1 nucleic acids structure and functions. Structure and functions of nucleic acids

Nucleic acids play an important role in the cell, ensuring its vital activity and reproduction. These properties make it possible to call them the second most important biological molecules after proteins. Many researchers even put DNA and RNA in first place, implying their main importance in the development of life. However, they are destined to take second place after proteins, because the basis of life is precisely the polypeptide molecule.

Nucleic acids are a different level of life, much more complex and interesting due to the fact that each type of molecule performs a specific job. This should be looked into in more detail.

Concept of nucleic acids

All nucleic acids and RNA) are biological heterogeneous polymers that differ in the number of chains. DNA is a double-stranded polymer molecule that contains the genetic information of eukaryotic organisms. Circular DNA molecules may contain hereditary information from some viruses. These are HIV and adenoviruses. There are also 2 special types of DNA: mitochondrial and plastid (found in chloroplasts).

RNA has many more types, which is due to the different functions of the nucleic acid. There is nuclear RNA, which contains the hereditary information of bacteria and most viruses, matrix (or messenger RNA), ribosomal and transport. All of them are involved in either storage or gene expression. However, what functions nucleic acids perform in a cell should be understood in more detail.

Double stranded DNA molecule

This type of DNA is a perfect system for storing hereditary information. A double-stranded DNA molecule is a single molecule consisting of heterogeneous monomers. Their task is to form hydrogen bonds between the nucleotides of another chain. It itself consists of a nitrogenous base, an orthophosphate residue and a five-carbon monosaccharide deoxyribose. Depending on what type of nitrogenous base underlies a particular DNA monomer, it has its own name. Types of DNA monomers:

• deoxyribose with an orthophosphate residue and an adenyl nitrogenous base;
• thymidine nitrogenous base with deoxyribose and an orthophosphate residue;
• a cytosine nitrogenous base, deoxyribose and an orthophosphate residue;
• orthophosphate with deoxyribose and a guanine nitrogen residue.

In writing, to simplify the diagram, the adenyl residue is designated as “A”, the guanine residue is “G”, the thymidine residue is “T”, and the cytosine residue is “C”. It is important that genetic information is transferred from a double-stranded DNA molecule to messenger RNA. It has few differences: here the carbohydrate residue is not deoxyribose, but ribose, and instead of the thymidyl nitrogenous base in RNA there is a uracil one.

Structure and functions of DNA

DNA is built on the principle of a biological polymer, in which one chain is created in advance according to a given template, depending on the genetic information of the parent cell. DNA nucleodides are connected here by covalent bonds. Then, other nucleotides are added to the nucleotides of a single-stranded molecule. If in a single-stranded molecule the beginning is represented by the nucleotide adenine, then in the second (complementary) chain it will correspond to thymine. Cytosine is complementary to guanine. In this way, a double-stranded DNA molecule is built. It is located in the nucleus and stores hereditary information, which is encoded by codons - triplets of nucleotides. Functions of double-stranded DNA:

• preservation of hereditary information received from the parent cell;
• gene expression;
• obstacle to mutational changes.

The importance of proteins and nucleic acids

It is believed that the functions of proteins and nucleic acids are common, namely: they are involved in gene expression. The nucleic acid itself is their storage location, and the protein is the end result of reading information from the gene. The gene itself is a section of one integral DNA molecule packaged in a chromosome, in which information about the structure of a particular protein is recorded using nucleotides. One gene codes for the amino acid sequence of only one protein. It is the protein that will implement hereditary information.

Classification of RNA species

The functions of nucleic acids in a cell are very diverse. And they are most numerous in the case of RNA. However, this polyfunctionality is still relative, because one type of RNA is responsible for one of the functions. The following types of RNA exist:

• nuclear RNA of viruses and bacteria;
• messenger (messenger) RNA;
• ribosomal RNA;
• messenger RNA of plasmids (chloroplasts);
• ribosomal RNA of chloroplasts;
• mitochondrial ribosomal RNA;
• mitochondrial messenger RNA;
• transfer RNA.

Functions of RNA

This classification contains several types of RNA, which are divided depending on their location. However, in functional terms, they should be divided into only 4 types: nuclear, information, ribosomal and transport. The function of ribosomal RNA is protein synthesis based on the nucleotide sequence of messenger RNA. In this case, amino acids are “brought” to ribosomal RNA, “strung” on messenger RNA, through transport ribonucleic acid. This is how synthesis occurs in any organism that has ribosomes. The structure and functions of nucleic acids ensure both the preservation of genetic material and the creation of protein synthesis processes.

Mitochondrial nucleic acids

While almost everything is known about the functions in the cell performed by nucleic acids located in the nucleus or cytoplasm, there is still little information about mitochondrial and plastid DNA. Specific ribosomal as well as messenger RNAs were also found here. Nucleic acids DNA and RNA are present here even in the most autotrophic organisms.

Perhaps the nucleic acid entered the cell through symbiogenesis. This path is considered by scientists to be the most likely due to the lack of alternative explanations. The process is considered as follows: a symbiotic autotrophic bacterium entered the cell at a certain period. As a result, this one lives inside the cell and provides it with energy, but gradually degrades.

On initial stages evolutionary development, it is likely that the symbiont anucleate bacterium drove mutation processes in the nucleus of the host cell. This allowed the genes responsible for storing information about the structure of mitochondrial proteins to be incorporated into the nucleic acid of the host cell. However, so far there is not much information about what functions nucleic acids of mitochondrial origin perform in the cell.

