Central dogma. Central dogma of molecular genetics
The main postulate molecular biology
There are three processes in molecular biology
Acquired form
This more common form of orotataciduria can be observed:
If there is any defect enzymes for the synthesis of urea, except for carbamoyl phosphate synthetase. At the same time, carbamoyl phosphate of mitochondria (normally used for the formation of urea) leaves them and is used for excessive synthesis of orotic acid. The disease is usually accompanied by hyperammonemia,
· In the treatment of gout with allopurinol, which can be converted into oxypurinol mononucleotide, which is an inhibitor of orotate decarboxylase, which again leads to the accumulation of orotate.
The main figure of matrix biosyntheses is RNA and DNA nucleic acids. They are polymeric molecules containing five types of nitrogenous bases, two types of pentoses, and phosphoric acid residues. Nitrogenous bases in nucleic acids ah can be purine ( adenine,guanine) and pyrimidine ( cytosine, uracil(only in RNA), thymine(DNA only)). Depending on the structure of the carbohydrate, they release ribonucleic acids- contain ribose (RNA), and deoxyribonucleic acids- contain deoxyribose (DNA).
The term " matrix biosynthesis"refers to the ability of a cell to synthesize polymer molecules such as nucleic acids and proteins, based on the template - matrices... This ensures an accurate transfer of the most complex structure from existing molecules to newly synthesized ones.
In the overwhelming majority of cases, the transfer of hereditary information from the mother cell to the daughter cell is carried out using DNA ( replication). To use genetic information, the cell itself needs RNAs formed on the DNA matrix ( transcription). Further, RNAs are directly involved in all stages of the synthesis of protein molecules ( broadcast), providing the structure and activity of the cell.
Based on the above central dogma of molecular biology, according to which the transfer of genetic information is carried out only from nucleic acid (DNA and RNA). The recipient of the information can be another nucleic acid (DNA or RNA) and a protein.
Hybridization is already widely used
If the DNA solution is heated above 90 ° C or the pH is shifted to a sharply alkaline or sharply acidic side, then hydrogen bonds between the DNA strands are destroyed and the double helix untwist. Is happening DNA denaturation or, in another way, melting... If you remove the aggressive factor, then there is renaturation or annealing... During annealing, the DNA strands “seek out” complementary regions from each other and, in the end, fold back into a double helix.
If melting and annealing of a DNA mixture is carried out in one "test tube", for example, human and mice, then some portions of the mouse DNA strands will reunite with complementary portions of the human DNA strands to form hybrids... The number of such sites depends on the degree of relatedness of the species. The closer the species are to each other, the more areas of complementarity of DNA strands. This phenomenon is called DNA-DNA hybridization.
If RNA is present in the solution, then you can carry out DNA-RNA hybridization... Such hybridization helps to establish the proximity of certain DNA sequences to any RNA.
DNA-DNA and DNA-RNA hybridization is used as effective remedy in molecular genetics, forensic medicine, anthropology to establish the genetic relationship between species.
What information is recorded in the DNA molecule, and how is this information decrypted or decoded? In the early twentieth century in 1902, Archibald Garrod suggested that some hereditary diseases are due to innate metabolic errors. In the 1930s, in the works of Beadle and Ephrussi, carried out on Drosophila, it was convincingly shown that mutations block certain stages of the biosynthesis of the final product. And finally, in 1952, direct evidence of A. Garrod's assumption was found on the example of a well-known hereditary human disease - type 1 glycogenosis. It was shown that the disease develops due to a decrease in the activity of only one enzyme - glucose-6-phosphatase. This is how the most important position was formulated: "one gene - one enzyme", which was later named central dogma of molecular genetics... Later it was shown that this position is true not only for enzymes, but also for other proteins. The modern formulation of the central dogma of molecular genetics is as follows: “ one gene - one polypeptide chain", Since many proteins consist of different polypeptide chains, each of which is encoded by its own. But this position turns out to be true not for all genes. The end products of about a quarter of human genes are not proteins, but ribonucleic acids ().
