Life of DNA molecules. What is DNA - deoxyribonucleic acid Structure of DNA genetics

DNA is a universal source and keeper of hereditary information, which is recorded using a special sequence of nucleotides; it determines the properties of all living organisms.

The average molecular weight of a nucleotide is assumed to be 345, and the number of nucleotide residues can reach several hundred, thousand, and even millions. DNA is mostly found in the nuclei of cells. Slightly found in chloroplasts and mitochondria. However, the DNA of the cell nucleus is not one molecule. It consists of many molecules that are distributed on different chromosomes, their number varies depending on the organism. These are the structural features of DNA.

History of the discovery of DNA

The structure and functions of DNA were discovered by James Watson and Francis Crick, and they were even awarded the Nobel Prize in 1962.

But the Swiss scientist Friedrich Johann Miescher, who worked in Germany, was the first to discover nucleic acids. In 1869, he studied animal cells - leukocytes. To obtain them, he used bandages with pus, which he got from hospitals. Mischer washed out leukocytes from the pus and isolated protein from them. During these studies, the scientist was able to establish that in leukocytes, in addition to proteins, there is something else, some substance unknown at that time. It was a thread-like or flocculent sediment that was released if an acidic environment was created. The precipitate immediately dissolved when alkali was added.

Using a microscope, the scientist discovered that when leukocytes are washed with hydrochloric acid, nuclei remain from the cells. Then he concluded that there is an unknown substance in the nucleus, which he called nuclein (the word nucleus in translation means nucleus).

After conducting a chemical analysis, Miescher found out that the new substance contains carbon, hydrogen, oxygen and phosphorus. At that time, little was known about organophosphorus compounds, so Friedrich believed that he had discovered a new class of compounds found in the cell nucleus.

Thus, in the 19th century the existence of nucleic acids was discovered. However, at that time no one could even think about the important role they played.

Substance of heredity

The structure of DNA continued to be studied, and in 1944 a group of bacteriologists led by Oswald Avery received evidence that this molecule deserves serious attention. The scientist spent many years studying pneumococci, organisms that caused pneumonia, or lung disease. Avery conducted experiments mixing pneumococci that cause disease with those that are safe for living organisms. First, the pathogenic cells were killed, and then those that did not cause diseases were added to them.

The research results amazed everyone. There were living cells that, after interacting with dead ones, learned to cause disease. The scientist found out the nature of the substance that is involved in the process of transmitting information to living cells from dead ones. The DNA molecule turned out to be this substance.

Structure

So, it is necessary to understand what structure the DNA molecule has. The discovery of its structure was a significant event; it led to the formation of molecular biology - a new branch of biochemistry. DNA is found in large quantities in the nuclei of cells, but the size and number of molecules depend on the type of organism. It has been established that the nuclei of mammalian cells contain many of these cells, they are distributed along the chromosomes, there are 46 of them.

While studying the structure of DNA, in 1924 Feulgen first established its localization. Evidence obtained from experiments showed that DNA is located in mitochondria (1-2%). Elsewhere, these molecules can be found during viral infection, in basal bodies, and also in the eggs of some animals. It is known that the more complex the organism, the greater the mass of DNA. The number of molecules present in a cell depends on the function and is usually 1-10%. The least of them is found in myocytes (0.2%), the most in germ cells (60%).

The structure of DNA has shown that in the chromosomes of higher organisms they are associated with simple proteins - albumins, histones and others, which together form DNP (deoxyribonucleoprotein). Typically, a large molecule is unstable, and in order for it to remain intact and unchanged during evolution, a so-called repair system has been created, which consists of enzymes - ligases and nucleases, which are responsible for the “repair” of the molecule.

Chemical structure of DNA

DNA is a polymer, a polynucleotide, consisting of a huge number (up to tens of thousands of millions) of mononucleotides. The structure of DNA is as follows: mononucleotides contain nitrogenous bases - cytosine (C) and thymine (T) - from pyrimidine derivatives, adenine (A) and guanine (G) - from purine derivatives. In addition to nitrogenous bases, the human and animal molecule contains 5-methylcytosine, a minor pyrimidine base. Nitrogenous bases bind to phosphoric acid and deoxyribose. The structure of DNA is shown below.

