Nucleic acids are macromolecular substances consisting of mononucleotides, which are connected to each other in a polymer chain using 3",5" - phosphodiester bonds and packed in cells in a certain way.
Nucleic acids are biopolymers of two varieties: ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Each biopolymer consists of nucleotides that differ in carbohydrate residue (ribose, deoxyribose) and one of the nitrogenous bases (uracil, thymine). Accordingly, nucleic acids got their name.
Structure of deoxyribonucleic acid
Nucleic acids have primary, secondary and tertiary structures.
Primary structure of DNA
The primary structure of DNA is a linear polynucleotide chain in which the mononucleotides are connected by 3", 5" phosphodiester bonds. The starting material for assembling a nucleic acid chain in a cell is the nucleoside 5'-triphosphate, which, as a result of the removal of β and γ residues of phosphoric acid, is able to attach the 3'-carbon atom of another nucleoside. Thus, the 3" carbon atom of one deoxyribose covalently binds to the 5" carbon atom of another deoxyribose via a single phosphoric acid residue and forms a linear polynucleotide chain of nucleic acid. Hence the name: 3", 5"-phosphodiester bonds. Nitrogenous bases do not take part in the connection of nucleotides of one chain (Fig. 1.).
Such a connection, between the phosphoric acid residue of one nucleotide and the carbohydrate of the other, leads to the formation of a pentose-phosphate backbone of the polynucleotide molecule, on which nitrogenous bases are added one after the other from the side. Their sequence in the chains of nucleic acid molecules is strictly specific for cells of different organisms, i.e. has a specific character (Chargaff's rule).
A linear DNA chain, the length of which depends on the number of nucleotides included in the chain, has two ends: one is called the 3 "end and contains a free hydroxyl, and the other, the 5" end, contains a phosphoric acid residue. The circuit is polar and can be 5"->3" and 3"->5". An exception is circular DNA.
The genetic "text" of DNA is made up of code "words" - triplets of nucleotides called codons. DNA segments containing information about the primary structure of all types of RNA are called structural genes.
Polynucleoditic DNA chains reach gigantic sizes, so they are packed in a certain way in the cell.
Studying the composition of DNA, Chargaff (1949) established important regularities concerning the content of individual DNA bases. They helped uncover the secondary structure of DNA. These patterns are called Chargaff's rules. Chargaff rules
These rules say that when building DNA, a rather strict correspondence (pairing) must be observed not for purine and pyrimidine bases in general, but specifically for thymine with adenine and cytosine with guanine. Based on these rules, among other things, in 1953 Watson and Crick proposed a model of the secondary structure of DNA, called the double helix (Fig.). |
Secondary structure of DNA
The secondary structure of DNA is a double helix, the model of which was proposed by D. Watson and F. Crick in 1953.
Prerequisites for creating a DNA model
As a result of initial analyzes, the idea was that DNA of any origin contains all four nucleotides in equal molar amounts. However, in the 1940s, E. Chargaff and his colleagues, as a result of the analysis of DNA isolated from various organisms, clearly showed that nitrogenous bases are contained in them in various quantitative ratios. Chargaff found that, although these ratios are the same for DNA from all cells of the same species of organisms, DNA from different species can differ markedly in the content of certain nucleotides. This suggested that the differences in the ratio of nitrogenous bases might be related to some biological code. Although the ratio of individual purine and pyrimidine bases in different DNA samples was not the same, when comparing the results of the analyzes, a certain pattern was revealed: in all samples, the total amount of purines was equal to the total amount of pyrimidines (A + G = T + C), the amount of adenine was equal to the amount of thymine (A = T), and the amount of guanine - the amount of cytosine (G = C). DNA isolated from mammalian cells was generally richer in adenine and thymine and relatively poorer in guanine and cytosine, while DNA from bacteria was richer in guanine and cytosine and relatively poorer in adenine and thymine. These data formed an important part of the factual material, on the basis of which the Watson-Crick DNA structure model was later built.
Another important indirect indication of the possible structure of DNA was L. Pauling's data on the structure of protein molecules. Pauling showed that several different stable configurations of the amino acid chain are possible in a protein molecule. One of the common configurations of the peptide chain - α-helix - is a regular helical structure. With such a structure, the formation of hydrogen bonds between amino acids located on adjacent turns of the chain is possible. Pauling described the α-helical configuration of the polypeptide chain in 1950 and suggested that DNA molecules also probably have a helical structure fixed by hydrogen bonds.
However, the most valuable information about the structure of the DNA molecule was provided by the results of X-ray diffraction analysis. X-rays, passing through a DNA crystal, undergo diffraction, that is, they are deflected in certain directions. The degree and nature of the deflection of the rays depend on the structure of the molecules themselves. The X-ray diffraction pattern (Fig. 3) gives the experienced eye a number of indirect indications regarding the structure of the molecules of the substance under study. Analysis of DNA X-ray diffraction patterns led to the conclusion that the nitrogenous bases (having a flat shape) are stacked like a stack of plates. X-ray patterns made it possible to identify three main periods in the structure of crystalline DNA: 0.34, 2, and 3.4 nm.
Watson-Crick DNA Model
Starting from Chargaff's analytical data, Wilkins' x-rays, and chemist's research, which provided information about the exact distances between atoms in a molecule, about the angles between the bonds of a given atom, and about the size of atoms, Watson and Crick began to build physical models of individual components of the DNA molecule at a certain scale. and "adjust" them to each other in such a way that the resulting system corresponds to various experimental data [show] .
Even earlier, it was known that adjacent nucleotides in a DNA chain are connected by phosphodiester bridges that link the 5'-carbon atom of deoxyribose of one nucleotide to the 3'-carbon atom of deoxyribose of the next nucleotide. Watson and Crick had no doubt that a period of 0.34 nm corresponds to the distance between successive nucleotides in a DNA strand. Further, it could be assumed that the period of 2 nm corresponds to the thickness of the chain. And in order to explain to what real structure the period of 3.4 nm corresponds, Watson and Crick, as well as Pauling earlier, assumed that the chain is twisted in the form of a spiral (or, more precisely, forms a helix, since the spiral in the strict sense of this the word is obtained when the turns form a conical rather than a cylindrical surface in space). Then the period of 3.4 nm will correspond to the distance between successive turns of this spiral. Such a spiral can be very dense or somewhat stretched, i.e., its turns can be flat or steep. Since the period of 3.4 nm is exactly 10 times the distance between consecutive nucleotides (0.34 nm), it is clear that each complete turn of the helix contains 10 nucleotides. From these data, Watson and Crick were able to calculate the density of a polynucleotide chain twisted into a helix with a diameter of 2 nm, with a distance between turns equal to 3.4 nm. It turned out that such a strand would have a density half that of the actual density of DNA, which was already known. I had to assume that the DNA molecule consists of two chains - that it is a double helix of nucleotides.
