Basic dogma of molecular biology. The central dogma of molecular biology. Basic postulate of molecular biology

In order not only to understand the significance of the structural features of the cell, but also, most importantly, to understand the functional functions of its individual components and the entire cell as a whole, in order to combine the study of cell morphology with the most important biochemical and genetic features of its structure and work, in order to study the cell specifically with positions of modern cell biology, it is necessary to at least briefly recall the basic molecular biological principles, and once again briefly refer to the content of the central dogma of molecular biology.

The cell as such performs many different functions. As we have already said, some of them are general cellular, some are special, characteristic of special cell types. The main working mechanisms for performing these functions are proteins or their complexes with other biological macromolecules, such as nucleic acids, lipids and polysaccharides. For example, it is known that the processes of transport in the cell of various substances, from ions to macromolecules, are determined by the work of special proteins or lipoprotein complexes that are part of the plasma and other cellular membranes. Almost all processes of synthesis, decay, and rearrangement of various proteins, nucleic acids, lipids, and carbohydrates occur as a result of the activity of protein-enzymes specific for each individual reaction. The synthesis of individual biological monomers, nucleotides, amino acids, fatty acids, sugars and other compounds is also carried out by a huge number of specific enzymes - proteins. Contraction, leading to cell motility or the movement of substances and structures within cells, is also carried out by special contractile proteins. Many cell reactions in response to external factors (viruses, hormones, foreign proteins, etc.) begin with the interaction of these factors with special cellular receptor proteins.

Proteins are the main components of almost all cellular structures. The many chemical reactions within a cell are determined by many enzymes, each of which carries out one or more separate reactions. The structure of each individual protein is strictly specific, which is expressed in the specificity of their primary structure - in the sequence of amino acids along the polypeptide protein chain. Moreover, the specificity of this amino acid sequence is unmistakably repeated in all molecules of a given cellular protein.

Such correctness in reproducing an unambiguous sequence of amino acids in a protein chain is determined by the DNA structure of the gene region that is ultimately responsible for the structure and synthesis of a given protein. These ideas serve as the main postulate of molecular biology, its “dogma”. Information about the future protein molecule is transmitted to the sites of its synthesis (ribosomes) by an intermediary - messenger RNA (mRNA), the nucleotide composition of which reflects the composition and sequence of nucleotides of the gene region of DNA. A polypeptide chain is built in the ribosome, the sequence of amino acids in which is determined by the sequence of nucleotides in mRNA, the sequence of their triplets. Thus, the central dogma of molecular biology emphasizes the unidirectionality of information transfer: only from DNA to protein using an intermediate link - mRNA (DNA → mRNA → protein). For some RNA-containing viruses, the chain of information transmission can follow the pattern RNA → mRNA → protein. This does not change the essence of the matter, since the determining, determining link here is also the nucleic acid. The reverse pathways of determination from protein to nucleic acid to DNA or RNA are unknown.

In order to further move on to the study of cell structures associated with all stages of protein synthesis, we need to briefly dwell on the main processes and components that determine this phenomenon.

Currently, based on modern ideas about protein biosynthesis, the following general principle diagram of this complex and multi-stage process can be given (Fig. 16).

The main, “command” role in determining the specific structure of proteins belongs to deoxyribonucleic acid - DNA. The DNA molecule is an extremely long linear structure consisting of two intertwined polymer chains. The constituent elements - monomers - of these chains are four types of deoxyribonucleotides, the alternation or sequence of which along the chain is unique and specific for each DNA molecule and each of its sections. Different fairly long sections of the DNA molecule are responsible for the synthesis of different proteins. Thus, one DNA molecule can determine the synthesis of a large number of functionally and chemically different cell proteins. Only a certain section of the DNA molecule is responsible for the synthesis of each type of protein. Such a section of the DNA molecule associated with the synthesis of one protein in the cell is often referred to as a “cistron”. Currently, the concept of cistrons is considered to be equivalent to the concept of gene. The unique structure of a gene - the specific sequential arrangement of its nucleotides along the chain - contains all the information about the structure of one corresponding protein.

From the general diagram of protein synthesis it is clear (see Fig. 16) that the starting point from which the flow of information for the biosynthesis of proteins in the cell begins is DNA. Consequently, it is DNA that contains the primary record of information that must be preserved and reproduced from cell to cell, from generation to generation.

Briefly touching on the issue of where genetic information is stored, i.e. The following can be said about the localization of DNA in a cell. It has long been known that, unlike all other components of the protein synthesizing apparatus, DNA has a special, very limited localization: its location in the cells of higher (eukaryotic) organisms will be the cell nucleus. In lower (prokaryotic) organisms that do not have a formed cell nucleus, DNA is also mixed from the rest of the protoplasm in the form of one or several compact nucleotide formations. In full accordance with this, the nucleus of eukaryotes or the nucleoid of prokaryotes has long been considered as a receptacle for genes, as a unique cellular organelle that controls the implementation of the hereditary characteristics of organisms and their transmission over generations.

The main principle underlying the macromolecular structure of DNA is the so-called principle of complementarity (Fig. 17). As already mentioned, the DNA molecule consists of two intertwisted strands. These chains are linked to each other through the interaction of their opposing nucleotides. Moreover, for structural reasons, the existence of such a double-stranded structure is possible only if the opposite nucleotides of both chains are sterically complementary, i.e. will complement each other with their spatial structure. Such complementary - nucleotide pairs are the A-T pair (adenine-thymine) and the G-C pair (guanine-cytosine).

Consequently, according to this principle of complementarity, if in one chain of a DNA molecule we have a certain sequence of four types of nucleotides, then in the second chain the sequence of nucleotides will be uniquely determined, so that each A of the first chain will correspond to a T in the second chain, each T of the first chain - A in the second chain, for each G of the first chain - C in the second chain and for each C of the first chain - G in the second chain.

This structural principle underlying the double-stranded structure of the DNA molecule makes it easy to understand the exact reproduction of the original structure, i.e. accurate reproduction of information recorded in the chains of a molecule in the form of a specific sequence of four types of nucleotides. Indeed, the synthesis of new DNA molecules in a cell occurs only on the basis of existing DNA molecules. In this case, the two chains of the original DNA molecule begin to diverge at one end, and at each of the diverged single-stranded sections, the second chain begins to assemble from the free nucleotides present in the environment in strict accordance with the principle of complementarity. The process of divergence of the two chains of the original DNA molecule continues, and accordingly both chains are complemented by complementary chains. As a result (as can be seen in Fig. 17), instead of one, two DNA molecules appear, exactly identical to the original one. In each resulting “daughter” DNA molecule, one strand is entirely derived from the original one, and the other is newly synthesized.