Chargaff rules

The structure and biological role of DNA were studied by E. Chargaff in 1949. In the course of his research, he identified patterns that are observed in the quantitative distribution of nitrogenous bases:

1. ∑T + C = ∑A + G (that is, the number of pyrimidine bases is equal to the number of purine bases).
2. The number of adenine residues is always equal to the number of thymine residues, and the number of guanine is equal to cytosine.
3. The specificity coefficient has the formula: G+C/A+T. For example, for a person it is 1.5, for a bull it is 1.3.
4. The sum of “A + C” is equal to the sum of “G + T”, that is, there is as much adenine and cytosine as guanine and thymine.

DNA structure model

It was created by Watson and Crick. Phosphate and deoxyribose residues are located along the backbone of two polynucleotide chains twisted in a spiral manner. It was determined that the planar structures of pyrimidine and purine bases are located perpendicular to the chain axis and form, as it were, steps of a ladder in the form of a spiral. It has also been established that A is always connected to T using two hydrogen bonds, and G is attached to C by three of the same bonds. This phenomenon was given the name “principle of selectivity and complementarity.”

Levels of structural organization

A polynucleotide chain bent like a spiral is a primary structure that has a certain qualitative and quantitative set of mononucleotides linked by a 3’,5’-phosphodiester bond. Thus, each of the chains has a 3' end (deoxyribose) and a 5' end (phosphate). Regions that contain genetic information are called structural genes.

The double helix molecule is the secondary structure. Moreover, its polynucleotide chains are antiparallel and are linked by hydrogen bonds between the complementary bases of the chains. It has been established that each turn of this helix contains 10 nucleotide residues, its length is 3.4 nm. This structure is also supported by van der Waals interaction forces, which are observed between the bases of the same chain, including repulsive and attractive components. These forces are explained by the interaction of electrons in neighboring atoms. Electrostatic interaction also stabilizes the secondary structure. It occurs between positively charged histone molecules and a negatively charged DNA strand.

Tertiary structure is the winding of DNA strands around histones, or supercoiling. Five types of histones have been described: H1, H2A, H2B, H3, H4.

The folding of nucleosomes into chromatin is a quaternary structure, so a DNA molecule several centimeters long can fold up to 5 nm.

Functions of DNA

The main functions of DNA are:

1. Storage of hereditary information. The sequence of amino acids found in a protein molecule is determined by the order in which the nucleotide residues are located in the DNA molecule. It also encrypts all the information about the properties and characteristics of the organism.
2. DNA is capable of transmitting hereditary information to the next generation. This is possible due to the ability of replication - self-duplication. DNA is capable of breaking up into two complementary chains, and on each of them (in accordance with the principle of complementarity) the original nucleotide sequence is restored.
3. With the help of DNA, the biosynthesis of proteins, enzymes and hormones occurs.

Conclusion

The structure of DNA allows it to be the custodian of genetic information and also pass it on to future generations. What features does this molecule have?

1. Stability. This is possible due to glycosidic, hydrogen and phosphodiester bonds, as well as the mechanism of repair of induced and spontaneous damage.
2. Possibility of replication. This mechanism allows the diploid number of chromosomes to be maintained in somatic cells.
3. The existence of a genetic code. Through the processes of translation and transcription, the sequence of bases found in DNA is converted into a sequence of amino acids found in the polypeptide chain.
4. Capacity for genetic recombination. In this case, new combinations of genes are formed that are linked to each other.

Thus, the structure and functions of DNA allow it to play an invaluable role in living beings. It is known that the length of the 46 DNA molecules found in each human cell is almost 2 m, and the number of nucleotide pairs is 3.2 billion.

Content

The abbreviation cellular DNA is familiar to many from a school biology course, but few can easily answer what it is. Only a vague idea of ​​heredity and genetics remains in memory immediately after graduation. Knowing what DNA is and what impact it has on our lives can sometimes be very necessary.

DNA molecule

Biochemists distinguish three types of macromolecules: DNA, RNA and proteins. Deoxyribonucleic acid is a biopolymer that is responsible for transmitting data about the hereditary traits, characteristics and development of a species from generation to generation. Its monomer is a nucleotide. What are DNA molecules? It is the main component of chromosomes and contains the genetic code.