Probably, some proteins are synthesized in mitochondria, the structure of which is not yet encoded by nuclear DNA or RNA of the host. It is also likely that the cell needs its own protein synthesis mechanism only because many proteins synthesized in the cytoplasm cannot pass through the double membrane of the mitochondrion. At the same time, these organelles produce energy, and therefore, if there is a channel or a specific transporter for the protein, it will be enough for the movement of molecules and against the concentration gradient.

Plasmid DNA and RNA

Plastids (chloroplasts) also have their own DNA, which is probably responsible for implementing similar functions, as is the case with mitochondrial nucleic acids. It also has its own ribosomal, matrix and plastids. Moreover, plastids, judging by the number of membranes, and not by the number of biochemical reactions, are more complex. It happens that many plastids have 4 layers of membranes, which is explained by scientists in different ways.

One thing is clear: the functions of nucleic acids in cells have not yet been fully studied. It is not known what significance the mitochondrial protein synthesizing system and the similar chloroplastic system have. It is also not entirely clear why cells need mitochondrial nucleic acids if the proteins (not all, obviously) are already encoded in nuclear DNA (or RNA, depending on the organism). Although some facts force us to agree that the protein synthesizing system of mitochondria and chloroplasts is responsible for the same functions as nuclear DNA and cytoplasmic RNA. They store hereditary information, reproduce it and pass it on to daughter cells.

Summary

It is important to understand what functions nucleic acids of nuclear, plastid and mitochondrial origin perform in a cell. This opens up many prospects for science, because the symbiont mechanism, according to which many autotrophic organisms appeared, can be reproduced today. This will make it possible to obtain a new type of cell, perhaps even a human one. Although it is too early to talk about the prospects for the introduction of multimembrane plastid organelles into cells.

It is much more important to understand that in a cell nucleic acids are responsible for almost all processes. This is also the preservation of information about the structure of the cell. Moreover, it is much more important that nucleic acids perform the function of transferring hereditary material from parent cells to daughter cells. This guarantees the further development of evolutionary processes.

The content of the article

NUCLEIC ACIDS– biological polymer molecules that store all the information about an individual living organism, determining its growth and development, as well as hereditary characteristics transmitted to the next generation. Nucleic acids are found in the cell nuclei of all plant and animal organisms, which determined their name (lat. . nucleus - core).

Composition of the polymer chain of nucleic acids.

The polymer chain of nucleic acids is assembled from fragments of phosphoric acid H 3 PO 3 and fragments of heterocyclic molecules that are derivatives of furan. There are only two types of nucleic acids, each built on the basis of one of two types of such heterocycles - ribose or deoxyribose (Fig. 1).

Rice. 1. STRUCTURE OF RIBOSE AND DEOXYRIBOSE.

The name ribose (from Lat. . Rib - rib, paper clip) has the ending - ose, which indicates that it belongs to the class of sugars (for example, glucose, fructose). The second compound does not have an OH group (hydroxy group), which is marked in red in ribose. In this regard, the triple compound is called deoxyribose, i.e., ribose devoid of an oxy group.

The polymer chain, built from fragments of ribose and phosphoric acid, is the basis of one of the nucleic acids - ribonucleic acid (RNA). The term “acid” in the name of this compound is used because one of the acidic groups OH of phosphoric acid remains unsubstituted, which gives the entire compound a slightly acidic character. If deoxyribose is involved in the formation of the polymer chain instead of ribose, then deoxyribonucleic acid is formed, for which the well-known abbreviation DNA is commonly accepted.

DNA structure.

The DNA molecule serves as the starting point in the process of growth and development of the organism. In Fig. Figure 2 shows how two types of alternating starting compounds are combined into a polymer chain; it shows not the synthesis method, but circuit diagram assembly of a DNA molecule.

In the final version, the polymer DNA molecule contains nitrogen-containing heterocycles in the side frame. Four types of such compounds are involved in the formation of DNA, two of them are six-membered cycles, and two are condensed cycles, where a six-membered ring is fused to a five-membered one (Fig. 3).

Rice. 3. STRUCTURE OF NITROGEN-CONTAINING HETEROCYCLES, which are part of DNA

At the second stage of assembly, the nitrogen-containing heterocyclic compounds shown above are added to the free OH groups of deoxyribose, forming side pendants on the polymer chain (Fig. 4).

The molecules of adenine, thymine, guanine and cytosine attached to the polymer chain are designated by the first letters of the names of the original compounds, that is, A, T, G And C.

The polymer chain of DNA itself has a certain direction - when mentally moving along the molecule in the forward and reverse directions, the same groups that make up the chain are encountered along the way in different sequences. When moving in one direction from one phosphorus atom to another, first along the path there is a CH 2 group, and then two CH groups (oxygen atoms can be ignored); when moving in the opposite direction, the sequence of these groups will be reversed (Fig. 5) .

Rice. 5. ORIENTATION OF THE POLYMER CHAIN ​​OF DNA. When describing the order in which the attached heterocycles alternate, it is customary to use the direct direction, that is, from the CH 2 group to the CH groups.

The very concept of “strand direction” helps to understand how two DNA strands are arranged when they are combined, and is also directly related to protein synthesis.

At the next stage, two DNA molecules are combined, positioned so that the beginning and ends of the chains are directed in opposite directions. In this case, the heterocycles of the two chains face each other and are located in some optimal way, meaning that hydrogen bonds arise between pairs of C=O and NH 2 groups, as well as between є N and NH=, which are part of the heterocycles ( cm. HYDROGEN BOND). In Fig. Figure 6 shows how the two chains are positioned relative to each other and how hydrogen bonds arise between the heterocycles. The most important detail is that the pairs connected by hydrogen bonds are strictly defined: fragment A always interacts with T, and the fragment G– always with C. The strictly defined geometry of these groups leads to the fact that these pairs fit each other extremely precisely (like a key to a lock), a pair A-T connected by two hydrogen bonds, and the pair G-C- three connections.