The next task was, of course, to elucidate the spatial relationship between the two strands forming the double helix. Having tried a number of strand arrangements on their physical model, Watson and Crick found that the best fit for all available data is one in which the two polynucleotide helices run in opposite directions; in this case, chains consisting of sugar and phosphate residues form the surface of a double helix, and purines and pyrimidines are located inside. The bases located opposite each other, belonging to two chains, are connected in pairs by hydrogen bonds; it is these hydrogen bonds that hold the chains together, thus fixing the overall configuration of the molecule.
The DNA double helix can be thought of as a helical rope ladder, with the rungs remaining horizontal. Then two longitudinal ropes will correspond to chains of sugar and phosphate residues, and the crossbars will correspond to pairs of nitrogenous bases connected by hydrogen bonds.
As a result of further study of possible models, Watson and Crick came to the conclusion that each "crossbar" should consist of one purine and one pyrimidine; at a period of 2 nm (corresponding to the diameter of the double helix), there would not be enough room for two purines, and the two pyrimidines could not be close enough together to form proper hydrogen bonds. An in-depth study of the detailed model showed that adenine and cytosine, making up a combination of the right size, still could not be arranged in such a way that hydrogen bonds formed between them. Similar reports also forced the guanine-thymine combination to be excluded, while the combinations adenine-thymine and guanine-cytosine were found to be quite acceptable. The nature of hydrogen bonds is such that adenine pairs with thymine, and guanine pairs with cytosine. This concept of specific base pairing made it possible to explain the "Chargaff rule", according to which in any DNA molecule the amount of adenine is always equal to the content of thymine, and the amount of guanine is always equal to the amount of cytosine. Two hydrogen bonds form between adenine and thymine, and three between guanine and cytosine. Due to this specificity in the formation of hydrogen bonds against each adenine in one chain, thymine is in the other; in the same way, only cytosine can be placed against each guanine. Thus, the chains are complementary to each other, that is, the sequence of nucleotides in one chain uniquely determines their sequence in the other. The two chains run in opposite directions and their phosphate end groups are at opposite ends of the double helix.
As a result of their research, in 1953 Watson and Crick proposed a model for the structure of the DNA molecule (Fig. 3), which remains relevant to the present. According to the model, a DNA molecule consists of two complementary polynucleotide chains. Each DNA strand is a polynucleotide consisting of several tens of thousands of nucleotides. In it, neighboring nucleotides form a regular pentose-phosphate backbone due to the combination of a phosphoric acid residue and deoxyribose by a strong covalent bond. The nitrogenous bases of one polynucleotide chain are arranged in a strictly defined order against the nitrogenous bases of the other. The alternation of nitrogenous bases in the polynucleotide chain is irregular.
The arrangement of nitrogenous bases in the DNA chain is complementary (from the Greek "complement" - addition), i.e. against adenine (A) is always thymine (T), and against guanine (G) - only cytosine (C). This is explained by the fact that A and T, as well as G and C, strictly correspond to each other, i.e. complement each other. This correspondence is given by the chemical structure of the bases, which allows the formation of hydrogen bonds in a pair of purine and pyrimidine. Between A and T there are two bonds, between G and C - three. These bonds provide partial stabilization of the DNA molecule in space. The stability of the double helix is directly proportional to the number of G≡C bonds, which are more stable than the A=T bonds.
The known sequence of nucleotides in one strand of DNA makes it possible, by the principle of complementarity, to establish the nucleotides of another strand.
In addition, it has been established that nitrogenous bases having an aromatic structure are located one above the other in an aqueous solution, forming, as it were, a stack of coins. This process of forming stacks of organic molecules is called stacking. The polynucleotide chains of the DNA molecule of the considered Watson-Crick model have a similar physicochemical state, their nitrogenous bases are arranged in the form of a stack of coins, between the planes of which van der Waals interactions (stacking interactions) occur.
Hydrogen bonds between complementary bases (horizontally) and stacking interaction between base planes in a polynucleotide chain due to van der Waals forces (vertically) provide the DNA molecule with additional stabilization in space.
The sugar-phosphate backbones of both chains are turned outward, and the bases are inward, towards each other. The direction of the strands in DNA is antiparallel (one of them has the direction 5"->3", the other - 3"->5", i.e. the 3"-end of one strand is located opposite the 5"-end of the other.). The chains form right helixes with a common axis. One turn of the helix is 10 nucleotides, the size of the turn is 3.4 nm, the height of each nucleotide is 0.34 nm, the diameter of the helix is 2.0 nm. As a result of the rotation of one strand around the other, a major groove (about 20 Å in diameter) and a minor groove (about 12 Å) are formed in the DNA double helix. This form of the Watson-Crick double helix was later called the B-form. In cells, DNA usually exists in the B form, which is the most stable.
Functions of DNA
The proposed model explained many of the biological properties of deoxyribonucleic acid, including the storage of genetic information and the diversity of genes, provided by a wide variety of consecutive combinations of 4 nucleotides and the fact of the existence of a genetic code, the ability to self-reproduce and transmit genetic information, provided by the replication process, and the implementation of genetic information in the form of proteins, as well as any other compounds formed with the help of enzyme proteins.
Basic functions of DNA.
- DNA is the carrier of genetic information, which is ensured by the fact of the existence of the genetic code.
- Reproduction and transmitted genetic information in generations of cells and organisms. This function is provided by the replication process.
- Implementation of genetic information in the form of proteins, as well as any other compounds formed with the help of enzyme proteins. This function is provided by the processes of transcription and translation.
Forms of organization of double-stranded DNA
DNA can form several types of double helixes (Fig. 4). Currently, six forms are already known (from A to E and Z-form).
Structural forms of DNA, as established by Rosalind Franklin, depend on the saturation of the nucleic acid molecule with water. In studies of DNA fibers using X-ray diffraction analysis, it was shown that the X-ray diffraction pattern radically depends on at what relative humidity, at what degree of water saturation of this fiber, the experiment takes place. If the fiber was sufficiently saturated with water, then one radiograph was obtained. When dried, a completely different X-ray pattern appeared, very different from the X-ray pattern of a high-moisture fiber.