DNA structure

Previously, scientists imagined that the DNA structure model was periodic, where identical groups of nucleotides (combinations of phosphate and sugar molecules) were repeated. A certain combination of nucleotide sequences provides the ability to “encode” information. Thanks to research, it has become clear that the structure differs in different organisms.

American scientists Alexander Rich, David Davis and Gary Felsenfeld are especially famous in studying the question of what DNA is. They presented a description of a three-helix nucleic acid in 1957. 28 years later, scientist Maxim Davidovich Frank-Kamenitsky demonstrated how deoxyribonucleic acid, which consists of two helices, folds into an H-shape of 3 strands.

The structure of deoxyribonucleic acid is double-stranded. In it, nucleotides are connected in pairs to form long polynucleotide chains. These chains make possible the formation of a double helix using hydrogen bonds. The exception is viruses that have a single-stranded genome. There are linear DNA (some viruses, bacteria) and circular (mitochondria, chloroplasts).

DNA composition

Without knowledge of what DNA is made of, there would be no medical advances. Each nucleotide is made up of three parts: a pentose sugar residue, a nitrogenous base, and a phosphoric acid residue. Based on the characteristics of the compound, the acid can be called deoxyribonucleic or ribonucleic. DNA contains a huge number of mononucleotides of two bases: cytosine and thymine. In addition, it contains pyrimidine derivatives, adenine and guanine.

There is a definition in biology called DNA - junk DNA. Its functions are still unknown. An alternative version of the name is “non-coding”, which is not correct, because it contains coding proteins and transposons, but their purpose is also a mystery. One of the working hypotheses suggests that a certain amount of this macromolecule contributes to the structural stabilization of the genome with respect to mutations.

Where is

The location inside the cell depends on the characteristics of the species. In single-celled organisms, DNA is located in the membrane. In other living beings it is located in the nucleus, plastids and mitochondria. If we talk about human DNA, it is called a chromosome. True, this is not entirely true, because chromosomes are a complex of chromatin and deoxyribonucleic acid.

Role in the cage

The main role of DNA in cells is the transmission of hereditary genes and the survival of the future generation. Not only the external data of the future individual, but also its character and health depend on it. Deoxyribonucleic acid is in a supercoiled state, but for a high-quality life process it must be untwisted. Enzymes help her with this - topoisomerases and helicases.

Topoisomerases are nucleases; they are capable of changing the degree of torsion. Another of their functions is participation in transcription and replication (cell division). Helicases break hydrogen bonds between bases. There are ligase enzymes, which “cross-link” broken bonds, and polymerases, which are involved in the synthesis of new polynucleotide chains.

How DNA is deciphered

This abbreviation for biology is familiar. The full name of DNA is deoxyribonucleic acid. Not everyone can say this the first time, so DNA decoding is often omitted in speech. There is also the concept of RNA - ribonucleic acid, which consists of amino acid sequences in proteins. They are directly related, and RNA is the second most important macromolecule.

Human DNA

Human chromosomes are separated within the nucleus, making human DNA the most stable, complete carrier of information. During genetic recombination, the helices are separated, sections are exchanged, and then the connection is restored. Due to DNA damage, new combinations and patterns are formed. The whole mechanism promotes natural selection. It is still unknown how long it has been responsible for genome transmission and what its metabolic evolution has been.

Who opened

The first discovery of the structure of DNA is attributed to the English biologists James Watson and Francis Crick, who in 1953 revealed the structural features of the molecule. It was found by the Swiss doctor Friedrich Miescher in 1869. He studied the chemical composition of animal cells using leukocytes, which accumulate en masse in purulent lesions.

Miescher was studying methods for washing white blood cells, isolated proteins when he discovered that there was something else besides them. A sediment of flakes formed at the bottom of the dish during processing. Having examined these deposits under a microscope, the young doctor discovered nuclei that remained after treatment with hydrochloric acid. It contained a compound that Friedrich called nuclein (from the Latin nucleus - nucleus).