Hydrogen bonds are noticeably weaker than ordinary valence bonds, but due to their large number along the entire polymer molecule, the connection of the two chains becomes quite strong. A DNA molecule contains tens of thousands of groups A, T, G And C and the order of their alternation within one polymer molecule can be different, for example, in a certain section of the chain the sequence can look like: - A-A-T-G-C-G-A-T-. Since the interacting groups are strictly defined, the opposite section of the second polymer molecule will necessarily have the sequence - T-T-A-C-G-C-T-A-. Thus, knowing the order of arrangement of heterocycles in one chain, one can indicate their placement in another chain. From this correspondence it follows that the total number of groups in a double DNA molecule A equal to the number of groups T, and the number of groups G– quantity C(E. Chargaff's rule).

Two DNA molecules connected by hydrogen bonds are shown in Fig. 5 in the form of two flat-lying chains, but in reality they are arranged differently. The true direction in space of all bonds, determined by bond angles and contracting hydrogen interactions, leads to a certain bending of the polymer chains and rotation of the heterocycle plane, which is approximately shown in the first video fragment of Fig. 7 using the structural formula. The entire spatial structure can be conveyed much more accurately only with the help of three-dimensional models (Fig. 7, second video fragment). In this case, a complex picture arises, so it is customary to use simplified images, which are especially widely used when depicting the structure of nucleic acids or proteins. In the case of nucleic acids, polymer chains are depicted in the form of flat ribbons, and heterocyclic groups A, T, G And C– in the form of side rods or simple valence strokes having various colors, or containing at the end letter designations corresponding heterocycles (Fig. 7, third video fragment).

When the entire structure is rotated around the vertical axis (Fig. 8), the helical shape of two polymer molecules is clearly visible, as if wound on the surface of the cylinder; this is the well-known double helix of DNA.

With such a simplified image, the main information does not disappear - the order of grouping alternation A, T, G And C, which determines the individuality of each living organism, all information is recorded in a four-letter code.

The structure of the polymer chain and the obligatory presence of four types of heterocycles are the same for all representatives of the living world. All animals and higher plants have the number of pairs A – T always somewhat more than a couple G – C. The difference between mammalian DNA and plant DNA is that mammals have a pair A – T along the entire length of the chain occurs slightly more often (approximately 1.2 times) than the pair G – C. In the case of plants, the preference for the first pair is much more pronounced (approximately 1.6 times).

DNA is one of the largest polymer molecules known today; in some organisms, its polymer chain consists of hundreds of millions of units. The length of such a molecule reaches several centimeters, which is a very large value for molecular objects. Because Since the cross section of the molecule is only 2 nm (1 nm = 10–9 m), its proportions can be compared to a railway rail tens of kilometers long.

Chemical properties of DNA.

In water, DNA forms viscous solutions; when such solutions are heated to 60 ° C or when exposed to alkalis, the double helix breaks up into two component chains, which can again unite if we return to the original conditions. Under slightly acidic conditions, hydrolysis occurs, as a result of which the –P-O-CH 2 fragments are partially broken down to form the –P-OH and HO-CH 2 fragments, respectively, resulting in the formation of monomeric, dimeric (double) or trimeric (triple) acids, which are links from which the DNA chain was assembled (Fig. 9).

Rice. 9. FRAGMENTS OBTAINED BY DNA CLEAVATION.

Deeper hydrolysis makes it possible to separate the deoxyribose sections from the phosphoric acid, as well as the group G from deoxyribose, i.e., disassemble the DNA molecule into its constituent components in more detail. Under the action of strong acids (in addition to the decomposition of fragments –P(O)-O-CH 2 -), groups are also split off A And G. The action of other reagents (for example, hydrazine) makes it possible to separate groups T And C. A more delicate cleavage of DNA into components is carried out using a biological preparation - deoxyribonuclease, isolated from the pancreas (end - aza always indicates that the substance is a catalyst of biological origin - an enzyme). The initial part of the name is deoxyribonuclease- indicates which compound this enzyme breaks down. All of these methods of DNA cleavage are focused, first of all, on a detailed analysis of its composition.

The most important information, contained in a DNA molecule, is the order of alternation of groups A, T, G And C, it is obtained using specially developed techniques. For this purpose, a wide range of enzymes has been created that find a strictly defined sequence in the DNA molecule, for example, C-T-G-C-A-G(as well as the corresponding sequence on the opposite chain G-A-C-G-T-C) and isolate it from the chain. This property is possessed by the enzyme Pst I (trade name, it is formed from the name of that microorganism P rovidencia st uartii, from which this enzyme is obtained). When using another enzyme Pal I, it is possible to find the sequence G-G-C-C. Next, the results obtained from the action of a wide range of different enzymes according to a pre-developed scheme are compared, as a result it is possible to determine the sequence of such groups on a certain DNA section. Now such techniques have been brought to the stage of widespread use; they are used in a wide variety of areas far from scientific biochemical research, for example, in identifying the remains of living organisms or establishing the degree of relationship.

RNA structure

is in many ways reminiscent of DNA, the difference is that in the main chain phosphoric acid fragments alternate with ribose, and not with deoxyribose (Fig.). The second difference is that a uracil heterocycle ( U) instead of thymine ( T), other heterocycles A, G And C the same as for DNA. Uracil differs from thymine in the absence of a methyl group attached to the ring, in Fig. 10 this methyl group is highlighted in red.

Rice. 10. DIFFERENCE THYMINE FROM URACIL– the absence of a methyl group in the second compound, highlighted in red in thymine.