Just like DNA, it is made up of four types of randomly alternating nucleotides. True, the function of T is performed by another nucleotide - Y (uracil) - Fig. 15. The second important structural difference is that another sugar is located at the base of the RNA - not deoxyribose, but ribose. Ribose also contains 5 carbon atoms, however, unlike deoxyribose, the hydrogen atom on the second carbon atom in ribose is replaced by a hydroxyl group (-OH). RNAs function as single-stranded structures, although they are capable of forming double-stranded structures, in particular, with DNA molecules.
Let us analyze in more detail how the transition from DNA to a polypeptide chain occurs - Fig. 17.
Figure 17. Central dogma of molecular genetics
The first step towards decoding information in the DNA molecule is transcription- synthesis of RNA molecules complementary to certain regions in the DNA molecule. Transcription occurs in the nuclei of cells and is carried out by an enzyme - RNA polymerase... Those parts of the DNA molecule that are transcribed are just genes. RNA molecules that are formed as a result of transcription are called preRNA or, more precisely, the primary RNA transcript. A series of modifications transforms preRNA into informational or messenger RNA - mRNA. Huge contribution the discovery and study of the role of mRNA was made by the studies of S. Brenner and F. Jacob, carried out in 1961 on microorganisms. During the processing of preRNA, that is, the transition from preRNA to mRNA, changes occur at the ends of the molecule. it polyadenelation- attaching a polyA sequence to the 3'-end, and capping- attachment of guanosine-3-phosphate to the 5'-end of the preRNA molecule. Terminal modifications ensure the stabilization of mRNA and the possibility of its advancement to the desired organelles, primarily to the ribosomes. In prokaryotes, preRNA processing is limited only by these terminal modifications.
But in eukaryotes, including humans, one of the main semantic modifications during the transition from preRNA to mRNA is splicing... In order to determine what splicing is, you need to remember the discontinuous structure of most eukaryotic genes. Unlike prokaryotes, the coding regions of eukaryotic genes called exons, as a rule, interspersed with long non-coding sections - introns... In the process of transcription, both exons and introns are rewritten into a preRNA molecule. And then, in the course of preRNA processing, a mechanism of selective excision of introns and stitching of exons with the formation of mRNA operates. This is splicing - fig. 18. Since the total introns are, on average, much longer than exons, mRNA molecules can be ten times shorter than preRNA molecules.
Figure 18. Splicing
At the next stage, mRNA passes into the cytoplasm of the cell and is translated. Broadcast Is the synthesis of a polypeptide chain from an mRNA molecule. In fig. 19 shows the main stages of the broadcast.
Figure 19. Translation of mRNA
The broadcast takes place on ribosomes- small organelles, widely represented in cells. Ribosomes are composed of two main subunits ribosomal RNA (rRNA)... The most important participants in the translation process are molecules transport RNA (tRNA)... The tRNA molecules are shaped like a maple leaf (Fig. 20), and they are able to form a complex with one of the amino acids and transport it to the ribosome. Which amino acid will be transported by tRNA depends on the sequence of three nucleotides in a very important functional region of tRNA, which is called anticodon.
Figure 20. Transport RNA (tRNA)
In the process of translation, three nucleotides of mRNA, which are called codon or coding triplet, are part of the ribosome. This is a signal that the tRNA in which the anticodon is complementary to this codon is approaching the ribosomal complex, and it delivers its amino acid. After this, further advancement of the ribosome along the mRNA occurs, and the next codon is included in it. This is a signal of approaching the ribosomal complex of another tRNA, in which the anticodon is complementary to the next codon. And this new tRNA delivers the next amino acid to the ribosomal complex, which forms peptide bonds with the previous one. Thus, there is a cross-linking of amino acids on the ribosome with the formation of a polypeptide chain.
So, a polypeptide chain is a sequence of amino acids interconnected by peptide bonds. A mature protein differs from a polypeptide chain primarily in the presence of a tertiary spatial structure. During protein maturation, that is, during protein processing, dozens of biochemicals can occur on one polypeptide chain. chemical reactions... Protein processing is highly specific for different proteins, and its study is beyond the scope of our course.
The transition from the sequence of nucleotides in mRNA to the sequence of amino acids in the polypeptide chain is based on genetic code(Table 3) or the correspondence of a sequence of three nucleotides in mRNA to a specific amino acid in a protein.