It must be emphasized that the potential ability for accurate reproduction lies in the double-stranded complementary structure of DNA itself, and the discovery of this, of course, constitutes one of the main achievements of biology.

However, the problem of DNA reproduction (reduplication) is not limited to stating the potential ability of its structure to accurately reproduce its nucleotide sequence. The fact is that DNA itself is not a self-replicating molecule at all. To carry out the process of synthesis - DNA reproduction according to the scheme described above - the activity of a special enzymatic complex called DNA polymerase is necessary. It is this enzyme that carries out the sequential process of divergence of two chains from one end of the DNA molecule to the other with the simultaneous polymerization of free nucleotides on them according to the complementary principle. Thus, DNA, like a matrix, only sets the order of arrangement of nucleotides in the synthesized chains, and the process itself is carried out by the protein. The work of the enzyme during DNA reduplication is one of the most interesting problems today. Probably, the DNA polymerase actively crawls along the double-stranded DNA molecule from one end to the other, leaving behind a bifurcated reduplicated “tail.” The physical principles of this protein’s operation are not yet clear.

However, DNA and its individual functional sections, which carry information about the structure of proteins, do not themselves directly participate in the process of creating protein molecules. The first step towards the realization of this information recorded in DNA chains is the so-called process of transcription, or “rewriting”. In this process, on one strand of DNA, as on a matrix, the synthesis of a chemically related polymer - ribonucleic acid (RNA) - occurs. The RNA molecule is a single chain, the monomers of which are four types of ribonucleotides, which are considered as a slight modification of the four types of deoxyribonucleotides of DNA. The sequence of location of the four types of ribonucleotides in the resulting RNA chain exactly repeats the sequence of location of the corresponding deoxyribonucleotides of one of the two DNA chains. In this way, the nucleotide sequence of genes is copied in the form of RNA molecules, i.e. the information recorded in the structure of a given gene is completely transcribed into RNA. A large, theoretically unlimited number of such “copies” - RNA molecules - can be removed from each gene. These molecules, rewritten in many copies as “copies” of genes and, therefore, carrying the same information as genes, disperse throughout the cell. They are already in direct contact with the protein-synthesizing particles of the cell and take a “personal” part in the processes of creating protein molecules. In other words, they move information from the place where it is stored to the places where it is implemented. Accordingly, these RNAs are designated as messenger RNAs (mRNAs) or messenger RNAs (mRNAs).

It was found that the mRNA chain is synthesized directly using the corresponding DNA section as a template. In this case, the synthesized mRNA chain exactly copies one of the two DNA chains in its nucleotide sequence (assuming that uracil (U) in RNA corresponds to its derivative thymine (T) in DNA). This occurs on the basis of the same structural principle of complementarity that determines DNA reduplication (Fig. 18). It turned out that when mRNA is synthesized on DNA in a cell, only one DNA strand is used as a template for the formation of an mRNA chain. Then each G of this DNA chain will correspond to a C in the RNA chain under construction, each C of the DNA chain will correspond to a G in the RNA chain, each T of the DNA chain will correspond to an A in the RNA chain, and each A of the DNA chain will correspond to a Y in the RNA chain. As a result, the resulting RNA strand will be strictly complementary to the template DNA strand and, therefore, identical in nucleotide sequence (taking T = Y) to the second DNA strand. In this way, information is “rewritten” from DNA to RNA, i.e. transcription. The “rewritten” combinations of nucleotides in the RNA chain already directly determine the arrangement of the corresponding amino acids they encode in the protein chain.

Here, as when considering DNA reduplication, it is necessary to point out its enzymatic nature as one of the most significant aspects of the transcription process. DNA, which is the matrix in this process, completely determines the location of nucleotides in the synthesized mRNA chain, all the specificity of the resulting RNA, but the process itself is carried out by a special protein - an enzyme. This enzyme is called RNA polymerase. Its molecule has a complex organization that allows it to actively move along the DNA molecule, while simultaneously synthesizing an RNA chain complementary to one of the DNA chains. The DNA molecule, which serves as a template, is not consumed or changed, remaining in its original form and always ready for such rewriting from it of an unlimited number of “copies” - mRNA. The flow of these mRNAs from DNA to ribosomes constitutes the flow of information that ensures the programming of the protein synthesizing apparatus of the cell, the entire set of its ribosomes.

Thus, the considered part of the diagram describes the flow of information coming from DNA in the form of mRNA molecules to intracellular particles that synthesize proteins. Now we turn to a different kind of flow - to the flow of the material from which the protein must be created. The elementary units - monomers - of a protein molecule are amino acids, of which there are about 20. To create (synthesis) a protein molecule, free amino acids present in the cell must be involved in the appropriate flow entering the protein-synthesizing particle, and there they are arranged in a chain in a certain unique way , dictated by messenger RNA. This involvement of amino acids - the building blocks of protein - is carried out by attaching free amino acids to special RNA molecules of a relatively small size. These RNAs, which serve to attach free amino acids to them, while not being informational, have a different - adapter - function, the meaning of which will be seen further. Amino acids are attached to one end of small chains of transfer RNA (tRNA), one amino acid per RNA molecule. For each such amino acid, the cell has its own specific adapter RNA molecules that attach only these amino acids. In this form, attached to RNA, amino acids enter protein-synthesizing particles.

The central point of the process of protein biosynthesis is the fusion of these two intracellular flows - the flow of information and the flow of material - in the protein-synthesizing particles of the cell. These particles are called ribosomes. Ribosomes are ultramicroscopic biochemical “machines” of molecular size, where specific proteins are assembled from incoming amino acid residues, according to the plan contained in messenger RNA. Although in Fig. 19 shows only one particle; each cell contains thousands of ribs. The number of ribosomes determines the overall intensity of protein synthesis in the cell. The diameter of one ribosomal particle is about 20 nm. By its chemical nature, a ribosome is a ribonucleoprotein: it consists of a special ribosomal RNA (this is the third class of RNA known to us in addition to messenger and adapter RNAs) and molecules of structural ribosomal protein. Together, this combination of several dozen macromolecules forms an ideally organized and reliable “machine” that has the ability to read the information contained in the mRNA chain and implement it in the form of a finished protein molecule of a specific structure. Since the essence of the process is that the linear arrangement of 20 different amino acids in a protein chain is uniquely determined by the location of four different nucleotides in the chain of a chemically completely different polymer - nucleic acid (mRNA), this process occurring in the ribosome is usually referred to as “translation” or “translation” - translation, as it were, from a four-letter alphabet of nucleic acid chains to a twenty-letter alphabet of protein (polypeptide) chains. As can be seen, all three known classes of RNA are involved in the translation process: messenger RNA, which is the object of translation; ribosomal RNA, which plays the role of organizer of the protein-synthesizing ribonucleoprotein particle - the ribosome; and adapter RNAs that perform the function of a translator.