A fragment of an RNA molecule is shown in Fig. 11, order of groupings A, U, G And C, and their quantitative ratio may be different.

Fig. 11. FRAGMENT OF AN RNA MOLECULE. The main difference from DNA is the presence of OH groups in ribose (red) and a uracil fragment (blue).

The polymer chain of RNA is approximately ten times shorter than that of DNA. An additional difference is that RNA molecules are not combined into double helices consisting of two molecules, but usually exist as a single molecule, which in some areas can form double-stranded helical fragments with itself, alternating with linear sections. In helical regions, the interaction of pairs is observed as strictly as in DNA. Pairs connected by hydrogen bonds and forming a helix ( A-U And G-C), appear in those areas where the arrangement of groups turns out to be favorable for such interaction (Fig. 12).

For the vast majority of living organisms, the quantitative content of pairs A-U more than G-C, in mammals 1.5–1.6 times, in plants – 1.2 times. There are several types of RNA, which have different roles in a living organism.

Chemical properties of RNA

resemble the properties of DNA, however, the presence of additional OH groups in ribose and the lower (compared to DNA) content of stabilized helical regions makes RNA molecules chemically more vulnerable. Under the action of acids or alkalis, the main fragments of the polymer chain P(O)-O-CH2 are easily hydrolyzed, groups A, U, G And C break off more easily. If it is necessary to obtain monomeric fragments (like those in Fig. 9), while retaining chemically linked heterocycles, delicate enzymes called ribonculeases are used.

Participation of DNA and RNA in protein synthesis

– one of the main functions of nucleic acids. Proteins are the most important components of every living organism. Muscles, internal organs, bone tissue, skin and hair of mammals consist of proteins. These are polymer compounds that are assembled in a living organism from various amino acids. In such an assembly, nucleic acids play a controlling role; the process takes place in two stages, and in each of them the determining factor is the mutual orientation of the nitrogen-containing heterocycles of DNA and RNA.

The main task of DNA is to store recorded information and provide it at the moment when protein synthesis begins. In this regard, the increased chemical stability of DNA compared to RNA is understandable. Nature has taken care to keep basic information as inviolable as possible.

At the first stage, part of the double helix opens, the freed branches diverge, and in groups A, T, G And C, which turned out to be accessible, the synthesis of RNA begins, called messenger RNA, since it, as a copy from the matrix, accurately reproduces the information recorded on the revealed DNA section. Opposite the group A, belonging to the DNA molecule, there is a fragment of the future messenger RNA containing the group U, all other groups are located opposite each other in exact accordance with how this occurs during the formation of a DNA double helix (Fig. 13).

According to this scheme, a polymer molecule of messenger RNA is formed, containing several thousand monomer units.

At the second stage, the template DNA moves from the cell nucleus to the perinuclear space - the cytoplasm. The resulting messenger RNA is accompanied by so-called transfer RNAs, which carry (transport) various amino acids. Each transfer RNA, loaded with a specific amino acid, approaches a strictly determined region of the messenger RNA; the desired location is detected using the same principle of group intercorrespondence A

An important detail is that the temporary interaction between messenger and transfer RNA occurs in only three groups, for example, the triad C-C-U matrix acid, only the corresponding triple can be suitable G-G-A transfer RNA, which certainly carries with it the amino acid glycine (Fig. 14). Likewise for the triad G-A-U only a set can come close C-U-A, transporting only the amino acid leucine. Thus, the sequence of groups in the messenger RNA indicates in what order the amino acids should be combined. In addition, the system contains additional regulatory rules in encoded form; some sequences from three groups of messenger RNA indicate that protein synthesis should stop at this point, i.e. the molecule has reached the required length.

Shown in Fig. 14 protein synthesis takes place with the participation of one more - the third type of RNA acids; they are part of ribosomes and therefore they are called ribosomal. The ribosome, which is an ensemble of certain ribosomal RNA proteins, ensures the interaction of the messenger and transfer RNA, playing the role of a conveyor belt that moves the messenger RNA one step after the connection of two amino acids has occurred.

The main meaning of the two-stage scheme shown in Fig. 13 and 14, is that the polymer chain of a protein molecule is assembled from various amino acids in the intended order and strictly according to the plan that was written in encoded form on a certain section of DNA. Thus, DNA represents the starting point of this entire programmed process.

In the process of life, proteins are constantly consumed, and therefore they are regularly reproduced according to the described scheme; the entire synthesis of a protein molecule, consisting of hundreds of amino acids, takes place in a living organism within approximately one minute.

The first studies of nucleic acids were carried out in the second half of the 19th century, the understanding that all information about a living organism is encrypted in DNA came in the mid-20th century, the structure of the double helix of DNA was established in 1953 by J. Watson and F. Crick based on data X-ray diffraction analysis, which is recognized as the largest scientific achievement 20th century. In the mid-70s of the 20th century. Methods for deciphering the detailed structure of nucleic acids appeared, and after that, methods for their targeted synthesis were developed. Today, not all processes occurring in living organisms involving nucleic acids are clear, and today this is one of the most intensively developing areas of science.

Mikhail Levitsky

1. Nucleic acids, their structure and functions

2. The main stages of protein biosynthesis. Genetic code, its main properties

3. Regulation of gene expression

1. Nucleic acids, their structure and functions

Nucleic acids are linear, unbranched heteropolymers whose monomers are nucleotides linked by phosphodiester bonds.

Nucleotides - these are organic substances whose molecules consist of a pentose residue (ribose or deoxyribose), to which a phosphoric acid residue and a nitrogenous base are covalently attached. Nitrogen bases in nucleotides are divided into two groups: purine (adenine and guanine) and pyrimidine (cytosine, thymine and uracil). Deoxyribonucleotides include deoxyribose and one of the nitrogenous bases: adenine (A), guanine (G), thymine (T), cytosine (C). Ribonucleotides include ribose and one of the nitrogenous bases: adenine (A), guanine (G), uracil (U), cytosine (C).