Table 3. Genetic code
Transport RNA molecules serve as the physical prototype of the genetic code. They provide the correspondence between the nucleotides in the mRNA and the amino acids in the protein. So, the genetic code is triplet and composed of four nucleotides. The number of possible combinations of four nucleotides of three per codon is 4 3 or 64. Of these 64 variants, three are signals for the termination of the translation process. it stop codons or nonsense codons... As soon as any of these options are included in the ribosome, the broadcast stops. The rest of the triplets encode 20 amino acids, and all amino acids, with the exception of methionine, are encoded not by one, but by several variants of the triplets. Leucine, for example, is encoded by six variants of triplets. This property of the genetic code is called degeneracy... Variation between triplets encoding the same amino acid and therefore called synonym codons or synonomic triplets, as a rule, goes to the third nucleotide in the codon.
Deciphering the genetic code, which is associated with the studies of M. Nirenberg, H. G. Koran and M. Messelson, carried out in 1966, also belongs to the category of the greatest discoveries in the field of molecular genetics, allowing the transition from gene analysis to the analysis of proteins and the study of cell functioning. as a whole interconnected system. Indeed, knowledge of the nucleotide sequence of the coding DNA allows one to unambiguously predict the amino acid sequence of the encoded protein. At the same time, knowledge of the amino acid sequence of the polypeptide chain does not allow to unambiguously predict the nucleotide sequence of mRNA or the coding region of a gene due to the degeneracy of the genetic code. For example, there is leucine in a protein, and you cannot tell which of the six possible synonomic triplets encodes this amino acid in the gene. You can only write all six possible triplets.
Why is methionine encoded by one variant of triplets? Because it is encoded by the ATG codon, which, in turn, is the site of the beginning of transcription or, as they say, transcription initiation site... Therefore, the translation of all proteins begins with methionine. It is an insignificant amino acid and is then cleaved off during protein processing. Thus, it is necessary to remember that ATG is the beginning of transcription, and methionine is the beginning of translation.
Surprisingly, the genetic code turned out to be the same for all living things, from viruses to humans. Versatility the genetic code is indisputable proof of the kinship of all life on Earth. At the same time, the most plausible hypothesis for the origin of life seems to be its introduction in the form of interaction of nucleic acids and proteins from somewhere outside. True, the question remains insoluble: how was life formed where it came from on Earth? In this place, it is most appropriate to pronounce the word God and talk about the divine nature of the origin of life on Earth. But this is no longer a matter of science, but of conviction. On the other hand, even 100 years ago, all the previously described and completely material facts would seem so fantastic that their explanation could only be made from the standpoint of the divine principle. We can only hope that our grandchildren or even great-grandchildren will find out where life came from on Earth.
The versatility of the genetic code is based on the possibility of genetically engineered manipulations with DNA molecules. You can, for example, isolate a human gene, include it in the DNA of a virus, introduce this genetic construct into a bacterial cell and be sure that bacterial cell will read the information written in the human gene, just as a human cell would do. Why? Because the genetic code is universal! One of the practical applications of these biotechnologies is genetic engineering production drugs, such as interferon and many others.
The main ones discussed in this section information processes, such as replication, transcription and translation, which ensure the transfer of genetic information within or between cells, are based on matrix processes, that is, such processes when one of the strands of DNA or RNA serves as a template for subsequent synthesis. Matrix processes also include repair, that is, the correction of defects arising from DNA replication and recombination- exchange between homologous (crossing over) or non-homologous DNA regions. The molecular basis of all matrix processes is currently well understood.
This hypothesis was successfully developed in the second half of the 20th century. Now we understand how information about chemical reactions in cells is transmitted from generation to generation and implemented to ensure the vital activity of the cell. All information in the cell is stored in the DNA molecule (deoxyribonucleic acid) - the famous double helix, or "twisted ladder". Important operational information is stored on the rungs of this ladder, each of which consists of two molecules of nitrogenous bases (see Acids and bases). These bases - adenine, guanine, cytosine, and thymine - are usually denoted by the letters A, G, C, and T. Reading information from one DNA strand gives you the sequence of the bases. Think of this sequence as a message written in an alphabet with only four letters. It is this message that determines the flow of chemical reactions in the cell and, therefore, the characteristics of the organism.