Rice. 19. Scheme of a functioning ribosome

The process of protein synthesis begins with the formation of amino acid compounds with adapter RNA molecules, or tRNA. In this case, the amino acid is first energetically “activated” due to its enzymatic reaction with the adenosine triphosphate (ATP) molecule, and then the “activated” amino acid is connected to the end of a relatively short tRNA chain, the increment in the chemical energy of the activated amino acid is stored in the form of the energy of the chemical bond between the amino acid and tRNA.

At the same time, the second problem is solved. The fact is that the reaction between an amino acid and a tRNA molecule is carried out by an enzyme designated as aminoacyl-tRNA synthetase. Each of the 20 amino acids has its own special enzymes that carry out a reaction involving only this amino acid. Thus, there are at least 20 enzymes (aminoacyl-tRNA synthetase), each of which is specific for one specific amino acid. Each of these enzymes can react not with any tRNA molecule, but only with those that carry a strictly defined combination of nucleotides in their chain. Thus, due to the existence of a set of such specific enzymes that distinguish, on the one hand, the nature of the amino acid and, on the other, the nucleotide sequence of the tRNA, each of the 20 amino acids is “assigned” only to certain tRNAs with a given characteristic nucleotide combination.

Schematically, some aspects of the protein biosynthesis process, as far as we represent them today, are given in Fig. 19. Here, first of all, it is clear that the messenger RNA molecule is connected to the ribosome or, as they say, the ribosome is “programmed” by the messenger RNA. At any given moment, only a relatively short segment of the mRNA chain is located directly in the ribosome itself. But it is precisely this segment that, with the participation of the ribosome, can interact with adapter RNA molecules. Here again the principle of complementarity plays a major role.

This is the explanation of the mechanism of why a given triplet of the mRNA chain corresponds to a strictly defined amino acid. The necessary intermediate, or adapter, when each amino acid “recognizes” its triplet on mRNA is adapter RNA (tRNA).

In Fig. Figure 19 shows that in the ribosome, in addition to the tRNA molecule with a suspended amino acid, there is another tRNA molecule. But, unlike the tRNA molecule discussed above, this tRNA molecule is attached at its end to the end of the protein (polypeptide) chain that is in the process of synthesis. This situation reflects the dynamics of events occurring in the ribosome during the synthesis of a protein molecule. This dynamic can be imagined as follows. Let's start with a certain intermediate moment, reflected in Fig. 19 and is characterized by the presence of a protein chain that has already begun to be built, tRNA attached to it and which has just entered the ribosome and associated with the triplet of a new tRNA molecule with its corresponding amino acid. Apparently, the very act of attaching a tRNA molecule to an mRNA triplet located at a given location on the ribosome leads to such mutual orientation and close contact between the amino acid residue and the protein chain under construction that a covalent bond arises between them. The connection occurs in such a way that the end of the protein chain under construction (attached to tRNA in Fig. 19) is transferred from this tRNA to the amino acid residue of the incoming aminoacyl-tRNA. As a result, the “right” tRNA, having played the role of a “donor”, will be free, and the protein chain will be transferred to the “acceptor”, i.e. to the “left” (incoming) aminoacyl-tRNA. As a result, the protein chain will be extended by one amino acid and attached to the “left” tRNA. Following this, the “left” tRNA, together with the triplet of mRNA nucleotides associated with it, is transferred to the right, then the previous “donor” tRNA molecule will be displaced from here and leave the ribosomes. In its place, a new tRNA will appear with a protein chain under construction, extended by one amino acid residue, and the mRNA chain will be advanced relative to the ribosome by one triplet to the right. As a result of the movement of the mRNA chain one triplet to the right, the next vacant triplet (UUU) will appear in the ribosome, and the corresponding tRNA with an amino acid (phenylalanyl-tRNA) will immediately join it according to the complementary principle. This will again cause the formation of a covalent (peptide) bond between the protein chain under construction and the phenylalanine residue and, following this, the movement of the mRNA chain one triplet to the right with all the ensuing consequences, etc. In this way, the messenger RNA chain is pulled sequentially, triplet by triplet, through the ribosome, as a result of which the mRNA chain is “read” by the ribosome as a whole, from beginning to end. At the same time and in conjunction with this, a sequential, amino acid by amino acid, growth of the protein chain occurs. Accordingly, tRNA molecules with amino acids enter the ribosome one after another, and tRNA molecules without amino acids exit. Finding themselves in solution outside the ribosome, free tRNA molecules again combine with amino acids and again carry them into the ribosome, thus cycling themselves without destruction or change.

Lecture no.

Number of hours: 2

Central Dogma of Molecular Biology

1) Transcription

2) Broadcast

In the early 50s, F. Crick formulated the central dogma of molecular biology. According to this concept, genetic information from DNA to proteins is transmitted via RNA according to the following scheme: DNA - RNA - protein.

The first stage of biosynthesis occurs in the nucleus and is called transcriptions (rewriting).

Transcription- biosynthesis of RNA molecules on a DNA matrix. This process is catalyzed by the enzyme RNA polymerase. The enzyme recognizes the start signtranscriptions - promoter- and joins him. The promoter is oriented in such a way that RNA polymerase passes through a given genetic region in a certain direction. The enzyme unwinds the double helix of DNA and copies, starting from the promoter, one of its chains. As RNA polymerase moves, the growing RNA strand moves away from the template and the DNA double helix behind the enzyme is restored. During the process of transcription, pro-m-RNA is synthesized - the precursor of mature m-RNA involved in translation. Pro-m-RNA is large in size and contains fragments that do not code for the synthesis of the polypeptide chain. These fragments are called introns, the coding fragments are called exons. The process of cutting out introns and splicing exons in strict order is called splicing. During the process of fusion, mature m-RNA is formed. Transport of m-RNA from the nucleus to the cytoplasm occurs through nuclear pores. Mature eukaryotic mRNAs usually encode only one polypeptide chain.