The properties of DNA are determined by its structure:

1. Versatility- the principles of DNA construction are the same for all organisms.

2. Specificity- determined by the ratio of nitrogenous bases: A + T,

which is specific to each species. So in humans it is 1.35, in bacteria - 0.39

Specificity depends on:

number of nucleotides

type of nucleotides

arrangement of nucleotides in the DNA chain

2. Replication or DNA self-duplication: DNA↔DNA. The genetic program of cellular organisms is written in the nucleotide sequence of DNA. To preserve the unique properties of the organism, it is necessary to accurately reproduce this sequence in each subsequent generation. During cell division, the DNA content must double so that each daughter cell can receive the full spectrum of DNA, i.e. in any dividing human somatic cell, 6.4 * 10 9 nucleotide pairs must be copied. The process of DNA doubling is called replication. Replication refers to template synthesis reactions. During replication, each of the two DNA strands serves as a template for the formation of a complementary (daughter) strand. It occurs during the S-period of the interphase of the cell cycle. The high reliability of the replication process guarantees virtually error-free transmission of genetic information over a number of generations. The trigger signal for the start of DNA synthesis in the S-period is the so-called S - factor (specific proteins). Knowing the replication rate and the length of the eukaryotic chromosome, one can calculate the replication time, which theoretically amounts to several days, but in practice replication is carried out in 6–12 hours. It follows from this that replication in eukaryotes simultaneously begins in several places on one DNA molecule.

The unit of replication is the replicon. A replicon is a section of DNA where replication occurs. The number of replicons per interphase chromosome in eukaryotes can reach 100 or more. In a mammalian cell there can be 20 - 30 thousand replicons, in humans - approximately 50 thousand. At a fixed rate of chain growth (in eukaryotes - 100 nucleotides per second), multiple initiation ensures a greater speed of the process and a decrease in the time required for duplication of extended chromosome sections, those. in eukaryotes it is carried out polyreplicon replication. (Fig. 21)

The replicon contains all the necessary genes and regulatory sequences that enable replication. Each replicon is activated once during cell division. Replication is controlled at the initiation stage. Once the duplication process has begun, it will continue until the entire replicon has been duplicated.

In prokaryotes, all DNA is one replicon.

Fig.21. Replication of eukaryotic chromosomal DNA. Replication proceeds in two directions from different origins of replication (Ori) with the formation of vesicles. The "bubble" or "eye" is an area of ​​replicated DNA within unreplicated DNA. (A. S. Konichev, G. A. Sevastyanova, 2005, p. 213)

Enzymes involved in the replication process are combined into a multienzymatic complex. 15 enzymes are involved in DNA replication in prokaryotes, and more than 30 in eukaryotes, i.e. replication is an extremely complex and super-precise multi-step enzymatic process. The enzymatic complexes include the following enzymes:

1) DNA polymerases (I, III), catalyze complementary copying, i.e. are responsible for the growth of the daughter chain. (Fig. 22) Prokaryotes replicate at a rate of 1000 nucleotides per second, and eukaryotes replicate at a rate of 100 nucleotides per second. The reduced rate of synthesis in eukaryotes is associated with difficult dissociation of histone proteins, which must be removed for DNA polymerase to advance along the DNA chain in the replication fork.

2) DNA - primase. DNA polymerases can lengthen a polynucleotide chain by joining existing nucleotides. Therefore, in order for DNA polymerase to begin DNA synthesis, it needs a primer or primer (from the English primer). DNA primase synthesizes such a primer, which is then replaced by DNA segments. (Fig. 22).

3) DNA ligase connects Okazaki fragments to each other through the formation of a phosphodiester bond.

4) DNA – helicase, unwinds the DNA helix, breaks the hydrogen bonds between them. As a result, two single differently directed DNA branches are formed (Fig. 22).

5) SSB - proteins that bind to single-stranded DNA and stabilize it, i.e. they create the conditions for complementary mating.

DNA replication does not begin at any random point on the molecule, but at specific locations called the origin of replication region(s) (Ori). They have specific nucleotide sequences, which facilitates the separation of chains (Fig. 21). As a result of the initiation of replication at the Ori point, one or two replication forks are formed - places where the parent DNA strands separate. The copying process continues until the DNA is completely duplicated or until the replication forks of two adjacent replication origins merge. The origins of replication in eukaryotes are scattered along the chromosome at a distance of 20,000 nucleotide pairs (Fig. 21).