The genes discovered by Gregor Mendel (see Mendel's Laws) are actually nothing more than sequences of base pairs on a DNA molecule. A genome a person - the totality of all his DNA - contains approximately 30,000-50,000 genes (see The Human Genome Project). In the most advanced organisms, including humans, genes are often separated by fragments of “meaningless,” non-coding DNA, while in simpler organisms, the gene sequence is usually continuous. In any case, the cell knows how to read the information contained in the genes. In humans and other highly developed organisms, DNA is wrapped around a molecular backbone, with which it forms chromosome... All human DNA is located on 46 chromosomes.
Exactly the same as information with hard disk stored in the office of the plant must be transmitted to all devices in the workshops of the plant, the information stored in the DNA must be translated by means of cellular technical support into chemical processes in the "body" of the cell. The main role in this chemical translation belongs to molecules ribonucleic acid, RNA. Mentally cut the double-stranded “ladder” -DNA lengthwise into two halves, separating the “steps”, and replace all the molecules of thymine (T) with similar molecules of uracil (Y) - and you get an RNA molecule. When it is necessary to translate a gene, special cellular molecules "unwind" the DNA section containing this gene. Now the RNA molecules, floating in a huge amount in the cell fluid, can attach to the free bases of the DNA molecule. In this case, just as in the DNA molecule, only certain bonds can be formed. For example, only guanine (G) of an RNA molecule can bind to cytosine (C) of a DNA molecule. After all the bases of the RNA line up along the DNA, special enzymes assemble from them a complete RNA molecule. The message recorded by the RNA bases also refers to the original DNA molecule as negative to positive. As a result of this process, the information contained in the DNA gene is rewritten into RNA.
This class of RNA molecules is called matrix, or messenger RNAs(mRNA, or mRNA). Since mRNAs are much shorter than the entire DNA in the chromosome, they can penetrate through the nuclear pores into the cytoplasm of the cell. This is how mRNAs transfer information from the nucleus ("leading center") to the "body" of the cell.
In the "body" of the cell are RNA molecules of two other classes, and they both play a key role in the final assembly of the protein molecule encoded by the gene. One of them - ribosomal RNA, or rRNA. They are part of a cellular structure called the ribosome. The ribosome can be compared to the assembly line.
Others are located in the "body" of the cell and are called transport RNA, or tRNA. These molecules are structured as follows: on the one hand there are three nitrogenous bases, and on the other, there is a site for the attachment of an amino acid (see Proteins). These three bases on the tRNA molecule can bind to the paired bases of the mRNA molecule. (There are 64 tRNA molecules - four to the third power - and each of them can attach to only one triplet of free bases on the mRNA.) Thus, the process of protein assembly is the attachment of a particular tRNA molecule carrying an amino acid to an mRNA molecule. Eventually, all the tRNA molecules will attach to the mRNA, and on the other side of the tRNA, a chain of amino acids will line up, arranged in a specific order.
The amino acid sequence is known to be the primary structure of a protein. Other enzymes complete the assembly, and the final product is a protein whose primary structure is determined by a message written on the gene of the DNA molecule. This protein then folds into its final form and can act as an enzyme (see Catalysts and Enzymes) that catalyzes one chemical reaction in the cell.
Although different messages are recorded on the DNA of different living organisms, they are all recorded using the same genetic code - in all organisms, each triplet of bases on DNA corresponds to the same amino acid in the resulting protein. This similarity of all living organisms is the most powerful proof of the theory of evolution, since it implies that humans and other living organisms descended from the same biochemical ancestor.
Lecture no.
Number of hours: 2
Central dogma of molecular biology
1) T rancription
2) Broadcast
In the early 1950s, F. Crick formulated the central dogma of molecular biology. According to this concept, genetic information from DNA to proteins is transmitted through RNA according to the 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 sign of the beginningtranscriptions - promoter- and joins him. The promoter is oriented in such a way that the RNA polymerase passes through a given genetic site in a certain direction. The enzyme unwinds the double helix of DNA and copies, starting from the promoter, one of its strands. As the RNA polymerase moves, the growing RNA strand moves away from the template and the DNA double helix behind the enzyme is repaired. In the process of transcription, pro-m-RNA is synthesized - the precursor of the mature m-RNA involved in translation. Pro-mRNA is large and contains fragments that do not encode the synthesis of the polypeptide chain. These fragments were named introns, the coding fragments are called exons. The process of cutting out introns and splicing them in strict exon order is called splicing. In the process of fusion, a mature mRNA is formed. The transport of mRNA from the nucleus to the cytoplasm is carried out through the nuclear pores. Mature eukaryotic mRNAs usually encode only one polypeptide chain.