The next stage of biosynthesis occurs in the cytoplasm on ribosomes and is called translation.

Broadcast- synthesis of polypeptide chains of proteins on an m-RNA matrix according to the genetic code. During the translation process, information about the structure of the protein is translated from the nucleotide code of m-RNA into a specific sequence of amino acids in the synthesized proteins. Protein biosynthesis is carried out by a complex macromolecular complex. Amino acids are delivered to ribosomes by tRNA. During protein synthesis, m-RNA is part of a polyribosome (from several to 100 ribosomes are simultaneously synthesized on it).

Thus, transcription and translation are spatially separated. Transcription occurs in the nucleus, and translation occurs in the cytoplasm.

The cell as such has a huge number of diverse functions, as we have already said, some of them are general cellular, some are special, characteristic of special cell types. The main working mechanisms for performing these functions are proteins or their complexes with other biological macromolecules, such as nucleic acids, lipids and polysaccharides. Thus, it is known that the processes of transport in the cell of various substances, from ions to macromolecules, are determined by the work of special proteins or lipoprotein complexes in the plasma and other cellular membranes. Almost all processes of synthesis, breakdown, and rearrangement of various proteins, nucleic acids, lipids, and carbohydrates occur as a result of the activity of protein-enzymes specific for each individual reaction. Syntheses of individual biological monomers, nucleotides, amino acids, fatty acids, sugars, etc. are also carried out by a huge number of specific enzymes - proteins. Contraction, leading to cell motility or the movement of substances and structures within cells, is also carried out by special contractile proteins. Many cell reactions in response to external factors (viruses, hormones, foreign proteins, etc.) begin with the interaction of these factors with special cellular receptor proteins.

Such correctness in reproducing an unambiguous sequence of amino acids in a protein chain is determined by the DNA structure of the gene region that is ultimately responsible for the structure and synthesis of a given protein. These ideas serve as the main postulate of molecular biology, its “dogma”. Information about the future protein molecule is transmitted to the sites of its synthesis (ribosomes) by an intermediary - messenger RNA (mRNA), the nucleotide composition of which reflects the composition and sequence of nucleotides of the gene region of DNA. A polypeptide chain is built in the ribosome, the sequence of amino acids in which is determined by the sequence of nucleotides in mRNA, the sequence of their triplets. Thus, the central dogma of molecular biology emphasizes the unidirectionality of information transfer: only from DNA to protein, with the help of an intermediate, mRNA (DNA® mRNA ® protein). For some RNA-containing viruses, the information transmission chain can follow the RNA – mRNA – protein scheme. This does not change the essence of the matter, since the determining, determining link here is also the nucleic acid. The reverse pathways of determination from protein to nucleic acid to DNA or RNA are unknown.

Currently, based on modern ideas about protein biosynthesis, the following general principle diagram of this complex and multi-stage process can be given (Fig. 16).

The main, “command” role in determining the specific structure of proteins belongs to deoxyribonucleic acid – DNA. The DNA molecule is an extremely long linear structure consisting of two intertwined polymer chains. The constituent elements - monomers - of these chains are four types of deoxyribonucleotides, the alternation or sequence of which along the chain is unique and specific for each DNA molecule and each of its sections. Different fairly long sections of the DNA molecule are responsible for the synthesis of different proteins. Thus, one DNA molecule can determine the synthesis of a large number of functionally and chemically different cell proteins. Only a certain section of the DNA molecule is responsible for the synthesis of each type of protein. Such a section of the DNA molecule associated with the synthesis of one particular protein in the cell is often referred to as a “cistron”. Currently, the concept of cistrons is considered to be equivalent to the concept of gene. The unique structure of a gene—the specific sequential arrangement of its nucleotides along the chain—contains all the information about the structure of one corresponding protein.

Briefly touching on the issue of where genetic information is stored, i.e. The following can be said about the localization of DNA in a cell. It has long been known that, unlike all other components of the protein synthesizing apparatus, DNA has a special, very limited localization: its location in the cells of higher (eukaryotic) organisms will be the cell nucleus. In lower (prokaryotic) organisms that do not have a formed cell nucleus, DNA is also mixed from the rest of the protoplasm in the form of one or more compact nucleotide formations. In full accordance with this, the nucleus of eukaryotes or the nucleoid of prokaryotes has long been considered as a receptacle for genes, as a unique cellular organelle that controls the implementation of the hereditary characteristics of organisms and their transmission over generations.

The basic principle underlying the macromolecular structure of DNA is the so-called complementarity principle (Fig. 17). As already mentioned, the DNA molecule consists of two intertwisted strands. These chains are linked to each other through the interaction of their opposing nucleotides. Moreover, for structural reasons, the existence of such a double-stranded structure is possible only if the opposite nucleotides of both chains are sterically complementary, i.e. will complement each other with their spatial structure. Such complementary - nucleotide pairs are the A-T pair (adenine-thymine) and the G-C pair (guanine-cytosine).

Consequently, according to this principle of complementarity, if in one chain of a DNA molecule we have a certain sequence of four types of nucleotides, then in the second chain the sequence of nucleotides will be uniquely determined, so that each A of the first chain will correspond to a T in the second chain, each T of the first chain will correspond to an A in the second chain, to each G of the first chain - C in the second chain and to each C of the first chain - G in the second chain.

It can be seen that the indicated structural principle underlying the double-stranded structure of the DNA molecule makes it easy to understand the exact reproduction of the original structure, i.e. accurate reproduction of information recorded in the chains of a molecule in the form of a specific sequence of 4 types of nucleotides. Indeed, the synthesis of new DNA molecules in a cell occurs only on the basis of existing DNA molecules. In this case, the two chains of the original DNA molecule begin to diverge at one end, and at each of the diverged single-stranded sections, the second chain begins to assemble from the free nucleotides present in the environment in strict accordance with the principle of complementarity. The process of divergence of the two chains of the original DNA molecule continues, and accordingly both chains are complemented by complementary chains. As a result, as can be seen in the diagram, instead of one, two DNA molecules appear, exactly identical to the original one. In each resulting “daughter” DNA molecule, one strand appears to be entirely derived from the original one, while the other is newly synthesized.

The main thing that needs to be emphasized once again is that the potential ability for accurate reproduction is inherent in the double-stranded complementary structure of DNA itself, and the discovery of this, of course, constitutes one of the main achievements of biology.

