In the nucleus, they are carriers of hereditary information. DNA is the carrier of hereditary information

2.1.1. DNA is the carrier of hereditary information

“The importance of DNA is so great that no knowledge of it will be complete.” F.Crick.

DNA - deoxyribonucleic acid - is a biological macromolecule, the carrier of genetic information in all eukaryotic and prokaryotic cells and in many viruses.

In 1928, F. Griffith discovered the phenomenon of transformation in pneumococci (transformation of the properties of bacteria). He showed that cells of non-virulent strains of bacteria (rough without capsules) acquire the properties of virulent (smooth with capsules) strains killed by heat. The nature of the transforming agent was established by Avery, McLeod and McCarthy in 1944; it turned out to be DNA. Thus, the discovery and study of transformation proved the role of DNA as a material carrier of hereditary information (Fig. 2.1).

Rice. 2.1. Transforming factor is DNA

A three-dimensional model of the spatial structure of double-stranded DNA was described in the April journal Nature in 1953 by J. Watson, Francis Crick and Maurice Wilkins. These studies formed the basis of molecular biology, which studies the basic properties and manifestations of life at the molecular level.

The structure of DNA is a polymer, the structural unit of which is the nucleotide (Fig. 2.2).

The nucleotide consists of a purine nitrogenous base: adenine (A) or guanine (G) or pyrimidine: cytosine (C) or thymine (T), a deoxyribose carbohydrate (five-carbon sugar ring) and a phosphoric acid residue (HPO~). The DNA double helix is ​​right-handed. 10 base pairs make a complete 360° rotation, so each base pair is rotated 36 degrees around the helix relative to the next pair. The phosphate groups are located on the outside of the helices, and the bases are on the inside and are located at intervals of 34 nm. The chains are held together by hydrogen bonds between the bases and are twisted around each other and around a common axis.

Rice. 2.2. Structure of DNA.

An important role in the development of the DNA model was played by the observations of Chargaff (1949) that the quantitative ratios of gaunine are always equal to the content of cytosine, and the content of adenine corresponds to the content of thymine. This position was called "Chargaff's rule":

those. the proportion of purine and pyrimidine bases is always equal.

Chargaff proposed a specificity coefficient to characterize the nucleotide composition of DNA, taking into account the proportion of guanine-cytosine pairs:

Nucleotides are connected into a polynucleotide chain by bonds between the 5" position of one pentose end and the 3" position of the next pentose ring through a phosphate group to form phosphodiester bridges, i.e. the sugar-phosphate backbone of DNA consists of 5-3" bonds. Genetic information is written in a sequence of nucleotides in the direction from the 5" end to the 3" end - this strand is called sense DNA, genes are located here. The second strand in the 3-5" direction is considered antisense, but is a necessary “standard” for storing genetic information. The antisense strand plays an important role in the processes of replication and repair (restoring the structure of damaged DNA). The bases in antiparallel strands form complementary pairs due to hydrogen bonds: A+T; G+C. Thus, the structure of one strand determines the nucleotide sequence of the other strand. Therefore, the sequences of bases in DNA strands are always antiparallel and complementary.

The principle of complementarity is universal for the processes of replication and transcription.

Currently, several modifications of the DNA molecule have been described.

DNA polymorphism-

is the ability of a molecule to take on different configurations. Currently, 6 forms have been described, some of which can only exist in vitro (in vitro):

B-shape- has a standard structure, practically corresponding to the DNA model, which was proposed by Watson, Crick and Wilkins, under physiological conditions (low salt concentration, high degree of hydration) is the dominant structural type.

A-shape - found in more dehydrated environments and at higher levels of potassium and sodium ions. Interesting from a biological point of view, because its information is close to the structure of double-stranded DNA, or for DNA-RNA duplexes.

C-shape- has fewer base shapes per turn than the B-form. All DNA can be found in these three forms, regardless of the nucleotide sequence. The following forms are characteristic only of DNA molecules with certain sequences in base pairs.

D- and E-form- extreme variants of the same form are possible; they have the smallest number of base pairs per turn (8 and 7.5). Found only in DNA molecules that do not contain guanine.