Molecule of high humidity DNA is called B-shape. Under physiological conditions (low salt concentration, high degree of hydration), the dominant structural type of DNA is the B-form (the main form of double-stranded DNA is the Watson-Crick model). The helix pitch of such a molecule is 3.4 nm. There are 10 complementary pairs per turn in the form of twisted stacks of "coins" - nitrogenous bases. The stacks are held together by hydrogen bonds between two opposite "coins" of the stacks, and are "coiled" with two ribbons of the phosphodiester backbone twisted into a right-handed helix. The planes of the nitrogenous bases are perpendicular to the axis of the helix. Neighboring complementary pairs are rotated relative to each other by 36°. The helix diameter is 20Å, with the purine nucleotide occupying 12Å and the pyrimidine nucleotide occupying 8Å.
DNA molecule of lower moisture is called A-form. The A-form is formed under conditions of less high hydration and at a higher content of Na + or K + ions. This wider right-handed conformation has 11 base pairs per turn. The planes of nitrogenous bases have a stronger inclination to the axis of the helix, they deviate from the normal to the axis of the helix by 20°. This implies the presence of an internal void with a diameter of 5 Å. The distance between adjacent nucleotides is 0.23 nm, the length of the coil is 2.5 nm, and the diameter of the helix is 2.3 nm.
Initially, the A-form of DNA was thought to be less important. However, later it turned out that the A-form of DNA, as well as the B-form, is of great biological importance. The RNA-DNA helix in the template-seed complex, as well as the RNA-RNA helix and RNA hairpin structures have the A-form (the 2'-hydroxyl group of ribose does not allow RNA molecules to form the B-form). The A-form of DNA is found in spores. It has been established that the A-form of DNA is 10 times more resistant to UV rays than the B-form.
The A-form and B-form are called the canonical forms of DNA.
Forms C-E also right-handed, their formation can only be observed in special experiments, and, apparently, they do not exist in vivo. The C-form of DNA has a structure similar to B-DNA. The number of base pairs per turn is 9.33, and the length of the helix is 3.1 nm. The base pairs are inclined at an angle of 8 degrees relative to the perpendicular position to the axis. The grooves are close in size to the grooves of B-DNA. In this case, the main groove is somewhat smaller, and the minor groove is deeper. Natural and synthetic DNA polynucleotides can pass into the C-form.
| Table 1. Characteristics of some types of DNA structures | |||
| Spiral type | A | B | Z |
| Spiral pitch | 0.32 nm | 3.38 nm | 4.46 nm |
| Spiral twist | Right | Right | Left |
| Number of base pairs per turn | 11 | 10 | 12 |
| Distance between base planes | 0.256 nm | 0.338 nm | 0.371 nm |
| Glycosidic bond conformation | anti | anti | anti-C syn-G |
| Furanose ring conformation | C3 "-endo | C2 "-endo | C3 "-endo-G C2 "-endo-C |
| Groove width, small/large | 1.11/0.22 nm | 0.57/1.17 nm | 0.2/0.88 nm |
| Groove depth, small/large | 0.26/1.30 nm | 0.82/0.85 nm | 1.38/0.37 nm |
| Spiral diameter | 2.3 nm | 2.0 nm | 1.8 nm |
Structural elements of DNA
(non-canonical DNA structures)
Structural elements of DNA include unusual structures limited by some special sequences:
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Z-form of DNA was discovered in 1979 while studying the hexanucleotide d(CG)3 - . It was opened by MIT professor Alexander Rich and his staff. The Z-form has become one of the most important structural elements of DNA due to the fact that its formation was observed in DNA regions where purines alternate with pyrimidines (for example, 5'-HCHCHC-3'), or in repeats 5'-CHCHCH-3' containing methylated cytosine. An essential condition for the formation and stabilization of Z-DNA was the presence in it of purine nucleotides in the syn-conformation, alternating with pyrimidine bases in the anti-conformation.
Natural DNA molecules mostly exist in the right B form unless they contain sequences like (CG)n. However, if such sequences are part of the DNA, then these regions, when the ionic strength of the solution or cations that neutralize the negative charge on the phosphodiester backbone, can change into the Z-form, while other DNA regions in the chain remain in the classical B-form. The possibility of such a transition indicates that the two strands in the DNA double helix are in a dynamic state and can unwind relative to each other, passing from the right form to the left one and vice versa. The biological consequences of this lability, which allows conformational transformations of the DNA structure, are not yet fully understood. It is believed that Z-DNA regions play a role in the regulation of the expression of certain genes and take part in genetic recombination.
The Z-form of DNA is a left-handed double helix, in which the phosphodiester backbone is zigzag along the axis of the molecule. Hence the name of the molecule (zigzag)-DNA. Z-DNA is the least twisted (12 base pairs per turn) and thinnest known in nature. The distance between adjacent nucleotides is 0.38 nm, the coil length is 4.56 nm, and the Z-DNA diameter is 1.8 nm. In addition, the appearance of this DNA molecule is distinguished by the presence of a single groove.
The Z-form of DNA has been found in prokaryotic and eukaryotic cells. To date, antibodies have been obtained that can distinguish between the Z-form and the B-form of DNA. These antibodies bind to specific regions of the giant chromosomes of Drosophila (Dr. melanogaster) salivary gland cells. The binding reaction is easy to follow due to the unusual structure of these chromosomes, in which denser regions (disks) contrast with less dense regions (interdisks). Z-DNA regions are located in the interdiscs. It follows from this that the Z-form actually exists in natural conditions, although the sizes of the individual sections of the Z-form are not yet known.
(shifters) - the most famous and frequently occurring base sequences in DNA. A palindrome is a word or phrase that reads from left to right and vice versa in the same way. Examples of such words or phrases are: HUT, COSSACK, FLOOD, AND A ROSE FALLED ON AZOR'S PAWS. When applied to sections of DNA, this term (palindrome) means the same alternation of nucleotides along the chain from right to left and from left to right (like the letters in the word "hut", etc.).
A palindrome is characterized by the presence of inverted repeats of base sequences having a second-order symmetry with respect to two DNA strands. Such sequences, for obvious reasons, are self-complementary and tend to form hairpin or cruciform structures (Fig.). Hairpins help regulatory proteins to recognize the place where the genetic text of chromosome DNA is copied.