However, the problem of DNA reproduction (reduplication) is not limited to stating the potential ability of its structure to accurately reproduce its nucleotide sequence. The fact is that DNA itself is not a self-replicating molecule at all. To carry out the process of DNA synthesis and reproduction according to the scheme described above, the activity of a special enzymatic complex called DNA polymerase is required. Apparently, it is this enzyme that carries out the sequential process of separation of two chains from one end of the DNA molecule to the other with the simultaneous polymerization of free nucleotides on them according to the complementary principle. Thus, DNA, like a matrix, only sets the order of arrangement of nucleotides in the synthesized chains, and the process itself is carried out by the protein. The work of the enzyme during DNA reduplication is one of the most interesting problems today. Apparently, the DNA polymerase actively crawls along the double-stranded DNA molecule from one end to the other, leaving behind a forked, reduplicated “tail.” The physical principles of this protein’s operation are not yet clear.

However, DNA and its individual functional sections, which carry information about the structure of proteins, do not themselves directly participate in the process of creating protein molecules. The first step towards the realization of this information recorded in DNA chains is the so-called process of transcription, or “rewriting”. In this process, the synthesis of a chemically related polymer, ribonucleic acid (RNA), occurs on the DNA chain, as on a matrix. The RNA molecule is a single chain, the monomers of which are four types of ribonucleotides, which are considered as a slight modification of the four types of deoxyribonucleotides of DNA. The sequence of location of the four types of ribonucleotides in the resulting RNA chain exactly repeats the sequence of location of the corresponding deoxyribonucleotides of one of the two DNA chains. In this way, the nucleotide sequence of genes is copied in the form of RNA molecules, i.e. the information recorded in the structure of a given gene is completely transcribed into RNA. A large, theoretically unlimited number of such “copies” - RNA molecules - can be removed from each gene. These molecules, rewritten in many copies as “copies” of genes and therefore carrying the same information as genes, disperse throughout the cell. They are already in direct contact with the protein-synthesizing particles of the cell and take a “personal” part in the processes of creating protein molecules. In other words, they move information from the place where it is stored to the places where it is implemented. Accordingly, these RNAs are referred to as messenger or messenger RNAs, abbreviated as mRNA (or mRNA).

It was found that the messenger RNA chain is synthesized directly using the corresponding DNA section as a template. In this case, the synthesized mRNA chain exactly copies one of the two DNA chains in its nucleotide sequence (assuming that uracil (U) in RNA corresponds to its derivative thymine (T) in DNA). This occurs on the basis of the same structural principle of complementarity that determines DNA reduplication (Fig. 18). It turned out that when mRNA is synthesized on DNA in a cell, only one DNA strand is used as a template for the formation of an mRNA chain. Then each G of this DNA chain will correspond to a C in the RNA chain under construction, each C of the DNA chain will correspond to a G in the RNA chain, each T of the DNA chain will correspond to an A in the RNA chain, and each A of the DNA chain will correspond to a Y in the RNA chain. As a result, the resulting RNA strand will be strictly complementary to the template DNA strand and, therefore, identical in nucleotide sequence (taking T = Y) to the second DNA strand. In this way, information is “rewritten” from DNA to RNA, i.e. transcription. The “rewritten” combinations of nucleotides in the RNA chain already directly determine the arrangement of the corresponding amino acids they encode in the protein chain.

Here, as when considering DNA reduplication, it is necessary to point out its enzymatic nature as one of the most significant aspects of the transcription process. DNA, which is the matrix in this process, completely determines the location of nucleotides in the synthesized mRNA chain, all the specificity of the resulting RNA, but the process itself is carried out by a special protein - an enzyme. This enzyme is called RNA polymerase. Its molecule has a complex organization that allows it to actively move along the DNA molecule, while simultaneously synthesizing an RNA chain complementary to one of the DNA chains. The DNA molecule, which serves as a template, is not consumed or changed, remaining in its original form and being always ready for such rewriting from it of an unlimited number of “copies” - mRNA. The flow of these mRNAs from DNA to ribosomes constitutes the flow of information that ensures the programming of the protein synthesizing apparatus of the cell, the entire set of its ribosomes.

Thus, the considered part of the diagram describes the flow of information coming from DNA in the form of mRNA molecules to intracellular particles that synthesize proteins. Now we turn to a different kind of flow - to the flow of the material from which the protein must be created. The elementary units - monomers - of a protein molecule are amino acids, of which there are 20 different varieties. To create (synthesize) a protein molecule, free amino acids present in the cell must be involved in the appropriate flow entering the protein-synthesizing particle, and there they are arranged in a chain in a certain unique way, dictated by messenger RNA. This involvement of amino acids - the building blocks for protein creation - is carried out through the attachment of free amino acids to special RNA molecules of a relatively small size. These RNAs, which serve to attach free amino acids to them, will not be informational, but carry a different adapter function, the meaning of which will be seen further. Amino acids are attached to one end of small chains of transfer RNA (tRNA), one amino acid per RNA molecule.

For each type of amino acid in the cell, there are specific adapter RNA molecules that attach only this type of amino acid. In this form, visiting RNA, amino acids enter protein-synthesizing particles.

The central point of the process of protein biosynthesis is the fusion of these two intracellular flows - the flow of information and the flow of material - in the protein-synthesizing particles of the cell. These particles are called ribosomes. Ribosomes are ultramicroscopic biochemical “machines” of molecular size, where specific proteins are assembled from incoming amino acid residues, according to the plan contained in messenger RNA. Although this diagram (Fig. 19) shows only one particle, each cell contains thousands of ribsomes. The number of ribosomes determines the overall intensity of protein synthesis in the cell. The diameter of one ribosomal particle is about 20 nm. By its chemical nature, a ribosome is a ribonucleoprotein: it consists of a special ribosomal RNA (this is the third class of RNA known to us in addition to messenger and adapter RNAs) and molecules of structural ribosomal protein. Together, this combination of several dozen macromolecules forms an ideally organized and reliable “machine” that has the ability to read the information contained in the mRNA chain and implement it in the form of a finished protein molecule of a specific structure. Since the essence of the process is that the linear arrangement of 20 types of amino acids in a protein chain is uniquely determined by the location of four types of nucleotides in the chain of a chemically completely different polymer - nucleic acid (mRNA), this process occurring in the ribosome is usually referred to as “translation” or “translation” - translation, as it were, from a 4-letter alphabet of nucleic acid chains to a 20-letter alphabet of protein (polypeptide) chains. As can be seen, all three known classes of RNA are involved in the translation process: messenger RNA, which is the object of translation, ribosomal RNA, which plays the role of organizer of the protein-synthesizing ribonucleoprotein particle - ribosome, and adapter RNA, which performs the function of a translator.