Z-form- This is a zigzag shape, with alternating left- and right-handed helicity. This form is detected in the presence of a number of factors: high concentration of salts and the presence of specific cations; a high content of negative superturns in the DNA molecule and other Z-DNA occurs in areas enriched in G-C pairs. It has been shown that the Z-form of DNA can participate in the regulation of the expression of genes both closely located and significantly distant from the Z-sites, and also play a significant role in recombination processes.

Scottish scientist Arnott suggested: “It would be surprising if this ability of DNA to change its shape was not used in any way in living nature.”

Some of the forms can, under certain conditions associated with changes in salt concentration and degree of hydration, transform into each other, for example, A<->IN; and also Z <-> B. It is assumed that mutual transitions of A- and B-forms regulate the functioning of genes. It is significant that V There are regions of human DNA that are potentially capable of transforming into the Z-form, which are dispersed in the human genome.

It is assumed that in human cells there are conditions that stabilize the Z-form (Murry et al., 1993).

Table 2.1 Structural properties of some types of DNA

Knowledge of the structure and function of DNA is necessary to understand the essence of some genetic processes that are template-based. It was clear that DNA itself cannot play the role of a template in the synthesis of proteins from amino acids, because almost all of it is found in chromosomes located in the nucleus, while most, if not all, cellular proteins are synthesized in the cytoplasm. Thus, the genetic information contained in DNA must be transferred to some intermediate molecule, which would be transported into the cytoplasm and participate in the synthesis of polypeptide chains. The idea that RNA could be such an intermediate molecule began to be seriously considered as soon as the structure of the double helix of DNA was discovered. First, cells that synthesized large amounts of protein contained a lot of RNA. Secondly, it seemed even more important that the sugar-phosphate “skeletons” of DNA and RNA are extremely similar and it would be easy to imagine how the synthesis of single RNA chains on single-stranded DNA occurs with the formation of unstable hybrid molecules, one chain of which is represented DNA and the other RNA. The relationships between DNA, RNA and protein in 1953 were presented as follows:

DNA replication.....transcription - -----> RNA... translation......-> protein,

where single strands of DNA serve as templates for the synthesis of complementary DNA molecules (replication). In turn, RNA molecules serve as templates for the sequential connection of amino acids to form polypeptide chains of proteins in the process of translation, so named because the “text” written in the “language” of nucleotides is translated (translated) into the “language” of amino acids. A group of nucleotides that code for one amino acid is called codon.

Deoxyribonucleic acid(DNA) is a material carrier of genetic information. This is a high molecular weight natural compound contained in the nuclei of cells of living organisms. DNA molecules together with histone proteins form a substance chromosomes. Histones are part of cell nuclei and are involved in maintaining and changing the structure of chromosomes at different stages of the cell cycle and in regulating gene activity. Individual sections of DNA molecules correspond to specific genes. A DNA molecule consists of two polynucleotide chains twisted around one another into a spiral (Fig. 7.1). The chains are built from a large number of monomers of four types - nucleotides, the specificity of which is determined by one of four nitrogenous bases: adenine(A), thymine(T), cytosine(C) and guanine(G). The combination of three adjacent nucleotides in a DNA chain forms genetic code. Violation of the nucleotide sequence in the DNA chain leads to hereditary changes in the body - mutations. DNA is accurately reproduced during cell division, which ensures the transmission of hereditary characteristics and specific forms of metabolism over a series of generations of cells and organisms.

Rice. 7.1. Structure of a DNA molecule.

The structural model of DNA in the form of a double helix was proposed in 1953 by the American biochemist J. Watson (b. 1928) and the English biophysicist and geneticist F. Crick (b. 1916). The Watson–Crick model made it possible to explain many properties and biological functions of the DNA molecule. For deciphering the genetic code, J. Watson, F. Crick and the English biophysicist M. Wilkins (b. 1916), who was the first to obtain a high-quality X-ray photograph of a DNA molecule, were awarded the 1962 Nobel Prize.