In cases where an inverted repeat is present in the same DNA strand, such a sequence is called a mirror repeat. Mirror repeats do not have self-complementary properties and therefore are not capable of forming hairpin or cruciform structures. Sequences of this type are found in almost all large DNA molecules and can range from just a few base pairs to several thousand base pairs.
The presence of palindromes in the form of cruciform structures in eukaryotic cells has not been proven, although a number of cruciform structures have been found in vivo in E. coli cells. The presence of self-complementary sequences in RNA or single-stranded DNA is the main reason for the folding of the nucleic chain in solutions into a certain spatial structure, characterized by the formation of many "hairpins".
H-form of DNA- this is a helix that is formed by three strands of DNA - the triple helix of DNA. It is a complex of the Watson-Crick double helix with the third single-stranded DNA strand, which fits into its large groove, with the formation of the so-called Hoogsteen pair.
The formation of such a triplex occurs as a result of the addition of the DNA double helix in such a way that half of its section remains in the form of a double helix, and the second half is disconnected. In this case, one of the disconnected spirals forms a new structure with the first half of the double helix - a triple helix, and the second turns out to be unstructured, in the form of a single-filament section. A feature of this structural transition is a sharp dependence on the pH of the medium, the protons of which stabilize the new structure. Due to this feature, the new structure was called the H-form of DNA, the formation of which was found in supercoiled plasmids containing homopurine-homopyrimidine regions, which are a mirror repeat.
In further studies, the possibility of structural transition of some homopurine-homopyrimidine double-stranded polynucleotides was established with the formation of a three-stranded structure containing:
- one homopurine and two homopyrimidine strands ( Py-Pu-Py triplex) [Hoogsteen interaction].
The constituent blocks of the Py-Pu-Py triplex are canonical isomorphic CGC+ and TAT triads. Stabilization of the triplex requires protonation of the CGC+ triad, so these triplexes are dependent on the pH of the solution.
- one homopyrimidine and two homopurine strands ( Py-Pu-Pu triplex) [inverse Hoogsteen interaction].
The constituent blocks of the Py-Pu-Pu triplex are the canonical isomorphic CGG and TAA triads. An essential property of Py-Pu-Pu triplexes is the dependence of their stability on the presence of doubly charged ions, and different ions are needed to stabilize triplexes of different sequences. Since the formation of Py-Pu-Pu triplexes does not require protonation of their constituent nucleotides, such triplexes can exist at neutral pH.
Note: the direct and reverse Hoogsteen interaction is explained by the symmetry of 1-methylthymine: a 180 ° rotation leads to the fact that the place of the O4 atom is occupied by the O2 atom, while the system of hydrogen bonds is preserved.
There are two types of triple helixes:
- parallel triple helixes in which the polarity of the third strand is the same as that of the homopurine chain of the Watson-Crick duplex
- antiparallel triple helixes, in which the polarities of the third and homopurine chains are opposite.
G-quadruplex- 4-stranded DNA. Such a structure is formed if there are four guanines, which form the so-called G-quadruplex - a round dance of four guanines.
The first hints of the possibility of the formation of such structures were obtained long before the breakthrough work of Watson and Crick - as early as 1910. Then the German chemist Ivar Bang discovered that one of the components of DNA - guanosic acid - forms gels at high concentrations, while other components of DNA do not have this property.
In 1962, using the X-ray diffraction method, it was possible to establish the cell structure of this gel. It turned out to be composed of four guanine residues, linking each other in a circle and forming a characteristic square. In the center, the bond is supported by a metal ion (Na, K, Mg). The same structures can be formed in DNA if it contains a lot of guanine. These flat squares (G-quartets) are stacked to form fairly stable, dense structures (G-quadruplexes).
Four separate strands of DNA can be woven into four-stranded complexes, but this is rather an exception. More often, a single strand of nucleic acid is simply tied into a knot, forming characteristic thickenings (for example, at the ends of chromosomes), or double-stranded DNA forms a local quadruplex at some guanine-rich site.
The most studied is the existence of quadruplexes at the ends of chromosomes - on telomeres and in oncopromoters. However, a complete understanding of the localization of such DNA in human chromosomes is still not known.
All these unusual structures of DNA in the linear form are unstable compared to the B-form of DNA. However, DNA often exists in a ring form of topological tension when it has what is known as supercoiling. Under these conditions, non-canonical DNA structures are easily formed: Z-forms, "crosses" and "hairpins", H-forms, guanine quadruplexes, and the i-motif.
- Supercoiled form - noted when released from the cell nucleus without damage to the pentose-phosphate backbone. It has the form of supertwisted closed rings. In the supertwisted state, the DNA double helix is "twisted on itself" at least once, i.e. it contains at least one supercoil (takes the shape of a figure eight).
- Relaxed state of DNA - observed with a single break (break of one strand). In this case, the supercoils disappear and the DNA takes the form of a closed ring.
- The linear form of DNA is observed when two strands of the double helix are broken.
Tertiary structure of DNA
Tertiary structure of DNA is formed as a result of additional twisting in space of a double-stranded molecule - its supercoiling. Supercoiling of the DNA molecule in eukaryotic cells, in contrast to prokaryotes, is carried out in the form of complexes with proteins.
Almost all eukaryotic DNA is located in the chromosomes of the nuclei, only a small amount of it is found in mitochondria, and in plants and in plastids. The main substance of the chromosomes of eukaryotic cells (including human chromosomes) is chromatin, consisting of double-stranded DNA, histone and non-histone proteins.
Histone proteins of chromatin
Histones are simple proteins that make up up to 50% of chromatin. In all the studied cells of animals and plants, five main classes of histones were found: H1, H2A, H2B, H3, H4, differing in size, amino acid composition and charge (always positive).
Mammalian histone H1 consists of a single polypeptide chain containing approximately 215 amino acids; the sizes of other histones vary from 100 to 135 amino acids. All of them are spiralized and twisted into a globule with a diameter of about 2.5 nm, contain an unusually large amount of positively charged amino acids lysine and arginine. Histones can be acetylated, methylated, phosphorylated, poly(ADP)-ribosylated, and histones H2A and H2B can be covalently linked to ubiquitin. What is the role of such modifications in the formation of the structure and performance of functions by histones has not yet been fully elucidated. It is assumed that this is their ability to interact with DNA and provide one of the mechanisms for regulating the action of genes.
Histones interact with DNA mainly through ionic bonds (salt bridges) formed between the negatively charged phosphate groups of DNA and the positively charged lysine and arginine residues of histones.