In some cases, various derivatives of the listed nitrogenous bases are found in cells - minor bases that are part of minor nucleotides.

Free nucleotides and substances similar to them play an important role in metabolism. For example, NAD (nicotinamide adenine dinucleotide) and NADP (nicotinamide adenine dinucleotide phosphate) serve as electron and proton carriers.

Free nucleotides are capable of adding 1...2 more phosphorus groups, forming high-energy compounds. The universal source of energy in the cell is ATP – adenosine triphosphoric acid, consisting of adenine, ribose and three phosphoric (pyrophosphoric) acid residues. The hydrolysis of one terminal pyrophosphate bond releases about 30.6 kJ/mol (or 8.4 kcal/mol) of free energy, which can be used by the cell. Such a pyrophosphate bond is called macroergic (high-energy).

In addition to ATP, there are other high-energy compounds based on nucleotides: GTP (contains guanine; is involved in the biosynthesis of proteins and glucose), UTP (contains uracil; is involved in the synthesis of polysaccharides).

Nucleotides are capable of forming cyclic forms, for example, cAMP, cCMP, cGMP. Cyclic nucleotides act as regulators of various physiological processes.

Nucleic acids

There are two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Nucleic acids provide storage, reproduction and implementation of genetic (hereditary) information. This information is reflected (encoded) in the form of nucleotide sequences. In particular, the nucleotide sequence reflects the primary structure of proteins (see below). The correspondence between amino acids and the nucleotide sequences encoding them is called genetic code. The unit of the genetic code of DNA and RNA is a triplet - a sequence of three nucleotides.

Nucleic acids - it's chemical active substances. They form various compounds with proteins - nucleoproteins, or nucleoproteins.

Deoxyribonucleic acid (DNA) is a nucleic acid whose monomers are deoxyribonucleotides. DNA is the primary carrier of hereditary information. This means that all information about the structure, functioning and development of individual cells and the entire organism is recorded in the form of DNA nucleotide sequences.

Nucleic acids were discovered by Miescher in 1868. However, only in 1924 did Feulgen prove that DNA is an essential component of chromosomes. In 1944, Avery, McLeod and McCarthy established that DNA plays a crucial role in the storage, transmission and implementation of hereditary information.

There are several types of DNA: A, B, Z, T forms. Of these, the B-form is usually found in cells - a double right-handed helix, which consists of two strands (or chains) interconnected by hydrogen bonds. Each strand is represented by alternating deoxyribose and phosphoric acid residues, with a nitrogenous base covalently attached to the deoxyribose. In this case, the nitrogenous bases of the two DNA strands are directed towards each other and, due to the formation of hydrogen bonds, form complementary pairs: A=T (two hydrogen bonds) and G≡C (three hydrogen bonds). Therefore, the nucleotide sequences of these chains uniquely correspond to each other. The length of the double helix turn is 3.4 nm, the distance between adjacent pairs of nitrogenous bases is 0.34 nm, and the diameter of the double helix is ​​1.8 nm.

The length of DNA is measured by the number of nucleotide pairs (abbreviated as bp). The length of one DNA molecule ranges from several thousand bp (abbreviated as tbp) to several million bp (mpp). For example, in the simplest viruses the DNA length is approximately 5 kb, in the most complex viruses - over 100 kb, in E. coli ~ 3.8 mb, in yeast ~ 13.5 mb, in the Drosophila fly ~ 105 mb, in humans ~ 2900 MPN (DNA sizes are given for the minimum set of chromosomes - haploid). The length of DNA can also be expressed in conventional metric units of length: the total length of the DNA molecule in E. coli is ~ 1.3 mm, and the length of the DNA molecule in the first human chromosome is ~ 16 cm, and the length of DNA in the entire human genome (in 23 chromosomes) ~ 1 meter. In eukaryotic cells, DNA exists in the form of nucleoprotein complexes, which include histone proteins.

Replication (self-duplication) of DNA - This is one of the most important biological processes that ensure the reproduction of genetic information. As a result of the replication of one DNA molecule, two new molecules are formed, which are an exact copy of the original molecule - the matrix. Each new molecule consists of two chains - one of the parent and one of the sister. This mechanism of DNA replication is called semi-conservative.

Reactions in which one heteropolymer molecule serves as a template (form) for the synthesis of another heteropolymer molecule with a complementary structure are called template-type reactions. If during the reaction molecules of the same substance that serve as the matrix are formed, then the reaction is called autocatalytic. If, during a reaction, molecules of another substance are formed on the matrix of one substance, then such a reaction is called heterocatalytic. Thus, DNA replication (i.e., DNA synthesis on a DNA template) is an autocatalytic template synthesis reaction.

Template-type reactions include, first of all, DNA replication (DNA synthesis on a DNA template), DNA transcription (RNA synthesis on a DNA template) and RNA translation (protein synthesis on an RNA template). However, there are other template-type reactions, for example, RNA synthesis on an RNA template and DNA synthesis on an RNA template. The last two types of reactions are observed when cells are infected with certain viruses. DNA synthesis on an RNA template (reverse transcription) is widely used in genetic engineering.

All matrix processes consist of three stages: initiation (beginning), elongation (continuation) and termination (ending).