DNA is an amazing natural formation with spiral symmetry. The long intertwined strands of the DNA chain structure are made up of sugar and phosphate molecules. Nitrogenous bases are attached to the sugar molecules, forming cross-links between the two helical strands. An elongated DNA molecule resembles a deformed spiral staircase. It is truly a macromolecule: its molecular weight can reach 10 9 . Despite its complex structure, the DNA molecule contains only four nitrogenous bases: A, T, C, G. Hydrogen bonds are formed between adenine and thymine. They are so structurally consistent with each other that adenine recognizes and binds to thymine, and vice versa. Cytosine and guanine are another pair of a similar type. In these nucleotide pairs, A is thus always associated with T, and C with G (Fig. 7.2). This connection corresponds the principle of complementarity. The number of base pairs: adenine-thymine and cytosine-guanine, for example, in humans is enormous: some researchers believe that there are 3 billion of them, while others believe that there are more than 3.5 billion.

The ability of nitrogenous bases to recognize their partner leads to the folding of sugar phosphate chains in the form of a double helix, the structure of which was experimentally determined as a result of X-ray observations. The interactions between nitrogenous bases are highly specific, so a helix can only form if the base sequences in both strands are completely identical.

A sugar phosphate group together with one of the nitrogenous bases A, T, C or G, forming nucleotide(Fig. 7.3) can be represented as a kind of building block. The DNA molecule consists of such blocks. The sequence of nucleotides encodes information in a DNA molecule. It contains information necessary, for example, for the production of proteins needed by a living organism.

A DNA molecule can be copied in a process catalyzed by enzymes replication, which consists in doubling it. During replication, hydrogen bonds are broken to form single chains that serve as a template for the enzymatic synthesis of the same sequences of building blocks. The replication process thus involves the breaking of old and the formation of new hydrogen bonds. At the beginning of replication, two opposing strands begin to unwind and separate from one another (Fig. 7.4). At the point of unwinding, the enzyme attaches new chains to two old ones according to the principle of complementarity: T in the new chain is located opposite A in the old one, etc., as a result, two identical double helices are formed. Due to the relative weakness of such bonds, replication occurs without breaking the stronger covalent bonds in the sugar-phosphate chains. Coding of genetic information and replication of the DNA molecule are interconnected essential processes necessary for the development of a living organism.

Genetic information is encoded by the sequence of DNA nucleotides. Fundamental work on deciphering the genetic code was carried out by American biochemists M. Nirenberg (b. 1927), X. Korana (b. 1922) and R. Holley (b. 1922); Nobel Prize winners 1968 Three consecutive nucleotides make up a unit of genetic code called codon. Each codon encodes one or another amino acid, the total number of which is 20. A DNA molecule can be represented as a sequence of letters-nucleotides that form a text from a large number of them, for example, ASAT-TGGAG... Such a text contains information that determines the specifics of each organism: human, dolphin, etc. The genetic code of all living things, be it a plant, an animal or a bacterium, is the same. For example, the codon GGU codes for the amino acid glycine in all organisms. This feature of the genetic code, together with the similarity of the amino acid composition of all proteins, indicates the biochemical unity of life, which, apparently, reflects the origin of all living beings from a single ancestor.

Genetic information is encoded in DNA. The genetic code was elucidated by M. Nirenberg and H.G. the Koran, for which they were awarded the Nobel Prize in 1968.

Genetic code- a system for the arrangement of nucleotides in nucleic acid molecules that controls the sequence of amino acids in the polypeptide molecule.

Basic tenets of the code:

1) The genetic code is triplet. The mRNA triplet is called a codon. A codon encodes one amino acid.

2) The genetic code is degenerate. One amino acid is encrypted by more than one codon (from 2 to 6). The exceptions are methionine and tryptophan (AUG, GUG). In codons for one amino acid, the first two nucleotides are most often the same, but the third varies.

3) Codons do not overlap. The nucleotide sequence is read in one direction in a row, triplet by triplet.

4) The code is unambiguous. A codon codes for a specific amino acid.

5) AUG is the start codon.

6) There are no punctuation marks inside the gene - stop codons: UAG, UAA, UGA.

7) The genetic code is universal, it is the same for all organisms and viruses.

The discovery of the structure of DNA, the material carrier of heredity, contributed to the solution of many issues: gene reproduction, the nature of mutations, protein biosynthesis, etc.

The mechanism of genetic code transmission contributed to the development of molecular biology, as well as genetic engineering and gene therapy.

DNA is located in the nucleus and is part of chromatin, as well as mitochondria, centrosomes, plastids, and RNA is in the nucleoli, cytoplasmic matrix, and ribosomes.