Non-histone proteins of chromatin
Non-histone proteins, unlike histones, are very diverse. Up to 590 different fractions of DNA-binding nonhistone proteins have been isolated. They are also called acidic proteins, since acidic amino acids predominate in their structure (they are polyanions). The specific regulation of chromatin activity is associated with a variety of non-histone proteins. For example, enzymes essential for DNA replication and expression can bind to chromatin transiently. Other proteins, say those involved in various regulatory processes, bind to DNA only in specific tissues or at certain stages of differentiation. Each protein is complementary to a specific sequence of DNA nucleotides (DNA site). This group includes:
- a family of site-specific zinc finger proteins. Each "zinc finger" recognizes a specific site consisting of 5 nucleotide pairs.
- a family of site-specific proteins - homodimers. A fragment of such a protein in contact with DNA has a "helix-turn-helix" structure.
- high mobility proteins (HMG proteins - from English, high mobility gel proteins) are a group of structural and regulatory proteins that are constantly associated with chromatin. They have a molecular weight of less than 30 kD and are characterized by a high content of charged amino acids. Due to their low molecular weight, HMG proteins are highly mobile during polyacrylamide gel electrophoresis.
- enzymes of replication, transcription and repair.
With the participation of structural, regulatory proteins and enzymes involved in the synthesis of DNA and RNA, the nucleosome thread is converted into a highly condensed complex of proteins and DNA. The resulting structure is 10,000 times shorter than the original DNA molecule.
Chromatin
Chromatin is a complex of proteins with nuclear DNA and inorganic substances. Most of the chromatin is inactive. It contains densely packed, condensed DNA. This is heterochromatin. There are constitutive, genetically inactive chromatin (satellite DNA) consisting of non-expressed regions, and facultative - inactive in a number of generations, but under certain circumstances capable of expressing.
Active chromatin (euchromatin) is uncondensed, i.e. packed less tightly. In different cells, its content ranges from 2 to 11%. In the cells of the brain, it is the most - 10-11%, in the cells of the liver - 3-4 and kidneys - 2-3%. There is an active transcription of euchromatin. At the same time, its structural organization makes it possible to use the same DNA genetic information inherent in a given type of organism in different ways in specialized cells.
In an electron microscope, the image of chromatin resembles beads: spherical thickenings about 10 nm in size, separated by filamentous bridges. These spherical thickenings are called nucleosomes. The nucleosome is the structural unit of chromatin. Each nucleosome contains a 146 bp long supercoiled DNA segment wound to form 1.75 left turns per nucleosome core. The nucleosomal core is a histone octamer consisting of histones H2A, H2B, H3 and H4, two molecules of each type (Fig. 9), which looks like a disk with a diameter of 11 nm and a thickness of 5.7 nm. The fifth histone, H1, is not part of the nucleosomal core and is not involved in the process of DNA winding around the histone octamer. It contacts DNA at the points where the double helix enters and exits the nucleosomal core. These are intercore (linker) sections of DNA, the length of which varies depending on the type of cell from 40 to 50 nucleotide pairs. As a result, the length of the DNA fragment that is part of the nucleosomes also varies (from 186 to 196 nucleotide pairs).
The nucleosome contains about 90% of DNA, the rest of it is the linker. It is believed that nucleosomes are fragments of "silent" chromatin, while the linker is active. However, nucleosomes can unfold and become linear. Unfolded nucleosomes are already active chromatin. This clearly shows the dependence of the function on the structure. It can be assumed that the more chromatin is in the composition of globular nucleosomes, the less active it is. Obviously, in different cells the unequal proportion of resting chromatin is associated with the number of such nucleosomes.
On electron microscopic photographs, depending on the conditions of isolation and the degree of stretching, chromatin can look not only as a long thread with thickenings - "beads" of nucleosomes, but also as a shorter and denser fibril (fiber) with a diameter of 30 nm, the formation of which is observed during the interaction histone H1 associated with the linker region of DNA and histone H3, which leads to additional twisting of the helix of six nucleosomes per turn with the formation of a solenoid with a diameter of 30 nm. In this case, the histone protein can interfere with the transcription of a number of genes and thus regulate their activity.
As a result of the interactions of DNA with histones described above, a segment of the DNA double helix of 186 base pairs with an average diameter of 2 nm and a length of 57 nm turns into a helix with a diameter of 10 nm and a length of 5 nm. With the subsequent compression of this helix to a fiber with a diameter of 30 nm, the degree of condensation increases by another six times.
Ultimately, the packaging of the DNA duplex with five histones results in a 50-fold DNA condensation. However, even such a high degree of condensation cannot explain the almost 50,000-100,000-fold DNA compaction in the metaphase chromosome. Unfortunately, the details of the further packing of chromatin up to the metaphase chromosome are not yet known; therefore, only general features of this process can be considered.
Levels of DNA compaction in chromosomes
Each DNA molecule is packaged into a separate chromosome. Diploid human cells contain 46 chromosomes, which are located in the cell nucleus. The total length of the DNA of all the chromosomes of a cell is 1.74 m, but the diameter of the nucleus in which the chromosomes are packed is millions of times smaller. Such a compact packing of DNA in chromosomes and chromosomes in the cell nucleus is provided by a variety of histone and non-histone proteins interacting in a certain sequence with DNA (see above). Compaction of DNA in chromosomes makes it possible to reduce its linear dimensions by about 10,000 times - conditionally from 5 cm to 5 microns. There are several levels of compactization (Fig. 10).
- DNA double helix is a negatively charged molecule with a diameter of 2 nm and a length of several cm.
- nucleosomal level- chromatin looks in an electron microscope as a chain of "beads" - nucleosomes - "on a thread". The nucleosome is a universal structural unit that is found both in euchromatin and heterochromatin, in the interphase nucleus and metaphase chromosomes.
The nucleosomal level of compaction is provided by special proteins - histones. Eight positively charged histone domains form the core (core) of the nucleosome around which the negatively charged DNA molecule is wound. This gives a shortening by a factor of 7, while the diameter increases from 2 to 11 nm.
- solenoid level
The solenoid level of chromosome organization is characterized by the twisting of the nucleosomal filament and the formation of thicker fibrils 20-35 nm in diameter from it - solenoids or superbids. The solenoid pitch is 11 nm, and there are about 6-10 nucleosomes per turn. Solenoid packing is considered more probable than superbid packing, according to which a chromatin fibril with a diameter of 20–35 nm is a chain of granules, or superbids, each of which consists of eight nucleosomes. At the solenoid level, the linear size of DNA is reduced by 6-10 times, the diameter increases to 30 nm.