Fig.22. DNA replication (explanation in text). (B. Alberts et al., 1994, vol. 2, p. 82)

Enzyme – helicase– breaks hydrogen bonds, i.e. unwinds the double chain, forming two differently directed DNA branches (Fig. 22). Single-stranded regions are linked by special SSB proteins, which line up outside each mother chain and pull them away from each other. This makes nitrogenous bases available for binding to complementary nucleotides. At the convergence of these branches in the direction of DNA replication, the enzyme DNA polymerase is located, which catalyzes the process and controls the accuracy of complementary synthesis. The peculiarity of the work of this enzyme is its unidirectionality, i.e. construction daughter strand of DNA goes in the direction from 5" end to 3" . On one mother strand, daughter DNA synthesis occurs continuously(leading chain). She grows from 5" to 3" end in the direction of movement of the replication fork and therefore requires only one act of initiation. On another mother chain, the synthesis of the daughter chain occurs in the form of short fragments with the usual 5" - 3" polarity and with the help of enzymes - ligase they are stitched into one continuous lagging chain. Therefore, the synthesis of the lagging chain requires several acts (points) of initiation.

This synthesis method is called intermittent replication. Fragment regions synthesized on the lagging strand are named fragments in honor of the discoverer Okazaki. They are found in all replicating DNA, both prokaryotes and eukaryotes. Their length corresponds to 1000–2000 nucleotides in prokaryotes and 100–200 in eukaryotes. Thus, as a result of replication, 2 identical DNA molecules are formed, in which one strand is the mother strand, the other is newly synthesized. This type of replication is called semi-conservative. The assumption about this method of replication was made by J. Watson and F. Crick, and proven in 1958. M. Meselson And F. Stalem. After replication, chromatin is a system of 2 decompacted DNA molecules united by a centromere.

During the replication process, errors can occur, which occur with the same frequency in prokaryotes and eukaryotes - one per 10 8 -10 10 nucleotides, i.e. on average 3 errors per genome. This is proof of the high accuracy and coordination of replication processes.

Replication errors are corrected by DNA polymerase III (“proofreading mechanism”) or the repair system.

2. Reparation- this is the property of DNA to restore its integrity, i.e. repair damage. The transmission of hereditary information in an undistorted form is the most important condition for the survival of both an individual organism and the species as a whole. Most changes are harmful to the cell, either leading to mutations, blocking DNA replication, or causing cell death. DNA is constantly exposed to spontaneous (replication errors, disruption of nucleotide structure, etc.) and induced (UV irradiation, ionizing radiation, chemical and biological mutagens) environmental factors. In the course of evolution, a system has been developed that allows correcting violations in DNA - DNA repair system. As a result of its activity, per 1000 DNA damages, only one leads to mutations. Damage is any change in DNA that causes a deviation from the normal double-stranded structure:

1) the appearance of single-strand breaks;

2) removal of one of the bases, as a result of which its homologue remains unpaired;

3) replacement of one base in a complementary pair with another, incorrectly paired with the partner base;

4) the appearance of covalent bonds between the bases of one DNA strand or between bases on opposite strands.

Repair can take place before DNA doubling (pre-replicative repair) and after DNA doubling (post-replicative). Depending on the nature of the mutagens and the degree of DNA damage, light (photoreactivation), dark, SOS repair, etc. occur in the cell.

Think that photoreactivation occurs in the cell if DNA damage is caused by natural conditions (physiological characteristics of the organism, normal environmental factors, including ultraviolet rays). In this case, restoration of DNA integrity occurs with the participation of visible light: the repair enzyme is activated by visible light quanta, connects to damaged DNA, separates the pyrimidine dimers of the damaged area and restores the integrity of the DNA strand.