DNA replication - This is a complex process in which several dozen enzymes take part. The most important of them include DNA polymerases (several types), primases, topoisomerases, ligases and others. the main problem during DNA replication is that in different chains of one molecule, phosphoric acid residues are directed in different directions, but chain extension can only occur from the end that ends with an OH group. Therefore, in the replicated region, which is called the replication fork, the replication process proceeds differently on different chains. On one of the strands, called the leading strand, continuous DNA synthesis occurs on a DNA template. On the other strand, which is called the lagging strand, the binding of a primer, a specific RNA fragment, first occurs. The primer serves as a primer for the synthesis of a DNA fragment called the Okazaki fragment. Subsequently, the primer is removed, and the Okazaki fragments are stitched together into a single strand of the DNA ligase enzyme. DNA replication is accompanied by reparation—the correction of errors that inevitably occur during replication. There are many repair mechanisms.

Ribonucleic acid (RNA) is a nucleic acid whose monomers are ribonucleotides.

Within one RNA molecule there are several regions that are complementary to each other. Hydrogen bonds are formed between such complementary regions. As a result, double-stranded and single-stranded structures alternate in one RNA molecule, and the overall conformation of the molecule resembles a clover leaf on a petiole.

The nitrogenous bases that make up RNA are capable of forming hydrogen bonds with complementary bases in both DNA and RNA. In this case, nitrogenous bases form pairs A=U, A=T and G≡C. Thanks to this, information can be transferred from DNA to RNA, from RNA to DNA and from RNA to proteins.

There are three main types of RNA found in cells that perform different functions:

1. Information, or messenger RNA (mRNA, or mRNA). Makes up 5% of cellular RNA. Serves to transfer genetic information from DNA to ribosomes during protein biosynthesis. In eukaryotic cells, mRNA (mRNA) is stabilized by specific proteins. This makes it possible for protein biosynthesis to continue even if the nucleus is inactive.

2. Ribosomal, or ribosomal RNA (rRNA). Makes up 85% of cellular RNA. It is part of ribosomes, determines the shape of the large and small ribosomal subunits, and ensures contact of the ribosome with other types of RNA.

3. Transfer RNA (tRNA). Makes up 10% of cellular RNA. Transports amino acids to the corresponding site of mRNA in ribosomes. Each type of tRNA transports a specific amino acid.

There are other types of RNA in cells that perform auxiliary functions.

All types of RNA are formed as a result of template synthesis reactions. In most cases, one of the DNA strands serves as the template. Thus, RNA synthesis on a DNA template is a heterocatalytic reaction of the template type. This process is called transcription and is controlled by certain enzymes - RNA polymerases (transcriptases).

Nucleic acids– natural high-molecular biopolymers that ensure the storage and transmission of hereditary (genetic) information in living organisms.

Macromolecules of nucleic acids, with a molecular weight from 10,000 Daltons to several million, were discovered in 1869 by the Swiss chemist F. Miescher in the nuclei of leukocytes that are part of pus, hence the name (nucleus - nucleus).

Nucleic acids are polymers whose monomers are nucleotides . Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphoric acid residue. Long molecules are built from nucleotides - polynucleotides .

Nitrogenous

base

Connection between

phosphate and sugar

Rice. Nucleotide structure.

Sugar, which is part of the nucleotide, contains five carbon atoms, i.e. it represents pentose . Depending on the type of pentose present in the nucleotide, two types of nucleic acids are distinguished - ribonucleic acids (RNA), which contain ribose , and deoxyribonucleic acids (DNA) containing deoxyribose (C 5 H 10 O 4).

Reasons, both types of nucleic acids contain four different types: two of them belong to the class purines and two - to the class pyrimidines . Purines include adenine (A) and guanine (D), and to the number of pyrimidines – cytisine (C) and thymine (T) or uracil (U) (in DNA or RNA, respectively).

Nucleic acids are acids because their molecules contain phosphoric acid.

The role of nucleotides in the body is not limited to serving as the building blocks of nucleic acids; Some important coenzymes are also nucoeotides. Examples include adenosine triphosphate (ATP), nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP) and flavin adenine dinucleotide (FAD).

Nucleic acids

DNARNA

nuclear cytoplasmic mRNA tRNA rRNA

Currently, a large number of varieties of DNA and RNA are known, differing from each other in structure and significance in metabolism.

Example: E. coli bacteria contain about 1000 different nucleic acids, and animals and plants have even more.

Each type of organism contains its own set of these acids, characteristic only for it. DNA is localized primarily in the chromosomes of the cell nucleus (99% of all cell DNA), as well as in mitochondria and chloroplasts. RNA is part of the nucleoli, ribosomes of mitochondria, plastids and cytoplasm.

The DNA molecule is a universal carrier of genetic information in cells. It is thanks to the structure and functions of this molecule that traits are inherited - from parents to descendants, i.e. the universal property of living things – heredity – is realized. DNA molecules are the largest biopolymers.

Structure of DNA.

The structure of DNA molecules was deciphered in 1953 by J. Watson and F. Crick. For this discovery they received the Nobel Prize.

According to Watson–Crick DNA models, a DNA molecule consists of two polynucleotide chains twisted to the right around the same axes , forming double helix . The chains are arranged antiparallel, i.e. towards each other. Two polynucleotide chains are combined into a single DNA molecule using hydrogen bonds that arise between the nitrogenous base of nucleotides of different chains. In a polynucleotide chain, neighboring nucleotides are interconnected by covalent bonds that are formed between the deoxyribose in the DNA molecule (and ribose in RNA) of one and the phosphoric acid residue of another nucleotide.

Double helix chains complementary each other, since base pairing occurs in strict accordance: adenine combines with thymine, and guanine connects with cytosine.

As a result, in every organism Fig. Nucleotide pairing.

number adenylic nucleotides equal to the number thymidyl, and the number guanyl– number cytidyl. This pattern is called the “Chargaff rule”.