- loop level
The loop level is provided by non-histone site-specific DNA-binding proteins that recognize and bind to specific DNA sequences, forming loops of approximately 30-300 kb. The loop ensures gene expression, i.e. the loop is not only a structural, but also a functional formation. Shortening at this level occurs by 20-30 times. The diameter increases to 300 nm. Loop-like "lampbrush" structures in amphibian oocytes can be seen on cytological preparations. These loops appear to be supercoiled and represent DNA domains, probably corresponding to units of chromatin transcription and replication. Specific proteins fix the bases of the loops and, possibly, some of their internal regions. The loop-like domain organization facilitates the folding of chromatin in metaphase chromosomes into helical structures of higher orders.
- domain level
The domain level of chromosome organization has not been studied enough. At this level, the formation of loop domains is noted - structures of filaments (fibrils) 25-30 nm thick, which contain 60% protein, 35% DNA and 5% RNA, are practically invisible in all phases of the cell cycle with the exception of mitosis and are somewhat randomly distributed over cell nucleus. Loop-like "lampbrush" structures in amphibian oocytes can be seen on cytological preparations.
Loop domains are attached with their base to the intranuclear protein matrix in the so-called built-in attachment sites, often referred to as MAR / SAR sequences (MAR, from the English matrix associated region; SAR, from the English scaffold attachment regions) - DNA fragments several hundred long base pairs that are characterized by a high content (>65%) of A/T base pairs. Each domain appears to have a single origin of replication and functions as an autonomous supercoiled unit. Any loop domain contains many transcription units, the functioning of which is likely to be coordinated - the entire domain is either in an active or inactive state.
At the domain level, as a result of sequential packing of chromatin, the linear dimensions of DNA decrease by about 200 times (700 nm).
- chromosome level
At the chromosomal level, the prophase chromosome condenses into a metaphase one with the compaction of loop domains around the axial framework of non-histone proteins. This supercoiling is accompanied by phosphorylation of all H1 molecules in the cell. As a result, the metaphase chromosome can be depicted as densely packed solenoid loops coiled into a tight spiral. A typical human chromosome can contain up to 2600 loops. The thickness of such a structure reaches 1400 nm (two chromatids), while the DNA molecule is shortened by 104 times, i.e. from 5 cm stretched DNA to 5 µm.
Functions of chromosomes
In interaction with extrachromosomal mechanisms, chromosomes provide
- storage of hereditary information
- using this information to create and maintain cellular organization
- regulation of reading hereditary information
- self-duplication of genetic material
- the transfer of genetic material from a mother cell to daughter cells.
There is evidence that upon activation of a chromatin region, i.e. during transcription, histone H1 is reversibly removed from it first, and then the histone octet. This causes decondensation of chromatin, successive transition of a 30-nm chromatin fibril into a 10-nm filament, and its further unfolding into regions of free DNA, i.e. loss of nucleosomal structure.
DNA molecule - a secret source of life data
The progress of science leaves no doubt that living beings have an extremely complex structure and too perfect organization, the emergence of which cannot be considered accidental. This testifies to the fact that living beings are created by the Almighty Creator, who has the highest knowledge. Recently, for example, with the explanation of the perfect structure of the human gene, which has become a significant task of the Human Genome Project, the unique creation of God has once again come to the fore.
From the United States to China, scientists around the world have been trying to decipher the 3 billion chemical letters in the DNA molecule for about a decade and establish their sequence. As a result, 85% of the data contained in the DNA molecule of human beings could be sequenced. Although this development is exciting and important, Dr. Francis Collins, who heads the Human Genome Project, says that at this point in the study of the structure of the DNA molecule and in deciphering the information, only the first step has been taken.
In order to understand why deciphering this information takes so long, we must understand the nature of the information stored in the structure of the DNA molecule.
Secret structure of DNA molecule
In the production of a technological product or in the management of a plant, the most used tools are experience and the accumulation of knowledge acquired over many centuries.
How can a chain invisible to the eye, consisting of atoms assembled in the form of tracks, with a size of one billionth of a millimeter, have such a capacity of information and memory?
Added to this question is the following: if each of the 100 trillion cells in your body knows one million pages of information by heart, how many encyclopedic pages can you, as an intelligent and conscious person, remember in a lifetime? The most important thing is that the cell uses this information flawlessly, in an extremely planned and consistent way, in the right places and never makes mistakes. Even before a person is born into the world, his cells have already begun the process of his creation.
Almost everyone has heard about the existence of DNA molecules in living cells and knows that this molecule is responsible for the transmission of hereditary information. A huge bunch of different films, to one degree or another, build their plots on the properties of a small, but proud, very important molecule.
However, few people can at least roughly explain what exactly is part of the DNA molecule and how the processes of reading all this information about the "structure of the whole organism" function. Only a few are able to read “deoxyribonucleic acid” without hesitation.
Let's try to figure out what it consists of and what it looks like the most important molecule for each of us.
The structure of the structural link - nucleotide
The composition of the DNA molecule includes many structural units, since it is a biopolymer. A polymer is a macromolecule that consists of many small repeating fragments connected in series. Just like a chain is made up of links.
The structural unit of the DNA macromolecule is the nucleotide. The composition of the nucleotides of the DNA molecule includes the remains of three substances - phosphoric acid, saccharide (deoxyribose) and one of the four possible nitrogen-containing bases.
The composition of the DNA molecule includes nitrogenous bases: adenine (A), guanine (G), cytosine (C) and thymine (T).
The composition of the nucleotide chain is displayed by the alternation of the bases included in it: -AAGCGTTAGCACGT-, etc. The sequence can be any. This forms a single strand of DNA.
Helical molecule. The phenomenon of complementarity
The size of the human DNA molecule is monstrously huge (on the scale of other molecules, of course)! The genome of a single cell (46 chromosomes) contains approximately 3.1 billion base pairs. The length of the DNA chain, composed of such a number of links, is approximately two meters. It is difficult to imagine how such a bulky molecule can be placed within a tiny cell.
But nature took care of a more compact package and protection of its genome - two chains are interconnected by nitrogenous bases and form a well-known double helix. Thus, it is possible to reduce the length of the molecule by almost six times.