Dark repair (excision) observed after exposure to ionizing radiation, chemicals, etc. It involves removing the damaged area and restoring the normal structure of the DNA molecule (Fig. 23). This type of repair requires a second complementary strand of DNA. Dark repair is multi-stage, it involves a complex of enzymes, namely:

1) an enzyme that recognizes a damaged section of the DNA chain

2) DNA - endonuclease, makes a break in the damaged DNA strand

3) exonuclease removes the changed part of the DNA strand

4) DNA polymerase I synthesizes a new DNA section to replace the deleted one

5) DNA ligase joins the end of the old DNA strand with the newly synthesized one, i.e. closes the two ends of DNA (Fig. 23). 25 enzyme proteins take part in dark repair in humans.

In case of large DNA damage that threatens the life of cells, it turns on SOS reparation. SOS reparation was discovered in 1974. This type of repair is observed after exposure to large doses of ionizing radiation. A characteristic feature of SOS repair is the inaccuracy of restoration of the primary DNA structure, which is why it received the name error-prone reparations. The main goal of SOS repair is to maintain cell viability.

Disturbances in the repair system can lead to premature aging, the development of cancer, diseases of the autoimmune system, and death of a cell or organism.

Rice. 23. Repair of damaged DNA by replacing modified nucleotide residues (dark repair or excision repair). (M. Singer, P. Berg, 1998, vol. 1, p. 100)

Nucleic acids are complex, high-molecular biopolymers. These substances were first discovered in the cell nucleus, hence their name (from the Latin nucleus - nucleus). Later it was found that nucleic acids are also present in the cytoplasm of cells.

Many scientists took part in deciphering the structure of nucleic acids, such as F. Miescher, E. Chargaff, R. Franklin and others, but the American biochemist J. Watson and the English geneticist F. Crick managed to finally solve the structure of nucleic acids in 1953, for which they were awarded the Nobel Prize, and their discovery was recognized as one of the greatest discoveries of the 20th century.

There are two types of nucleic acids: DNA - deoxyribonucleic acids And RNA - ribonucleic acids. Their molecules are polymers whose monomers are nucleotides. The length of thread-like DNA molecules is enormous; in the cells of the human body it is several centimeters. The total length of DNA in the 26 pairs of human chromosomes is approximately 1.5 meters. RNA molecules are shorter - the length of each of them does not exceed 0.01 mm.

Nucleotides - monomers of nucleic acids, in turn, have a complex structure. Each nucleotide consists of three parts: a nitrogenous base, a simple pentose carbohydrate and a phosphoric acid residue:

DNA nucleotides differ in structure from RNA nucleotides. DNA molecules contain four types of nucleotides, which differ from each other in nitrogenous bases, among which are known: adenine, guanine, cytosine and thymine. Depending on which of the four types of nitrogenous bases is part of the DNA nucleotide, it is, respectively, called adenine, guanine, cytosine or thymine. Nucleotides are abbreviated as A, G, C, T. A carbohydrate that is part of LNA nucleotides. it is always the same - it is deoxyribose; a constant and unchanging component of all DNA nucleotides is the phosphoric acid residue. Thus, one of the DNA nucleotides, for example, adenine A, can be depicted schematically as follows:

Nucleotides are joined into one chain by forming covalent bonds between the deoxyribose of one and the phosphoric acid residue of the subsequent nucleotide (Fig. 1).

A DNA molecule consists of not one, but two chains of nucleotides, which are oriented towards each other by nitrogenous bases and between which hydrogen bonds occur. The number of such bonds between different nitrogenous bases is not the same, and, as a result, they can only be connected in pairs: the nitrogenous base adenine of one polynucleotide chain is always connected by two hydrogen bonds with thymine of the other chain, and guanine - by three hydrogen bonds with the nitrogenous base cytosine of the opposite polynucleotide chain. This ability to selectively combine nucleotides is called complementarity(from Latin complementum - addition).

Rice. 1. Structure of DNA

In space, the DNA molecule is a twisted double helix (secondary structure of DNA), which, in turn, undergoes further spatial packaging, forming a tertiary structure - a superhelix. This structure is characteristic of the DNA of eukaryotic chromosomes and is caused by the interaction between DNA and nuclear proteins. Thus, the length of the DNA of the largest human chromosome is 8 cm, but it is twisted so that, ultimately, it does not exceed 5 nm.

The main property of the DNA molecule is the ability to self-duplicate ( replication) (Fig. 2).