The strict correspondence of nucleotides located in paired antiparallel DNA strands is called complementarity. This property underlies the formation of new DNA molecules based on the original molecule.

Thus, the double helix is ​​stabilized by numerous hydrogen properties (two are formed between A and T, and three between G and C) and hydrophobic interactions.

Along the axis of the molecule, adjacent base pairs are located at a distance of 0.34 nm from one another. A full turn of the helix occurs per 3.4 nm, i.e., per 10 base pairs (one turn). The diameter of the spiral is 2 nm. The distance between the carbohydrate components of two paired nucleotides is 1.1 nm. The length of a nucleic acid molecule reaches hundreds of thousands of nanometers. This is significantly larger than the largest protein macromolecule, which, when unfolded, reaches a length of no more than 100-200 nm. The mass of a DNA molecule is 6*10 -12 g.

The process of doubling a DNA molecule is called replication . Replication occurs as follows. Under the action of special enzymes (helicase), hydrogen bonds between the nucleotides of two chains are broken. The spiral unwinds. According to the principle of complementarity, the corresponding DNA nucleotides are added to the released bonds in the presence of the enzyme DNA polymerase. This build-up can only occur in the direction 5"→3". This means the continuous ability to copy only one DNA strand (top in the figure). This process is called continuous replication. Copying another chain must start again each time, resulting in breaks in the chain. To eliminate them, an enzyme is needed - DNA ligase. This replication is called intermittent.

This method of DNA replication, proposed by Watson and Crick, is known as semi-conservative replication .

Consequently, the order of nucleotides in the “old” DNA chain determines the order of nucleotides in the “new” one, i.e. The “old” DNA chain is, as it were, a template for the synthesis of the “new” one. Such reactions are called matrix synthesis reactions ; they are characteristic only of living things.

Replication (reduplication) allows you to maintain the constancy of the DNA structure. The synthesized DNA molecule is absolutely identical to the original one in terms of nucleotide sequence. If, under the influence of various factors during the replication process, changes in the number and order of nucleotides occur in the DNA molecule, then mutations occur. The ability of DNA molecules to correct emerging changes and restore the original is called reparation .

Functions of DNA:

1) Storage of hereditary information.

DNA stores information as a sequence of nucleotides.

2) Reproduction and transmission of genetic information.

The ability to transmit information to daughter cells is ensured by the ability of chromosomes to divide into chromatids with subsequent reduplication of DNA molecules. It encodes genetic information about the sequence of amino acids in a protein molecule. A section of DNA that carries information about one polypeptide chain is called a gene.

3) Structural.

DNA is present in chromosomes as a structural component, i.e. is the chemical basis of chromosomal genetic material (gene).

4) DNA is the template for creating RNA molecules.

RNA is found in all living cells in the form of single-stranded molecules. It differs from DNA in that it contains pentose ribose (instead of deoxyribose), and as one of the pyrimidine bases - uracil (instead of thymine). There are three types of RNA. These are messenger RNA (mRNA, mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA). All three are synthesized directly from DNA, and the amount of RNA in each cell depends on the amount of protein produced by that cell.

In a chain of RNA, nucleotides are joined by forming covalent bonds (phosphodiester bonds) between the ribose of one nucleotide and the phosphoric acid residue of another.

Unlike DNA, RNA molecules are a single-stranded linear biopolymer consisting of nucleotides.

Double-stranded RNA serves to store and reproduce hereditary information in some viruses, i.e. They perform the functions of chromosomes - viral RNA.

Nucleotides of one RNA molecule can enter into complementary relationships with other nucleotides of the same chain, as a result of the formation of the secondary and tertiary structure of RNA molecules.

Rice. The structure of transfer RNA.

Ribisomal RNA(rRNA) makes up 85% of the total RNA of the cell, it is synthesized in the nucleolus, in combination with protein it is part of ribosomes, mitochondria (mitochondrial RNA) and plastids (plastid RNA). Contains from 3 to 5 thousand nucleotides. Protein synthesis occurs on ribosomes.

Functions: rRNA performs a structural function (part of ribosomes) and participates in the formation of the active center of ribosomes, where the formation of peptide bonds between amino acid molecules occurs in the process of protein biosynthesis.

Messenger RNA(mRNA) makes up 5% of all RNA in cells. It is synthesized during transcription in a specific section of the DNA molecule - a gene. The structure of mRNA is complementary to a section of DNA molecules that carries information about the synthesis of a specific protein. The length of mRNA depends on the length of the DNA section from which the information was read (can consist of 300-30,000 nucleotides)

Functions: mRNA carries information about protein synthesis from the nucleus to the cytoplasm to ribosomes and becomes a template for the synthesis of protein molecules.

Transfer RNA(tRNA) makes up about 10% of all RNA, is synthesized in the nucleolus, has a short chain of nucleotides and is located in the cytoplasm. It has a trefoil function. Each amino acid has its own family of tRNA molecules. They deliver amino acids contained in the cytoplasm to the ribosome.

Functions: At one end there is a triplet of nucleotides (anticodon) that codes for a specific amino acid. At the other end is a triplet of nucleotides to which an amino acid is attached. Each amino acid has its own tRNA.

Which we inherit from our ancestors. If you have children, your genetic information in their genome will be recombined and combined with your partner's genetic information. Your own genome is duplicated whenever each of your cells divides. In addition, nucleic acids contain specific segments called genes, which are responsible for the synthesis of all proteins in cells. Gene properties control the biological characteristics of your body.

General information

There are two classes of nucleic acids: (better known as DNA) and (better known as RNA).

