosomes identical to the parent.
- Students present their work on the board and show the phase of mitosis on the screen of the multimedia complex.
- Interactive part (computer program)
- Illustrated material for the video material.
Mitosis underlies growth, regeneration and vegetative reproduction of all eukaryotic organisms. Next, we will see how this happens at the moment of fragmentation of a fertilized egg - a process that underlies the formation of a multicellular embryo (demonstration of a microslide “egg fragmentation” on an electron and light microscope).
- Demonstration of the video fragment “Mitosis”
- Mitosis is a very significant process; scientists have spent a lot of time and effort to understand all the features of this process. For example, it was found that mitosis in plant and animal cells occurs with certain differences, and that there are factors that negatively affect its progress. In addition, in the literature you can see another form of division - direct or amitosis. Working with additional literature.
Group 1: task “Amitosis”
Select “reference” points from the text, i.e. In 4-5 positions indicate the main signs of amitosis. “Mitosis is the most common, but not the only type of cell division. In almost all eukaryotes, the so-called direct nuclear division, or amitosis, is found. During amitosis, chromosome condensation does not occur and a spindle is not formed, and the nucleus is divided by constriction or fragmentation, remaining in an interphase state. Cytokinesis always follows nuclear division, resulting in the formation of a multinucleated cell. Amitotic division is characteristic of cells that complete development: dying epithelial, follicular cells of the ovaries... Amitosis also occurs in pathological processes: inflammation, malignant neoplasms... after which the cells are not capable of mitotic division.”
Group 2: task “mitosis disorder”
Make logical pairs: type of impact - consequences.
“The correct course of mitosis can be disrupted by various external factors: high doses of radiation, certain chemicals. For example, under the influence of X-rays, the DNA of a chromosome can break, and the chromosomes also break. Such chromosomes are not able to move, for example in anaphase. Some chemicals that are not characteristic of living organisms (alcohols, phenols) disrupt the consistency of mitotic processes. Some chromosomes move faster, others slower. Some of them may not be included in child kernels at all. There are substances that prevent the formation of spindle filaments. They are called cytostatics, for example, colchicine and colcemid. By influencing the cell, division can be stopped at the prometaphase stage. As a result of this effect, a double set of chromosomes appears in the nucleus."
Group 3: task:
Restore the chronological sequence of studying cells, including the processes of mitosis. Present your answer in the form of a table:
“The study of cells began with the invention of the microscope. The first person to appreciate the enormous importance of this device was the English physicist and botanist Robert Hooke. He coined the term “cell” (1665). Biologists developed ideas about the self-reproduction of cells by the mid-19th century. In 1838 - 39, the botanist Schleiden and the zoologist Schwann combined the ideas of different scientists and formed the cell theory, which postulated “the basic unit of structure and function of living organisms is the cell.” Somewhat earlier, the nucleus was discovered by Robert Brown, he described this structure as a characteristic spherical body found in plant cells. In 1868, Haeckel established that the storage and transmission of hereditary characteristics is carried out by the nucleus. Ten years earlier, Rudolf Virchow had expanded cell theory by declaring “every cell is a cell.” In 1879, Boveri and Fleming described the events occurring in a cell that result in the formation of two identical cells.”
4th group. Assignment: “Difference between mitosis in plants and animals.”
After analyzing the text, find differences in the course of mitosis in plants and animals. Fill the table.
The most important event that occurs during mitosis is the uniform distribution of the duplicated chromosomes between the two daughter cells. Mitosis in plant and animal cells proceeds almost identically, but there are still differences. For example, plant cells do not have centrioles. At the end of telophase in plant cells, a phragmoplast is formed from the filaments of the spindle in the equatorial part, and ribosomes, mitochondria, and EPS move into the same area. All this leads to the formation of a cell plate, which subsequently divides the cell in two. This process is not observed in animals. There are also differences in cytokinesis, for example, only animals form a constriction. Mitoses in animals occur in various tissues and parts of the body, which cannot be said about plants. There, mitosis occurs in strictly defined places where the educational tissue is located, that is, in meristems. For example, at the tips of the root (growth zone), in the bud (growth cone), cambium.
5 group. Assignment: create a symbolic sign that would fit the topic of our lesson. Work in a notebook and on a sheet of paper using colored pencils.
- Student performances.
- Conclusions.
Today the lesson was devoted to the most important process - mitosis. We devoted enough time to the process itself, its features, and problems. The most important thing is that this process ensures the genetic stability of the species, as well as the processes of regeneration, growth, and asexual (vegetative) reproduction. The process is complex, multi-stage and very sensitive to environmental factors.