Before replication, the double helix of the DNA molecule unwinds and breaks up into two chains, each of which serves as a matrix (form) for the assembly of

principle complementarity new (child) chain. The material for building a new DNA chain is nucleotides, which are always present in the nucleus in a free state. This process takes place before cell division and underlies the doubling of the number of chromosomes.

Rice. 2. DNA double helix replication

The nucleotides of a DNA molecule encode the sequence of amino acids in a protein molecule - this is the main function of DNA - storing hereditary information. One amino acid in a protein molecule encodes 3 nucleotides of a DNA molecule. A gene is a section of a DNA molecule in which the amino acid sequence of one protein molecule is written.

To understand in detail the essence of the PCR diagnostic method, it is necessary to take a short excursion into the school biology course.

We also know from school textbooks that deoxyribonucleic acid (DNA) is a universal carrier of genetic information and hereditary characteristics in all organisms existing on Earth. The only exceptions are some microorganisms, for example, viruses - their universal carrier of genetic information is RNA - single-stranded ribonucleic acid.

Structure of the DNA molecule

The discovery of the DNA molecule occurred in 1953. Francis Crick and James Watson discovered the structure of the double helix of DNA, their work was subsequently awarded the Nobel Prize.

DNA is a double strand twisted into a helix. Each thread consists of “bricks” - nucleotides connected in series. Each DNA nucleotide contains one of four nitrogenous bases - guanine (G), adenine (A) (purines), thymine (T) and cytosine (C) (pyrimidines), associated with deoxyribose, which in turn has a phosphate group attached . Adjacent nucleotides are connected to each other in the chain by a phosphodiester bond formed by 3'-hydroxyl (3'-OH) and 5'-phosphate groups (5'-PO3). This property determines the presence of polarity in DNA, i.e., opposite directions, namely 5' and 3' ends: the 5' end of one strand corresponds to the 3' end of the second strand.

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DNA structure

The primary structure of DNA is the linear sequence of DNA nucleotides in a chain. The sequence of nucleotides in a DNA chain is written in the form of a letter DNA formula: for example - AGTCATGCCAG, the entry is made from the 5' to the 3' end of the DNA chain.

The secondary structure of DNA is formed due to the interactions of nucleotides (mostly nitrogenous bases) with each other, hydrogen bonds. The classic example of DNA secondary structure is the DNA double helix. DNA double helix is ​​the most common form of DNA in nature, consisting of two polynucleotide chains of DNA. The construction of each new DNA chain is carried out according to the principle of complementarity, i.e., each nitrogenous base of one DNA chain corresponds to a strictly defined base of another chain: in a complementary pair, T is opposite A, and C is opposite G, etc.

DNA synthesis. Replication

A unique property of DNA is its ability to double (replicate). In nature, DNA replication occurs as follows: with the help of special enzymes (gyrases), which serve as a catalyst (substances that accelerate the reaction), the helix unwinds in the cell in the area where replication should occur (DNA doubling). Next, the hydrogen bonds that bind the threads are broken and the threads diverge.

In the construction of a new chain, the active “builder” is a special enzyme - DNA polymerase. For DNA doubling, a stratum block or “foundation” is also required, which is a small double-stranded DNA fragment. This starting block, or more precisely, the complementary section of the parent DNA chain, interacts with the primer - a single-stranded fragment of 20-30 nucleotides. DNA replication or cloning occurs simultaneously on both strands. From one DNA molecule, two DNA molecules are formed, in which one strand is from the mother DNA molecule, and the second, daughter, newly synthesized.

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Thus, the process of DNA replication (doubling) includes three main stages:

• Unraveling of the DNA helix and divergence of strands
• Attaching primers
• Formation of a new DNA strand of the daughter strand

PCR analysis is based on the principle of DNA replication - DNA synthesis, which modern scientists have managed to recreate artificially: in the laboratory, doctors cause DNA to double, but not the entire DNA chain, but a small fragment of it.

Functions of DNA

The human DNA molecule is a carrier of genetic information, which is written in the form of a sequence of nucleotides using the genetic code. As a result of DNA replication described above, DNA genes are passed on from generation to generation.

Changes in the sequence of nucleotides in DNA (mutations) can lead to genetic disorders in the body.