The carrier of hereditary information in the cell is DNA, and RNA serves to transmit and implement genetic information in pro- and eukaryotes. With the help of mRNA, the process of translating the sequence of DNA nucleotides into a polypeptide occurs.

In some organisms, in addition to DNA, RNA can be the carrier of hereditary information, for example, in tobacco mosaic viruses, polio, and AIDS.

The monomers of nucleic acids are nucleotides. It has been established that in the chromosomes of eukaryotes, a giant double-stranded DNA molecule is formed by 4 types of nucleotides: adenyl, guanyl, thymidyl, cytosyl. Each nucleotide consists of a nitrogenous base (purine G + A or pyrimidine C + T), deoxyribose and a phosphoric acid residue.

Analyzing DNA of different origins, Chargaff formulated patterns of the quantitative ratio of nitrogenous bases - Chargaff's rules.

a) the amount of adenine is equal to the amount of thymine (A=T);

b) the amount of guanine is equal to the amount of cytosine (G=C);

c) the number of purines is equal to the number of pyrimidines (G+A = C+T);

d) the number of bases with 6-amino groups is equal to the number of bases with 6-keto groups (A+C = G+T).

At the same time, the ratio of bases A+T\G+C is a strictly species-specific coefficient (for humans - 0.66; mice - 0.81; bacteria - 0.41).

In 1953, a biologist J.Watson and physicist F.Crick a spatial molecular model of DNA was proposed.

The main postulates of the model are as follows:

1. Each DNA molecule consists of two long antiparallel polynucleotide chains forming a double helix twisted around a central axis (right-handed - B-form, left-handed - Z-form, discovered by A. Rich in the late 70s).

2. Each nucleoside (pentose + nitrogenous base) is located in a plane perpendicular to the helix axis.

3. Two polynucleotide chains are held together by hydrogen bonds formed between nitrogenous bases.

4. Pairing of nitrogenous bases is strictly specific; purine bases combine only with pyrimidine bases: A-T, G-C.

5. The sequence of the bases of one chain can vary significantly, but the nitrogenous bases of the other chain must be strictly complementary to them.

Polynucleotide chains are formed by covalent bonds between adjacent nucleotides through a phosphoric acid residue that connects the carbon in the fifth position of the sugar to the third carbon of the adjacent nucleotide. The chains have a direction: the beginning of the chain is 3 "OH - in the third position of the deoxyribose carbon a hydroxyl group OH is attached, the end of the chain is 5" F, a phosphoric acid residue is attached to the fifth carbon of deoxyribose.

The autosynthetic function of DNA is replication - autoreproduction. Replication is based on the principles of semi-conservatism, anti-parallelism, complementarity and discontinuity. The hereditary information of DNA is realized as a result of replication according to the type of template synthesis. It occurs in stages: binding, initiation, elongation, termination. The process is confined to the S-period of interphase. The enzyme DNA polymerase uses single-stranded DNA as a template and, in the presence of 4 nucleotides, a primer (RNA) builds a second DNA strand.

DNA synthesis is carried out according to the principle of complementarity. Phosphodiester bonds are formed between the nucleotides of the DNA chain due to the connections of the 3 "OH group of the very last nucleotide with the 5 "-phosphate of the next nucleotide, which must join the chain.

There are three main types of DNA replication: conservative, semi-conservative, dispersed.

Conservative - preservation of the integrity of the original double-chain molecule and synthesis of the daughter double-chain molecule. Half of the daughter molecules are built entirely from new material, and half are built entirely from the old parent material.

Semi-conservative – DNA synthesis begins with the attachment of the helicase enzyme to the origin of replication, which unwinds sections of DNA. DNA binding protein (DBP) is attached to each of the chains, preventing their connection. The unit of replication is the replicon - this is the region between two points at which the synthesis of daughter chains begins. The interaction of enzymes with the origin of replication is called initiation. This point moves along the chain (3 "OH → 5" F) and a replication fork is formed.