But at the same time, the second task is also being solved. The fact is that the reaction between an amino acid and a tRNA molecule is carried out by an enzyme designated as aminoacyl-tRNA synthetase. For each of the 20 types of amino acids, there are special enzymes that carry out a reaction involving only this amino acid. Thus, there are at least 20 enzymes (aminoacyl-tRNA synthetase), each of which is specific for one type of amino acid. Each of these enzymes can react not with any tRNA molecule, but only with those that carry a strictly defined combination of nucleotides in their chain. Thus, due to the existence of a set of such specific enzymes that distinguish, on the one hand, the nature of the amino acid and, on the other, the nucleotide sequence of the tRNA, each of the 20 types of amino acids turns out to be “assigned” only to a certain tRNA with a given characteristic nucleotide combination.

Schematically, some aspects of the protein biosynthesis process, as far as we represent them today, are given in Fig. 19.

Here, first of all, it is clear that the messenger RNA molecule is connected to the ribosome or, as they say, the ribosome is “programmed” by the messenger RNA. At any given moment, only a relatively short segment of the mRNA chain is located directly in the ribosome itself. But it is precisely this segment that, with the participation of the ribosome, can interact with adapter RNA molecules. And here again the main role is played by the principle of complementarity, already discussed twice above.

This is the explanation of the mechanism of why a given triplet of the mRNA chain corresponds to a strictly defined amino acid. It can be seen that the necessary intermediate link, or adapter, when each amino acid “recognizes” its triplet on mRNA is adapter RNA (tRNA).

Further on the diagram (see Fig. 19) it is clear that in the ribosome, in addition to the tRNA molecule just discussed with an attached amino acid, there is another tRNA molecule. But, unlike the tRNA molecule discussed above, this tRNA molecule is attached at its end to the end of a protein (polypeptide) chain that is in the process of synthesis. This situation reflects the dynamics of events occurring in the ribosome during the synthesis of a protein molecule. This dynamic can be imagined as follows. Let's start with a certain intermediate moment, reflected in the diagram and characterized by the presence of a protein chain that has already begun to be built, tRNA attached to it and which has just entered the ribosome and contacted the triplet of a new tRNA molecule with its corresponding amino acid. Apparently, the very act of attaching a tRNA molecule to an mRNA triplet located at a given location on the ribosome leads to such mutual orientation and close contact between the amino acid residue and the protein chain under construction that a covalent bond arises between them. The connection occurs in such a way that the end of the protein chain under construction, attached to the tRNA in the diagram, is transferred from this tRNA to the amino acid residue of the incoming aminoacyl-tRNA. As a result, the “right” tRNA, having played the role of a “donor”, will be free, and the protein chain will be transferred to the “acceptor” - the “left” (arrived) aminoacyl-tRNA, as a result the protein chain will be extended by one amino acid and attached to the “left” » tRNA. Following this, the “left” tRNA, together with the triplet of mRNA nucleotides associated with it, is transferred “to the right”, then the previous “donor” tRNA molecule will be displaced from here and leave the ribosomes, in its place a new tRNA will appear with a protein chain under construction, lengthened by one amino acid residue, and the mRNA chain will be advanced one triplet to the right relative to the ribosome. As a result of the movement of the mRNA chain one triplet to the right, the next vacant triplet (UUU) will appear in the ribosome, and the corresponding tRNA with an amino acid (phenylalanyl-tRNA) will immediately join it according to the complementary principle. This will again cause the formation of a covalent (peptide) bond between the protein chain under construction and the phenylalanine residue and, following this, the movement of the mRNA chain one triplet to the right with all the ensuing consequences, etc. In this way, the messenger RNA chain is pulled sequentially, triplet by triplet, through the ribosome, as a result of which the mRNA chain is “read” by the ribosome as a whole, from beginning to end. At the same time and in conjunction with this, a sequential, amino acid by amino acid, growth of the protein chain occurs. Accordingly, tRNA molecules with amino acids enter the ribosome one after another, and tRNA molecules without amino acids exit. Finding themselves in solution outside the ribosome, free tRNA molecules again combine with amino acids and again carry them into the ribosome, thus cycling themselves without destruction or change.

CellularCORE

1. General characteristics of the interphase nucleus. Kernel functions

1. General characteristics of the interphase nucleus

The nucleus is the most important component of the cell, which is found in almost all cells of multicellular organisms. Most cells have a single nucleus, but there are binucleate and multinucleate cells (for example, striated muscle fibers). Binuclearity and multinucleation are determined by the functional characteristics or pathological state of the cells. The shape and size of the nucleus are very variable and depend on the type of organism, type, age and functional state of the cell. On average, the volume of the nucleus is approximately 10% of the total volume of the cell. Most often, the core has a round or oval shape ranging from 3 to 10 microns in diameter. The minimum size of the nucleus is 1 micron (in some protozoa), the maximum is 1 mm (the eggs of some fish and amphibians). In some cases, there is a dependence of the shape of the nucleus on the shape of the cell. The nucleus usually occupies a central position, but in differentiated cells it can be shifted to the peripheral part of the cell. Almost all the DNA of a eukaryotic cell is concentrated in the nucleus.

The main functions of the kernel are:

1) Storage and transfer of genetic information;

2) Regulation of protein synthesis, metabolism and energy in the cell.

Thus, the nucleus is not only the repository of genetic material, but also the place where this material functions and reproduces. Therefore, disruption of any of these functions will lead to cell death. All this indicates the leading importance of nuclear structures in the processes of synthesis of nucleic acids and proteins.

One of the first scientists to demonstrate the role of the nucleus in the life of a cell was the German biologist Hammerling. Hammerling used large unicellular algae as an experimental object Acetobulariamediterranea and A.crenulata. These closely related species are clearly distinguished from each other by the shape of their “cap.” At the base of the stalk is the nucleus. In some experiments, the cap was separated from the lower part of the stem. As a result, it was found that a nucleus is necessary for the normal development of the cap. In other experiments, a stalk with a nucleus from one species of algae was connected to a stalk without a nucleus from another species. The resulting chimeras always developed a cap typical of the species to which the nucleus belonged.

The general structure of the interphase nucleus is the same in all cells. The core consists of nuclear envelope, chromatin, nucleoli, nuclear protein matrix and karyoplasm (nucleoplasm). These components are found in almost all non-dividing cells of eukaryotic single- and multicellular organisms.