The order of interaction of nitrogenous bases is strictly determined by the phenomenon of complementarity. Adenine can only bind to thymine, while cytosine can only bind to guanine. These complementary pairs fit together like a key and lock, like puzzle pieces.
Now let's calculate how much memory in a computer (well, or on a flash drive) all the information about this small (on the scale of our world) molecule should occupy. The number of base pairs is 3.1x10 9 . There are 4 values in total, which means that 2 bits of information are enough for one pair (2 2 values). We multiply all this by each other and we get 6200000000 bits, or 775000000 bytes, or 775000 kilobytes, or 775 megabytes. Which roughly corresponds to the capacity of a CD disc or the volume of some 40-minute film series in average quality.
Chromosome formation. Determination of the human genome
In addition to spiralization, the molecule is repeatedly subjected to compaction. The double helix begins to twist like a ball of thread - this process is called supercoiling and occurs with the help of a special histone protein, on which the chain is wound like a coil.
This process reduces the length of the molecule by another 25-30 times. Being subjected to several more levels of packaging, more and more compacted, one DNA molecule, together with auxiliary proteins, forms a chromosome.
All information that concerns the form, type and features of the functioning of our body is determined by a set of genes. A gene is a strictly defined section of a DNA molecule. It consists of an unchanged sequence of nucleotides. Moreover, the gene is rigidly determined not only by its composition, but also by its position relative to other parts of the chain.
Ribonucleic acid and its role in protein synthesis
In addition to DNA, there are other types of nucleic acids - messenger, transport and ribosomal RNA (ribonucleic acid). RNA chains are much smaller and shorter, which makes them able to penetrate the nuclear membrane.
The RNA molecule is also a biopolymer. Its structural fragments are similar to those that are part of DNA with a small exception of the saccharide (ribose instead of deoxyribose). There are four types of nitrogenous bases: familiar to us A, G, C and uracil (U) instead of thymine. The picture above shows all this clearly.
A DNA macromolecule is capable of transmitting information to RNA in an untwisted form. The unwinding of the helix occurs with the help of a special enzyme that separates the double helix into separate chains - like the halves of a zipper lock.
At the same time, a complementary RNA chain is created parallel to the DNA chain. After copying the information and getting from the nucleus into the environment of the cell, the RNA chain initiates the processes of synthesis of the protein encoded by the gene. Protein synthesis takes place in special cell organelles - ribosomes.
The ribosome, as it reads the chain, determines in which sequence the amino acids must be connected, one after the other - as information is read into the RNA. Then, the synthesized chain of amino acids takes a certain 3D shape.
This voluminous structural molecule is a protein capable of performing the encoded functions of enzymes, hormones, receptors and building material.
conclusions
For any living being, it is protein (protein) that is the end product of each gene. It is proteins that determine all the variety of forms, properties and qualities that are encrypted in our cells.
Dear blog readers, do you know where the DNA is, leave comments or reviews that you would like to know. Someone will find this very useful!
Deoxyribonucleic acids (DNA), high-polymer natural compounds contained in the nuclei of cells of living organisms; Together with proteins, histones form the substance of chromosomes. DNA is the carrier of genetic information, its individual sections correspond to certain genes. The DNA molecule consists of 2 polynucleotide chains twisted one around the other in a helix. The chains are built from a large number of monomers of 4 types - nucleotides, the specificity of which is determined by one of the 4 nitrogenous bases (adenine, guanine, cytosine, thymine). The combinations of three adjacent nucleotides in the DNA chain (triplets, or codons) make up the genetic code. Violations of the nucleotide sequence in the DNA chain lead to hereditary changes in the body - mutations. DNA is accurately reproduced during cell division, which ensures the transmission of hereditary traits and specific forms of metabolism in a number of generations of cells and organisms.
Deoxyribonucleic acids (DNA), nucleic acids containing deoxyribose as a carbohydrate component. DNA is the main component of the chromosomes of all living organisms; it represents the genes of all pro- and eukaryotes, as well as the genomes of many viruses. In the nucleotide sequence of DNA, genetic information is recorded (encoded) about all the features of the species and the characteristics of the individual (individual) - its genotype. DNA regulates the biosynthesis of components of cells and tissues, determines the activity of the organism throughout its life.
History of the discovery and study of DNA
Already in the middle of the 19th century, it was established that the ability to inherit certain characteristics of organisms is associated with the material contained in the cell nucleus. In 1868-72. Swiss biochemist I.F. Misher isolated a substance from pus cells (leukocytes) and salmon sperm, which he called nuclein, and later called deoxyribonucleic acid.
At the end of the 19th - beginning of the 20th centuries. thanks to the work of L. Kessel, P. Levene, E. Fisher and others, it was found that DNA molecules are linear polymer chains consisting of many thousands of monomers connected to each other - deoxyribonucleotides of four types. These nucleotides are formed by the residues of the five-carbon sugar deoxyribose, phosphoric acid, and one of the four nitrogenous bases: purines - adenine and guanine, and pyrimidines - cytosine and thymine. To designate bases, they began to use the initial letters of their names in English or Russian (in the Russian-language scientific literature) language: A, G (G), C (C) and T, respectively.
For a long time it was believed that DNA is found only in animal cells, until in the 1930s. Russian biochemist A. N. Belozersky did not show that DNA is an essential component of all living cells. The first evidence of the genetic role of DNA (as the substance of heredity) was obtained in 1944 by a group of American scientists (O. Avery and others), who, in experiments on bacteria, unambiguously established that with its help an inherited trait can be transferred from one cell to another.
By the middle of the 20th century the work of British scientists (A. Todd and others) finally elucidated the structure of nucleotides, which serve as monomeric links in the DNA molecule, and the type of internucleotide bond. All nucleotides are interconnected by a 3"-, 5"-phosphodiester bond in such a way that the phosphoric acid residue serves as a link between the 3"-carbon atom of deoxyribose of one nucleotide and the 5"-carbon atom of deoxyribose of another nucleotide. Based on this, the 3' end and the 5' end of the molecule are isolated in each DNA strand.