The synthesis of a new chain occurs intermittently with the formation of fragments 700-800-2000 nucleotide residues long. There is a start and end point for replication. The replicon moves along the DNA molecule and its new sections unwind. Each of the mother chains is a template for the daughter chain, which is synthesized according to the principle of complementarity. As a result of successive connections of nucleotides, the DNA chain is lengthened (elongation stage) with the help of the enzyme DNA ligase. When the required length of the molecule is reached, the synthesis stops - termination. In eukaryotes, thousands of replication forks operate at once. In prokaryotes, initiation occurs at one point in the DNA ring, with two replication forks moving in 2 directions. At the point where they meet, the two-stranded DNA molecules are separated.

Dispersed - the breakdown of DNA into nucleotide fragments, the new double-stranded DNA consists of spontaneously assembled new and parent fragments.

Eukaryotic DNA is similar in structure to prokaryotic DNA. The differences relate to: the amount of DNA by gene, the length of the DNA molecule, the order of alternation of nucleotide sequences, the shape of the fold (in eukaryotes it is linear, in prokaryotes it is circular).

Eukaryotes are characterized by DNA redundancy: the amount of DNA involved in coding is only 2%. Some of the excess DNA is represented by identical sets of nucleotides repeated many times (repeats). There are multiple and moderately repeating sequences. They form constitutive heterochromatin (structural). It is embedded between unique sequences. Redundant genes have 10 4 copies.

Metaphase chromosome (coiled chromatin) consists of two chromatids. The shape is determined by the presence of a primary constriction - the centromere. It divides the chromosome into 2 arms.

The location of the centromere determines the main shapes of chromosomes:

Metacentric,

Submetacentric,

Acrocentric,

Telocentric.

The degree of chromosome spiralization is not the same. Regions of chromosomes with weak spiralization are called euchromatic. This is an area of ​​high metabolic activity where DNA is composed of unique sequences. Zone with strong spiralization - heterochromatic region capable of transcription. Distinguish constitutive heterochromatin - genetic inert, does not contain genes, does not transform into euchromatin, and also optional, which can transform into active euchromatin. The terminal sections of the distal sections of chromosomes are called telomeres.

Chromosomes are divided into autosomes (somatic cells) and heterochromosomes (germ cells).

At the suggestion of Levitsky (1924), the diploid set of somatic chromosomes of a cell was called karyotype. It is characterized by the number, shape, and size of chromosomes. To describe the chromosomes of the karyotype according to the proposal of S.G. Navashina they are arranged in the form idiograms - systematic karyotype. In 1960, the Denver International Chromosome Classification was proposed, where chromosomes are classified according to the size and location of the centromere. In the karyotype of a human somatic cell, there are 22 pairs of autosomes and a pair of sex chromosomes. The set of chromosomes in somatic cells is called diploid, and in germ cells - haploid (it is equal to half the set of autosomes). In the human karyotype idiogram, chromosomes are divided into 7 groups, depending on their size and shape.

1 - 1-3 large metacentric.

2 - 4-5 large submetacentric.

3 - 6-12 and X chromosome are average metacentric.

4 - 13-15 average acrocentric.

5 - 16-18 relatively small meta-submetacentric.

6 - 19-20 small metacentric.

7 - 21-22 and the Y chromosome are the smallest acrocentric.

According to Paris classification chromosomes are divided into groups according to their size and shape, as well as linear differentiation.

Chromosomes have the following properties (chromosome rules):

1. Individualities - differences between non-homologous chromosomes.

2. Pairs.

3. Constancy of number - characteristic of each type.

4. Continuity - ability to reproduce.

Lesson plan

• Chromosome- self-reproducing structural element the nucleus of a cell containing DNA, which contains genetic (hereditary) information.
• The number, size and shape of chromosomes are strictly defined and specific to each species. Each chromosome consists of one or more pairs of chromonemas.
• There are homologous

and non-homologous chromosomes

a brief description of

• Chromosomes were first described in the 80s. 19th century in the form of compact rod-shaped bodies, detected under a microscope in the nucleus at a certain stage of cell division.
• Later it turned out that X. are constantly present in every cell, but their appearance changes significantly at different stages of the cell’s life.
• It has been established that chromosomes are a thread-like structure of enormous length (chromatin thread), which can twist to form a compact spiral (spiralize) or unwind (despiralize). Tight spiralization occurs before the start of cell division and ensures precise redistribution of X. among daughter cells.
• At the stage of mitotic division, chromosomes become visible under a light microscope. In them you can see a region called the centromere, to which special threads (spindle threads) are attached, which are involved in the “stretching” of chromosomes during cell division.
• The centromere is located in the center of the X., dividing it into two equal arms, or it can move towards one of the ends. In the latter case, they say that this X. is unequal in arm.
• As the latest advances in molecular genetics show, a chromosome is actually one long chromatin strand formed by a giant DNA molecule