2. Nuclear envelope, structure and functional significance

Nuclear envelope (karyolemma, karyoteca) consists of outer and inner nuclear membranes 7 nm thick. Between them is located perinuclear space width from 20 to 40 nm. The main chemical components of the nuclear envelope are lipids (13-35%) and proteins (50-75%). Small amounts of DNA (0-8%) and RNA (3-9%) are also found in the nuclear membranes. Nuclear membranes are characterized by a relatively low cholesterol content and high phospholipid content. The nuclear envelope is directly connected to the endoplasmic reticulum and the contents of the nucleus. Network-like structures are adjacent to it on both sides. The network-like structure lining the inner nuclear membrane has the appearance of a thin shell and is called nuclear lamina. The nuclear lamina supports the membrane and contacts chromosomes and nuclear RNAs. The network-like structure surrounding the outer nuclear membrane is much less compact. The outer nuclear membrane is studded with ribosomes involved in protein synthesis. The nuclear envelope contains numerous pores with a diameter of about 30-100 nm. The number of nuclear pores depends on the cell type, stage of the cell cycle and the specific hormonal situation. So the more intense the synthetic processes in the cell, the more pores there are in the nuclear membrane. Nuclear pores are rather labile structures, i.e., depending on external influences, they are capable of changing their radius and conductivity. The pore opening is filled with complexly organized globular and fibrillar structures. The collection of membrane perforations and these structures is called the nuclear pore complex. The complex complex of pores has octagonal symmetry. Along the border of the round hole in the nuclear envelope there are three rows of granules, 8 pieces in each: one row contains a means for constructing conceptual models of the nuclear side, the other is a means for constructing conceptual models of the cytoplasm side, the third is located in the central part of the pores. The size of the granules is about 25 nm. Fibrillar processes extend from the granules. Such fibrils, extending from peripheral granules, can converge in the center and create, as it were, a partition, a diaphragm, across the pore. In the center of the hole you can often see the so-called central granule.

Nuclear-cytoplasmic transport

The process of substrate translocation through a nuclear pore (in the case of import) consists of several stages. At the first stage, the transporting complex is anchored on a fibril facing the cytoplasm. The fibril then bends and moves the complex to the entrance to the nuclear pore channel. The actual translocation and release of the complex into the karyoplasm occurs. The reverse process is also known - the transfer of substances from the nucleus to the cytoplasm. This primarily concerns the transport of RNA synthesized exclusively in the nucleus. There is also another way of transporting substances from the nucleus to the cytoplasm. It is associated with the formation of outgrowths of the nuclear membrane, which can be separated from the nucleus in the form of vacuoles, and then their contents are poured out or released into the cytoplasm.

Thus, the exchange of substances between the nucleus and the cytoplasm occurs in two main ways: through pores and by lacing.

Functions of the nuclear membrane:

1. Barrier.This function is to separate the contents of the nucleus from the cytoplasm. As a result, the processes of RNA/DNA synthesis and protein synthesis become spatially separated.

2. Transport.The nuclear envelope actively regulates the transport of macromolecules between the nucleus and the cytoplasm.

3. Organizing.One of the main functions of the nuclear envelope is its participation in the creation of intranuclear order.

3. Structure and functions of chromatin and chromosomes

Hereditary material can be present in the cell nucleus in two structural and functional states:

1. Chromatin.It is a decondensed, metabolically active state designed to support transcription and reduplication processes in interphase.

2. Chromosomes.This is a maximally condensed, compact, metabolically inactive state intended for the distribution and transport of genetic material to daughter cells.

Chromatin.In the cell nucleus, zones of dense matter are identified that are well stained with basic dyes. These structures are called "chromatin" (from the Greek "chromo"– color, paint). The chromatin of interphase nuclei represents chromosomes that are in a decondensed state. The degree of chromosome decondensation may vary. Zones of complete decondensation are called euchromatin. With incomplete decondensation, areas of condensed chromatin called heterochromatin. The degree of chromatin decondensation in interphase reflects the functional load of this structure. The more “diffuse” the chromatin is distributed in the interphase nucleus, the more intense the synthetic processes in it. DecreaseRNA synthesis in cells is usually accompanied by an increase in zones of condensed chromatin.Maximum condensation of condensed chromatin is achieved during mitotic cell division. During this period, chromosomes do not perform any synthetic functions.

Chemically, chromatin consists of DNA (30-45%), histones (30-50%), non-histone proteins (4-33%) and a small amount of RNA.The DNA of eukaryotic chromosomes is linear molecules consisting of replicons of different sizes arranged in tandem (one after another). The average replicon size is about 30 microns. Replicons are sections of DNA that are synthesized as independent units. Replicons have a starting point and a terminal point for DNA synthesis. RNA represents all known cellular types of RNA that are in the process of synthesis or maturation. Histones are synthesized on polysomes in the cytoplasm, and this synthesis begins somewhat earlier than DNA reduplication. Synthesized histones migrate from the cytoplasm to the nucleus, where they bind to sections of DNA.

Structurally, chromatin is a filamentous complex of deoxyribonucleoprotein (DNP) molecules that consists of DNA associated with histones. The chromatin thread is a double helix of DNA surrounding a histone core. It consists of repeating units - nucleosomes. The number of nucleosomes is huge.

Chromosomes(from the Greek chromo and soma) are organelles of the cell nucleus that are carriers of genes and determine the hereditary properties of cells and organisms.

Chromosomes are rod-shaped structures of varying lengths with fairly constant thickness. They have a primary constriction zone that divides the chromosome into two arms.Chromosomes with equals are called metacentric, with shoulders of unequal length - submetacentric. Chromosomes with a very short, almost imperceptible second arm are called acrocentric.

In the region of the primary constriction there is a centromere, which is a disc-shaped lamellar structure. Bundles of microtubules of the mitotic spindle are attached to the centromere, running towards the centrioles. These bundles of microtubules take part in the movement of chromosomes to the poles of the cell during mitosis. Some chromosomes have a secondary constriction. The latter is usually located near the distal end of the chromosome and separates a small region, a satellite. Secondary constrictions are called nucleolar organizers. The DNA responsible for the synthesis of rRNA is localized here. The chromosome arms end in telomeres, the terminal regions. The telomeric ends of chromosomes are not able to connect with other chromosomes or their fragments. In contrast, broken ends of chromosomes can be attached to the same broken ends of other chromosomes.