Structure of DNA. Discovery of the "double helix"
In 1950, the American biochemist E. Chargaff discovered significant differences in the nucleotide composition of DNA from different sources. In addition, it turned out that the composition of nucleotides in a DNA molecule obeys a number of patterns, the main of which are the equality of the total number of purine and pyrimidine bases and the equality of the amount of adenine and tinine (A-T) and guanine and cytosine (G-C). In 1953, the American biochemist J. Watson and the English physicist F. Crick, based on X-ray diffraction analysis of DNA crystals (M. Wilkins laboratory) and based on Chargaff's data, proposed a three-dimensional model of its structure. According to this model, DNA molecules are two right-handed polynucleotide chains around a common axis, or a double helix. There are approximately 10 nucleotide residues per turn of the helix. The strands in this double helix are anti-parallel, that is, they point in opposite directions, so that the 3" end of one strand is opposite the 5" end of the other.
The backbones of the chains are formed by deoxyribose residues and negatively charged phosphate groups. They are on the outside of the double helix (facing the surface of the molecule). The poorly water-soluble (hydrophobic) purine and pyrimidine bases of both chains are oriented inward and are located perpendicular to the axis of the double helix.
The antiparallel polynucleotide chains of the DNA double helix are not identical in either base sequence or nucleotide composition. However, they are complementary to each other: wherever adenine appears in one chain, thymine will necessarily stand opposite it in the other chain, and cytosine of the other chain will necessarily stand opposite guanine in one chain. This means that the sequence of bases in one chain uniquely determines the sequence of bases in the other (complementary) chain of the molecule. Moreover, these base pairs form hydrogen bonds with each other (three bonds are present in the G-C pair and two between A-T). Hydrogen bonds and hydrophobic interactions play a major role in the stabilization of the DNA double helix.
Heating, significant changes in pH and a number of other factors cause the denaturation of the DNA molecule, leading to the separation of its chains. Under certain conditions, it is possible to completely restore the original (native) structure of the DNA molecule, its renaturation. The ability of complementary DNA chains to easily separate and then restore the original structure again underlies the self-reproduction of the DNA molecule, its replication (doubling): if two complementary DNA chains are divided, and then on each, as on a matrix, build new, strictly complementary chains, then the two newly formed molecules will be identical to the original. The discovery of this principle made it possible to explain the phenomenon of heredity at the molecular level.
Similarities and differences in the structure of natural DNA. Dimensions
Almost all natural DNA consists of two strands (with the exception of the single-stranded DNA of some viruses). In this case, DNA can be linear or circular (when the ends of the molecule are covalently closed). In prokaryotic cells, DNA is organized into one chromosome (nucleoid) and is represented by one circular macromolecule with a molecular weight of more than 10. In addition, some bacteria have one or more plasmids - small circular DNA molecules that are not associated with the chromosome. In eukaryotes, the bulk of DNA is located in the nucleus of the cell as part of the chromosomes (nuclear DNA). In each eukaryotic chromosome there is only one linear DNA molecule, but since in all eukaryotic cells (except sex) there is a double set of homologous chromosomes, then DNA is represented by two non-identical copies received by the body from the father and mother during the fusion of germ cells. The molecular weight of eukaryotic DNA is higher than that of prokaryotes (for example, in one of the chromosomes of the Drosophila fruit fly, it reaches 7.9 x 1010). In addition, the composition of mitochondria and chloroplasts includes circular DNA molecules with a molecular weight of 106-107. The DNA of these organelles is called cytoplasmic; it makes up approximately 0.1% of all cellular DNA.
The sizes of DNA molecules are usually expressed by the number of nucleotides that form them. These sizes vary from several thousand base pairs in bacterial plasmids and some viruses to many hundreds of thousands of base pairs in higher organisms. Such giant molecules must be packed extremely compactly in cells and viruses. For example, the length of the DNA nucleotide of Escherichia coli, consisting of approximately four million base pairs, is 1.4 mm, which is 700 times the size of the bacterial cell itself. The total length of all DNA in one single human cell is approximately 2 m. If we take into account that the adult human body consists of approximately 1013 cells, then the total length of all human DNA should be about 2x1013 m, or 2x1010 km (for comparison: the circumference of the globe - 4x104 km, and the distance from the Earth to the Sun - 1.44x108 km). How does the packaging of giant DNA molecules occur in a small volume of a cell or virus? The DNA double helix is not absolutely rigid, which makes it possible to form kinks, loops, supercoil structures, etc. In the bacterial nucleoid, this folding is supported by a small number of special proteins and, possibly, ribonucleic acids. In eukaryotic cells, with the help of a universal set of basic histone proteins and some non-histone proteins, DNA is converted into a very compact formation - chromatin, which is the main component of chromosomes. For example, the length of the DNA of the largest human chromosome is 8 cm, and in the composition of the chromosome, due to packaging, it does not exceed 8 nm.
Separate sections of DNA encoding the primary structure of a protein (polypeptide) and RNA are called genes. Hereditary information is recorded in a linear sequence of nucleotides. In different organisms, it is strictly individual and serves as the most important characteristic that distinguishes one DNA molecule from another and, accordingly, one gene from another. Animals of different species differ from each other because the DNA molecules of their cells have different nucleotide sequences, that is, they carry different information.
DNA biosynthesis
DNA biosynthesis occurs through replication, which ensures the exact copying of genetic information and its transmission from generation to generation. This process occurs with the participation of the enzyme DNA polymerase. A single-stranded (single-stranded) ribonucleic acid (RNA) molecule can also serve as a template for DNA synthesis, which occurs, for example, when cells are infected with retroviruses (including the AIDS virus). The life cycle of these viruses includes a reverse flow of information - from RNA to DNA. In this case, complementary copying of RNA into DNA is carried out using the reverse transcriptase enzyme. During the life of organisms, their DNA under the influence of external factors can undergo various damages (mutations) associated with a violation of the structure of nitrogenous bases. In the course of evolution, cells have developed protective mechanisms that ensure the restoration of its original structure - DNA repair.
Efficient methods have been developed for determining the nucleotide sequence in DNA molecules, thanks to which vast information has been accumulated about its primary structure in the genes of many viruses, some mitochondria and chloroplasts, as well as individual genes and fragments of large genomes. The nucleotide sequence of DNA of yeast, nematode worm (150 million base pairs) has been completely determined. Within the framework of the international program "Human Genome", the establishment of the nucleotide sequence of all DNA in the human genome (3 billion base pairs) has been basically completed.
Knowing the sequence of nucleotide alternation in a DNA molecule is important in the analysis of human hereditary diseases, in the isolation of individual genes and other functionally important DNA sections; it allows, using the genetic code, to unmistakably establish the primary structure of proteins encoded by certain genes. Information about the primary structure of DNA is widely used in genetic engineering to create recombinant DNA - molecules with desired properties, including DNA components from different organisms.