Number of chromosomes in different species

• The number of chromosomes in all cells of each type of organism is strictly constant and is an accurate characteristic of a given species

• Human (Homo sapiens) 46
• Gorilla 48
• Macaque (Macaca mulatta) 42

• Animals
• Cat (Felis domesticus) 38
• Dog (Canis familiaris) 78
• Horse 64
• Cow (Bovis domesticus) 120
• Chicken (Gallus domesticus) 78
• Pig 40
• Fruit fly (D.melanogaster) 8
• Mouse (Mus musculus) 40
• Yeast (S.cerevisiae) 32
• Nematoda 22/24
• Rat 42
• Fox 34
• Dove 16
• Carp 104
• Lamprey 174
• Frog (Rana pipiens) 26
• Myxomycetes 14
• Butterfly 380
• Silkworm 56
• Proteus (Necturus maculosis) 38
• Crayfish (Cambarus clarkii) 200
• Hydra 30
• Ascaris 2
• Bee 16
• Ant (Myrmecia pilosula) 2
• Grape snail 24
• Earthworm 36
• Crayfish 1 16
• Malarial plasmodium 2
• Radiolaria 1600

• Plants
• Clover 14
• Topol 38
• Corn (Zea mays) 20
• Peas 14
• Birch 84
• El 24
• Onion (Allium cepa) 16
• Arabidopsis (Arabidopsis thaliana) 10
• Potato (S.tuberosum) 48
• Lily 24
• Horsetail 216

• Gooseberry 16
• Cherry 32
• Rye 14
• Wheat 42
• Fern ~1200
• Linden heart-shaped 78
• Iris Russian 80
• Common Gladiolus 80
• Pannonian clover 84
• Lake blanket 90-180
• Alpine grit 96-180
• Japanese leaflet 104
• Male shieldweed 110
• Common sheep 144
• Common grasshopper 164
• Haplopappus 4
• Arabidopsis Tal 6

• The female ant subspecies Myrmecia pilosula has the smallest number of chromosomes, having a pair of chromosomes per cell. Males have only 1 chromosome in each cell.
• Highest number: The fern species Ophioglossum reticulatum has about 630 pairs of chromosomes, or 1,260 chromosomes per cell
• The upper limit of the number of chromosomes does not depend on the amount of DNA they contain: the American amphibian Amphiuma has ~30 times more DNA than a human, which is contained in 14 chromosomes

Bacterial chromosomes

• Prokaryotes (archaebacteria and bacteria, including mitochondria and plastids, which permanently reside in the cells of most eukaryotes) do not have chromosomes in the proper sense of the word.
• Most of them have only one DNA macromolecule in the cell, closed in a ring (this structure is called a nucleoid). Linear DNA macromolecules have been found in a number of bacteria. In addition to the nucleoid or linear macromolecules, DNA can be present in the cytoplasm of prokaryotic cells in the form of small DNA molecules closed in a ring, the so-called plasmids, which usually contain a small number of genes compared to the bacterial chromosome. The composition of plasmids may be variable; bacteria can exchange plasmids during the parasexual process.
• There is evidence that bacteria have proteins associated with nucleoid DNA, but no histones have been found in them.

Eukaryotic chromosomes

• Eukaryotic chromosomes have a complex structure. The basis of a chromosome is a linear DNA macromolecule (in the DNA molecules of human chromosomes there are from 50 to 245 million pairs of nitrogenous bases). When stretched, the length of a human chromosome can reach 5 cm. In addition to it, the chromosome includes five specialized histone proteins - H1, H2A, H2B, H3 and H4 and a number of non-histone proteins
• In interphase, chromatin is not condensed, but even at this time its threads are a complex of DNA and proteins
• In early interphase (G1 phase), the basis of each of the future chromosomes is one DNA molecule. In the synthesis (S) phase, DNA molecules enter the process of replication and double. In late interphase (G2 phase), the basis of each chromosome consists of two identical DNA molecules formed as a result of replication and connected to each other in the region of the centromeric sequence
• Before the division of the cell nucleus begins, the chromosome, represented at this moment by a chain of nucleosomes, begins to spiral, or pack, forming, with the help of the H1 protein, a thicker chromatin thread, or chromatid, d = 30 nm. As a result of further spiralization, the chromatid diameter reaches 700 nm by the time of metaphase. The condensed chromosome has the shape of the letter X (often with unequal arms) because the two chromatids resulting from replication are still connected to each other at the centromere (for more information about the fate of chromosomes during cell division, see the articles mitosis and meiosis)