DNA is a thread-like chain of genes that is necessary for the growth, development, functioning and reproduction of all known living organisms and most viruses.

Changes in the DNA of multicellular organisms will lead to changes in subsequent generations.

DNA is a biogenetic substrate found in all existing living things, from the simplest living organisms to highly organized mammals.

Many viral particles (virions) contain RNA as genetic material in their nucleus. However, it should be mentioned that viruses lie on the border between living and nonliving nature, since without the host’s cellular apparatus they remain inactive.

Historical reference

In 1869, Friedrich Miescher isolated the nuclei from leukocytes and discovered that they contained a phosphorus-rich substance, which he called nuclein.

Hermann Fischer discovered purine and pyrimidine bases in nucleic acids in the 1880s.

In 1884, R. Hertwig suggested that nucleins are responsible for the transmission of hereditary traits.

In 1899, Richard Altmann coined the term "core acid".

And later, in the 40s of the 20th century, scientists Kaspersson and Brachet discovered the connection between nucleic acids and protein synthesis.

Nucleotides

Polynucleotides are built from many nucleotides - monomers, connected together into chains.

In the structure of nucleic acids, nucleotides are distinguished, each of which contains:

• Nitrogenous base.
• Pentose sugar.
• Phosphate group.

Each nucleotide contains a nitrogen-containing aromatic base attached to a pentose (five-carbon) saccharide, which in turn is attached to a phosphoric acid residue. Such monomers combine with each other to form polymer chains. They are connected by covalent hydrogen bonds that occur between the phosphorus residue of one chain and the pentose sugar of the other chain. These bonds are called phosphodiester bonds. Phosphodiester bonds form the phosphate-carbohydrate framework (skeleton) of both DNA and RNA.

Deoxyribonucleotide

Let's consider the properties of nucleic acids located in the nucleus. DNA forms the chromosomal apparatus of the nucleus of our cells. DNA contains the “software instructions” for the normal functioning of the cell. When a cell reproduces its own kind, these instructions are passed on to the new cell during mitosis. DNA has the form of a double-stranded macromolecule, twisted into a double helix strand.

The nucleic acid contains a phosphate-deoxyribose saccharide skeleton and four nitrogenous bases: adenine (A), guanine (G), cytosine (C) and thymine (T). In a double-stranded helix, adenine forms a pair with thymine (A-T), guanine with cytosine (G-C).

In 1953, James D. Watson and Francis H.C. Crick proposed a three-dimensional structure of DNA based on low-resolution X-ray crystallographic data. They also cited biologist Erwin Chargaff's findings that in DNA, the amount of thymine is equivalent to the amount of adenine, and the amount of guanine is equivalent to the amount of cytosine. Watson and Crick, who earned the Nobel Prize in 1962 for their contributions to science, postulated that two strands of polynucleotides form a double helix. The threads, although identical, twist in opposite directions. The phosphate-carbon chains are located on the outside of the helix, and the bases lie on the inside, where they bind to bases on another chain through covalent bonds.

Ribonucleotides

The RNA molecule exists as a single-stranded helical strand. The structure of RNA contains a phosphate-ribose carbohydrate skeleton and nitrate bases: adenine, guanine, cytosine and uracil (U). When RNA is created during transcription on a DNA template, guanine forms a pair with cytosine (G-C) and adenine with uracil (A-U).

RNA fragments are used to replicate proteins within all living cells, allowing them to continue to grow and divide.

There are two main functions of nucleic acids. First, they help DNA by serving as messengers that transmit necessary hereditary information to the countless ribosomes in our body. The other main function of RNA is to deliver the correct amino acid needed by each ribosome to make a new protein. There are several different classes of RNA.

Messenger RNA (mRNA, or messenger mRNA) is a copy of the basic sequence of a section of DNA obtained as a result of transcription. Messenger RNA serves as an intermediary between DNA and ribosomes - cell organelles that accept amino acids from transfer RNA and use them to build a polypeptide chain.

Activates the reading of hereditary data from messenger RNA, as a result of which the process of translation of ribonucleic acid - protein synthesis - is launched. It also transports essential amino acids to sites where protein is synthesized.

Ribosomal RNA (rRNA) is the main building material of ribosomes. It binds the template ribonucleotide in a specific place where its information can be read, thereby starting the translation process.

MicroRNAs are small RNA molecules that act as regulators of many genes.

The functions of nucleic acids are extremely important for life in general and for each cell in particular. Almost all functions that a cell performs are regulated by proteins synthesized using RNA and DNA. Enzymes, protein products, catalyze all vital processes: respiration, digestion, all types of metabolism.

Differences between the structure of nucleic acids

Distinctive properties of nucleic acid bases

Adenine and guanine are purines in their properties. This means that their molecular structure includes two fused benzene rings. Cytosine and thymine, in turn, belong to pyrimidines, and have one benzene ring. RNA monomers build their chains using adenine, guanine and cytosine bases, and instead of thymine they add uracil (U). Each of the pyrimidine and purine bases has its own unique structure and properties, its own set of functional groups linked to the benzene ring.

In molecular biology, special one-letter abbreviations are used to designate nitrogenous bases: A, T, G, C, or U.

Pentose sugar

In addition to having a different set of nitrogenous bases, DNA and RNA monomers differ in the pentose sugar they contain. The pentaatomic carbohydrate in DNA is deoxyribose, while in RNA it is ribose. They are almost identical in structure, with only one difference: ribose attaches a hydroxyl group, while in deoxyribose it is replaced by a hydrogen atom.

conclusions

In evolution biological species and the continuity of life, the role of nucleic acids cannot be overestimated. As an integral part of all nuclei of living cells, they are responsible for the activation of all vital processes occurring in cells.