The size of chromosomes varies widely among different organisms. Thus, the length of chromosomes can vary from 0.2 to 50 microns. The smallest chromosomes are found in some protozoa and fungi. The longest ones are found in some orthopteran insects, amphibians and lilies. The length of human chromosomes is in the range of 1.5-10 microns.

The number of chromosomes in different objects also varies significantly, but is typical for each species of animal or plant. In some radiolarians, the number of chromosomes reaches 1000-1600. The record holder among plants for the number of chromosomes (about 500) is the grass fern; the mulberry tree has 308 chromosomes. The smallest number of chromosomes (2 per diploid set) is observed in the malarial plasmodium, a horse roundworm. In humans, the number of chromosomes is 46,in chimpanzees, cockroaches and peppers– 48, Drosophila fruit fly – 8, house fly – 12, carp – 104, spruce and pine – 24, pigeon – 80.

Karyotype (from the Greek Karion - kernel, kernel of a nut, operators - pattern, shape) is a set of characteristics of a chromosome set (number, size, shape of chromosomes) characteristic of a particular species.

Individuals of different sexes (especially animals) of the same species may differ in the number of chromosomes (the difference is most often one chromosome). Even in closely related species, chromosome sets differ from each other either in the number of chromosomes or in the size of at least one or more chromosomes.Therefore, the structure of the karyotype can be a taxonomic feature.

In the second half of the 20th century, chromosome analysis began to be introduced methods for differential chromosome staining. It is believed that the ability of individual chromosome regions to stain is associated with their chemical differences.

4. Nucleolus. Karyoplasm. Nuclear protein matrix

The nucleolus (nucleolus) is an essential component of the cell nucleus of eukaryotic organisms. However, there are some exceptions. Thus, nucleoli are absent in highly specialized cells, in particular in some blood cells. The nucleolus is a dense, rounded body 1-5 microns in size. Unlike cytoplasmic organelles, the nucleolus does not have a membrane that surrounds its contents. The size of the nucleolus reflects the degree of its functional activity, which varies widely in different cells. The nucleolus is a derivative of the chromosome. The nucleolus consists of protein, RNA and DNA. The concentration of RNA in the nucleoli is always higher than the concentration of RNA in other components of the cell. Thus, the concentration of RNA in the nucleolus can be 2-8 times higher than in the nucleus, and 1-3 times higher than in the cytoplasm. Due to the high RNA content, the nucleoli are well stained with basic dyes. The DNA in the nucleolus forms large loops called “nucleolar organizers.” The formation and number of nucleoli in cells depends on them. The nucleolus is heterogeneous in its structure. It reveals two main components: granular and fibrillar. The diameter of the granules is about 15-20 nm, the thickness of the fibrils– 6-8 nm. The fibrillar component can be concentrated in the central part of the nucleolus, and the granular component - along the periphery. Often the granular component forms filamentous structures - nucleolonemas with a thickness of about 0.2 μm. The fibrillar component of the nucleoli is the ribonucleoprotein strands of ribosome precursors, and the granules are the maturing ribosomal subunits. The function of the nucleolus is the formation of ribosomal RNA (rRNA) and ribosomes, on which the synthesis of polypeptide chains occurs in the cytoplasm. The mechanism of ribosome formation is as follows: a rRNA precursor is formed on the DNA of the nucleolar organizer, which is coated with protein in the nucleolar zone. In the nucleolar zone, the assembly of ribosomal subunits occurs. In actively functioning nucleoli, 1500-3000 ribosomes are synthesized per minute. Ribosomes from the nucleolus enter the membranes of the endoplasmic reticulum through pores in the nuclear envelope. The number and formation of nucleoli is associated with the activity of nucleolar organizers. Changes in the number of nucleoli can occur due to the fusion of nucleoli or due to shifts in the chromosomal balance of the cell. Nuclei usually contain several nucleoli. The nuclei of some cells (newt oocytes) contain a large number of nucleoli. This phenomenon is called amplification. It consists in the organization of quality management systems, so that over-replication of the nucleolar organizer zone occurs, numerous copies depart from the chromosomes and become additionally working nucleoli. This process is necessary for the accumulation of a huge number of ribosomes per egg. This ensures the development of the embryo in the early stages even in the absence of the synthesis of new ribosomes. Supernumerous nucleoli disappear after maturation of the egg cell.

The fate of the nucleolus during cell division. As r-RNA synthesis decays in prophase, the nucleolus loosens and ready-made ribosomes are released into the karyoplasm, and then into the cytoplasm. During chromosome condensation, the fibrillar component of the nucleolus and part of the granules are closely associated with their surface, forming the basis of the matrix of mitotic chromosomes. This fibrillar-granular material is transferred by chromosomes to daughter cells. In early telophase, matrix components are released as chromosomes decondense. Its fibrillar part begins to assemble into numerous small associates - prenuclei, which can unite with each other. As RNA synthesis resumes, the prenucleoli transform into normally functioning nucleoli.

Karyoplasm(from Greek< карион > – nut, kernel of a nut), or nuclear sap, in the form of a structureless semi-liquid mass surrounds the chromatin and nucleoli. Nuclear sap contains proteins and various RNAs.

Nuclear protein matrix (nuclear skeleton) - a framework intranuclear system that serves to maintain the general structure of the interphase nucleus, combining all nuclear components. It is an insoluble material remaining in the core after biochemical extractions. It does not have a clear morphological structure and consists of 98% proteins.

Such correctness in reproducing an unambiguous sequence of amino acids in a protein chain is determined by the DNA structure of the gene region that is ultimately responsible for the structure and synthesis of a given protein. These ideas serve as the main postulate of molecular biology, its “dogma”. Information about the future protein molecule is transmitted to the sites of its synthesis (ribosomes) by an intermediary - messenger RNA (mRNA), the nucleotide composition of which reflects the composition and sequence of nucleotides of the gene region of DNA. A polypeptide chain is built in the ribosome, the sequence of amino acids in which is determined by the sequence of nucleotides in mRNA, the sequence of their triplets. Thus, the central dogma of molecular biology emphasizes the unidirectionality of information transfer: only from DNA to protein, with the help of an intermediate, mRNA (DNA ® mRNA ® protein). For some RNA-containing viruses, the information transmission chain can follow the RNA – mRNA – protein scheme. This does not change the essence of the matter, since the determining, determining link here is also the nucleic acid. The reverse pathways of determination from protein to nucleic acid to DNA or RNA are unknown.