Male chromosome set of a diploid (ordinary) cell

Note! Both X and Y chromosomes are present

Female chromosome set of a diploid (ordinary) cell

Note! Only X chromosomes are present

Types of chromosome structure

• There are four types of chromosome structure:

• telocentric - rod-shaped chromosomes with a centromere located at the proximal end);
• acrocentric - rod-shaped chromosomes with a very short, almost imperceptible second arm);
• submetacentric - with shoulders of unequal length, resembling the letter L in shape);
• metacentric - V-shaped chromosomes with arms of equal length).

• The chromosome type is constant for each homologous chromosome and may be constant in all members of the same species or genus

1 - equal arms (metacentric);

2 - unequal shoulders (submetacentric);

3 - rod-shaped (acrocentric);

4 - chromosomes with secondary constriction.

Deoxyribonucleic acid

DNA– a biological polymer consisting of two helically twisted chains

DNA structure

• DNA- polymer
• Monomers - nucleotides
• Nucleotide- a chemical combination of residues of three substances: nitrogenous bases, carbohydrates, phosphoric acid residue

Nucleotide structure

Nitrogenous

grounds :

Cytazine

Phosphoric acid residue

Carbohydrate :

Deoxyribose

Macromolecular structure of DNA

• In 1953, J. Watson and F. Crick proposed a model of the structure of DNA. When constructing the structure, scientists were based on 4 groups of data:

• DNA is a polymer consisting of nucleotides linked by 3`-5`-phosphodiester bonds

2. The composition of DNA nucleotides obeys Chargaff’s rules:

(A + G) = (T + C); number of residues A=T, G = C

3. X-ray patterns of DNA fibers indicate that the molecule has a helical structure and contains more than one polynucleotide chain

4. Stability of the structure due to hydrogen bonds

Macromolecular structure of DNA.

• regular right-handed helix, consisting of 2 polynucleotide chains that are twisted relative to each other around a common axis;
• the chains have antiparallel orientation
• pyrimidine and purine bases are stacked at 0.34 nm intervals;
• the length of the helix turn is 3.40 nm.
• the presence of complementary pairs - bases that form pairs in which they are combined by hydrogen bonds

A gene is a section of a DNA molecule containing information about the structure of one protein-enzyme molecule.

It is the hereditary factor of any living body of nature.

Each cell synthesizes several thousand different protein molecules.

Proteins are short-lived, their existence is limited, after which they are destroyed.

Information about the sequence of amino acids in a protein molecule is encoded

as a sequence of nucleotides in DNA.

In addition to proteins, the DNA nucleotide sequence encodes information about ribosomal RNA and transfer RNA.

So, a sequence of nucleotides somehow codes for a sequence of amino acids.

The entire variety of proteins is formed from 20 different amino acids, and there are 4 types of nucleotides in DNA.

The DNA code must be triplet. It has been proven that exactly three nucleotides encode one amino acid, in this case it will be possible to encode

4 3 - 64 amino acids.

And since there are only 20 amino acids, some amino acids must be encoded by several triplets.

Properties of the genetic code:

• Triplety: Each amino acid is encoded by a triplet of nucleotides - codon .
• Uniqueness: a code triplet, a codon, corresponds to only one amino acid.
• Degeneracy(redundancy): one amino acid can be encoded by several (up to six) codons.

• Versatility: The genetic code is the same, the same amino acids are encoded by the same nucleotide triplets in all organisms on Earth.
• Non-overlapping: a nucleotide sequence has a reading frame of 3 nucleotides; the same nucleotide cannot be part of two triplets.