Appendix 5
- Brainstorming (reinforcement of learned material)
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№ |
Cell and its phases |
Total mass of all DNA molecules |
Number of chromosomes |
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In one non-dividing somatic cell |
6*10-9mg |
46 |
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In one somatic cell towards the end of interphase, before prophase |
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In the maternal somatic cell in its prophase and metaphase of mitosis |
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Mother somatic cell in anaphase |
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In one daughter somatic cell at the end of the telophase of mitosis |
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In two daughter somatic cells (sum) at the end of the telophase of mitosis |
Appendix 5
Testing: "Mitosis"
1. During what period of the mitotic cycle does the amount of DNA double?
2. During the synthetic period.
4. In metaphase.
2. During what period does active cell growth occur?
1. During the presynthetic period.
2. During the synthetic period.
3. During the post-synthetic period.
4. In metaphase.
3. At what period of the life cycle does a cell have a set of chromosomes and DNA 2n4c and prepare to divide?
1. During the presynthetic period.
2. During the synthetic period.
3. During the post-synthetic period.
4. In metaphase.
4. During what period of mitosis does chromosome spiralization begin and the nuclear membrane dissolves?
1. In anaphase.
2. In prophase.
3. In telophase.
4. In metaphase.
5. During what period of mitosis do chromosomes line up along the equator of the cell?
1. In prophase.
2. In metaphase.
3. In anaphase.
4. In telophase.
6. During what period of mitosis do chromatids move away from each other and become independent chromosomes?
1. In prophase.
2. In metaphase.
3. In anaphase.
4. In telophase.
*7. During what periods of mitosis is the number of chromosomes and DNA equal to 2n4c?
1. In prophase.
2. In metaphase.
3. In anaphase.
4. In telophase.
8. At what period of mitosis is the number of chromosomes and DNA equal to 4n4c?
1. In prophase.
2. In metaphase.
3. In anaphase.
4. In telophase.
9. What is the name of the inactive part of DNA in a cell?
1. Chromatin.
2. Euchromatin.
3. Heterochromatin.
4. All DNA in the cell is active.
*10. During what periods of the cell cycle is the number of chromosomes and DNA in a cell equal to 2n4c?
1. During the presynthetic period.
2. At the end of the synthetic period.
3. During the post-synthetic period.
4. In prophase.
5. In metaphase.
6. In anaphase.
7. In telophase.
There are several correct answers to the question.
Answers on the topic “Mitosis”:
Test 1. 2.
Test 2. 1.
Test 3. 3.
Test 4. 2.
Test 5. 2.
Test 6. 3.
*Test 7. 1, 2.
Test 8. 3.
Test 9. 3.
*Test 10. 2, 3, 4, 5.
Almost all of a cell's DNA is contained in the nucleus. DNA is a long linear polymer containing many millions of nucleotides. Four types of DNA nucleotides, different nitrogenous bases. Nucleotides are arranged in a sequence that represents a code form for recording hereditary information.
To implement this information, it is rewritten, or transcribed, into shorter chains of mRNA. The symbols of the genetic code in mRNA are triplets of nucleotides - codons. Each codon designates one of the amino acids. Each DNA molecule corresponds to a separate chromosome, and all the genetic information stored in the chromosomes of an organism is called genome.
The genome of higher organisms contains an excess amount of DNA; this is not related to the complexity of the organism. It is known that the human genome contains 700 times more DNA than the bacterium E. coli. At the same time, the genome of some amphibians and plants is 30 times larger than the human genome. In vertebrates, more than 90% of DNA is not essential. The information stored in DNA is organized, read, and replicated by a variety of proteins.
The main structural proteins of the nucleus are histone proteins, characteristic only of eukaryotic cells. Histones- small, strongly basic proteins. This property is due to the fact that they are enriched with the main amino acids - lysine and arginine. Histones are also characterized by the absence of tryptophan. They are among the most conservative of all known proteins; for example, H4 in cow and pea is distinguished by only two amino acid residues. The complex of proteins with DNA in the cell nuclei of eukaryotes is referred to as chromatin.
When observing cells using a light microscope, chromatin is detected in the nuclei as zones of dense matter that are well stained with basic dyes. An in-depth study of the structure of chromatin began in 1974, when the spouses Ada and Donald Olins described its basic structural unit and named it the nucleosome.
Nucleosomes allow a long chain of DNA molecules to be packaged more compactly. Thus, in each human chromosome, the length of the DNA strand is thousands of times greater than the size of the nucleus. In electron photographs, the nucleosome appears as a disc-shaped particle with a diameter of about 11 nm. Its core is a complex of eight histone molecules, in which the four histones H2A, H2B, H3 and H4 are represented by two molecules each. These histones form the internal part of the nucleosome - the histone core. A DNA molecule containing 146 nucleotide pairs is wound onto the histone core. It forms two incomplete turns around the histone core of the nucleosome, with 83 nucleotide pairs per turn. Each nucleosome is separated from the next by a DNA linker sequence, which can be up to 80 nucleotides in length. This structure resembles beads on a string.
Calculations show that human DNA, which has 6x10 9 nucleotide pairs, should contain 3x10 7 nucleosomes. In living cells, chromatin rarely has this appearance. Nucleosomes are linked together into even more compact structures. Most of the chromatin is in the form of fibrils with a diameter of 30 nm. This packaging is carried out using another histone H1. For each nucleosome there is one H1 molecule, which pulls together the linker region at the points where DNA enters and exits the histone core.
DNA packaging significantly reduces its length. Nevertheless, the average length of the chromatin thread of one chromosome at this stage should exceed the size of the nucleus by 100 times.
The higher order structure of chromatin is a series of loops, each containing approximately 20 to 100 thousand base pairs. At the base of the loop is a site-specific DNA-binding protein. Such proteins recognize certain nucleotide sequences (sites) of two distant sections of the chromatin thread and bring them together.