Properties of the genetic code:

• The presence of an initiator codon and terminator codons: and h 64 code triplets, 61 codons are coding, encode amino acids, and 3 are meaningless, do not encode amino acids that terminate the synthesis of the polypeptide during the work of the ribosome (UAA, UGA, UAG). In addition, there is an initiator codon (AUG) - methionine, from which the synthesis of any polypeptide begins.

Information in cells is DNA molecules (in some viruses and bacteriophages, RNA). The genetic functions of DNA were established in the 40s. XX century when studying transformation in bacteria. This phenomenon was first described in 1928 by F. Griffith while studying pneumococcal infection in mice. The virulence of pneumococci is determined by the presence of a capsular polysaccharide located on the surface of the bacterial cell wall. Virulent cells form smooth colonies, designated as S-colonies (from the English smooth - smooth). Avirulent bacteria, deprived of the capsular polysaccharide as a result of a gene mutation, form rough R-colonies (from the English rough - uneven).

As can be seen from the diagram, in one of the variants of the experiment, Griffith infected mice with a mixture of living cells of the R-strain and dead cells of the S-strain. The mice died, although the living bacteria were not infective. Live bacteria isolated from dead animals, when sown on the medium, formed smooth colonies, since they had a polysaccharide capsule. Consequently, transformation of avirulent cells of the R-strain into virulent cells of the S-strain occurred. The nature of the transforming agent remained unknown.

In the 40s In the laboratory of the American geneticist O. Avery, a DNA preparation purified from protein impurities was first obtained from cells of the S-strain of pneumococci. Having treated mutant R-strain cells with this drug, Avery and his colleagues (K. McLeod and M. McCarthy) reproduced Griffith’s result, i.e. achieved transformation: cells acquired the property of virulence. Thus, the chemical nature of the substance carrying out the transfer of information was established. This substance turned out to be DNA.

The discovery was quite unexpected, since until that time scientists tended to attribute genetic functions to proteins. One of the reasons for this error was the lack of knowledge about the structure of the DNA molecule. Nucleic acids were discovered in the nuclei of pus cells in 1869. chemist I. Mischer, and their chemical composition was studied. However, until the 40s. XX century scientists mistakenly believed that DNA is a monotonous polymer in which the same sequence of 4 nucleotides alternates (AGCT). In addition, nucleic acids were considered extremely conservative compounds with low functional activity, while proteins had a number of properties necessary to perform genetic functions: polymorphism, lability, and the presence of various chemically active groups in their molecules. And therefore, Avery and his colleagues began to be accused of incorrect conclusions, of insufficient purification of the DNA preparation from protein impurities. However, improvements in purification techniques have made it possible to confirm the transforming function of DNA. Scientists were able to transfer the ability to form other types of capsular polysaccharides in pneumococci, and also obtain transformation in other types of bacteria for many characteristics, including resistance to antibiotics. The significance of the discovery of American geneticists is difficult to overestimate. It served as an incentive to study nucleic acids, primarily DNA, in scientific laboratories in many countries.

Following the evidence of transformation in bacteria, the genetic functions of DNA were confirmed in bacteriophages (bacterial viruses). In 1952, A. Hershey and S. Chase infected Escherichia coli cells with T2 phage. When added to a bacterial culture, this virus is first adsorbed on the surface of the cell and then injects its contents into it, which causes cell death and the release of new phage particles. The authors of the experiment radioactively labeled either the T2 phage DNA (32P) or the protein (35S). Phage particles were mixed with bacterial cells. Unadsorbed particles were removed. Infected bacteria were then separated from the empty shells of phage particles by centrifugation. It turned out that the 35S tag is associated with the virus shells, which remain on the cell surface, and, therefore, viral proteins do not enter the cell. Most of the 32P tag ended up inside the infected bacteria. Thus, it was found that the infectious properties of bacteriophage T2 are determined by its DNA, which penetrates the bacterial cell and serves as the basis for the formation of new phage particles. This experiment also showed that the phage uses the resources of the host cell to reproduce itself.

So, by the beginning of the 50s. XX century sufficient evidence has accumulated to indicate that DNA is the carrier of genetic information. In addition to the direct evidence outlined above, this conclusion was supported by indirect data on the nature of DNA localization in the cell, the constancy of its quantity, metabolic stability and susceptibility to mutagenic effects. All this stimulated research into the structure of this molecule.

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