The next stage of biosynthesis occurs in the cytoplasm on the ribosomes and is called translation.
Broadcast- synthesis of polypeptide chains of proteins on the m-RNA matrix according to the genetic code. In the process of translation, information about the structure of the protein is translated from the nucleotide code of the mRNA 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 the t-RNA ribosomes. During protein synthesis, m-RNA is part of the polyribosome (from several to 100 ribosomes are simultaneously synthesized on it).
Thus, transcription and translation are spatially separated. Transcription takes place in the nucleus, and translation takes place 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 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 composition of the plasma and other cell membranes. Almost all processes of synthesis, decay, rearrangement of various proteins, nucleic acids, lipids, carbohydrates occur as a result of the activity of proteins-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 to the movement of substances and structures within cells, is also carried out by special contractile proteins. Many reactions of cells 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. Many chemical reactions inside the cell are determined by many enzymes, each of which leads 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 this cellular protein.
Such correctness in the reproduction of the unambiguous sequence of amino acids in the protein chain is determined by the DNA structure of that gene region, which is ultimately responsible for the structure and synthesis of this protein. These ideas serve as the basic postulate of molecular biology, its "dogma". Information about the future protein molecule is transmitted to the places 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 amino acid sequence of which is determined by the sequence of nucleotides in mRNA, the sequence of their triplets. Thus, the central dogma of molecular biology emphasizes the unidirectional transmission of information: only from DNA to protein, using an intermediate link, mRNA (DNA® mRNA ® protein). For some RNA viruses, the information transmission chain can follow the RNA - mRNA - protein pattern. This does not change the essence of the matter, since the determining, defining 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 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.
At present, based on modern concepts of protein biosynthesis, the following general schematic diagram this complex and multi-stage process (Fig. 16).
The main, "command" role in determining the specific structure of proteins belongs to deoxyribonucleic acid - DNA. A 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. Various rather 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. For the synthesis of each one type of protein, only a certain part of the DNA molecule is responsible. This part of the DNA molecule associated with the synthesis of one protein in the cell is often referred to as "cistron". Currently, the concept of cistron is considered as equivalent to the concept of a gene. The unique structure of a gene - in a certain sequential arrangement of its nucleotides along the chain - contains all the information about the structure of one corresponding protein.
It can be seen from the general scheme of protein synthesis (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 upon the question of the place of storage of genetic information, i.e. about the localization of DNA in the cell, we can say the following. 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 formalized 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 repository of genes, as a unique cellular organoid that controls the implementation of hereditary characteristics of organisms and their transmission in generations.
The basic principle underlying the macromolecular structure of DNA is the so-called principle of complementarity (Fig. 17). As already mentioned, a DNA molecule consists of two intertwined chains. These chains are linked to each other through the interaction of their opposing nucleotides. In this case, for structural reasons, the existence of such a double-stranded structure is possible only if the opposite nucleotides of both strands are sterically complementary, i.e. will complement each other with their spatial structure. Such complementary - complementary - base pairs are pair AT(adenine-thymine) and a pair of H-C (guanine-cytosine).
Therefore, according to this principle of complementarity, if in one chain of the DNA molecule we have a certain sequence of four kinds 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 T in the second chain, each T of the first chain - A in the second chain, each G of the first chain is a C in the second chain, and each C of the first chain is G in the second chain.
It can be seen that 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 the 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 already existing DNA molecules. In this case, two strands of the original DNA molecule begin to diverge from one of the ends, and at each of the diverged single-stranded regions, the second strand begins to assemble from the free nucleotides present in the medium in strict accordance with the principle of complementarity. The process of divergence of two strands of the original DNA molecule continues, and, accordingly, both strands are complemented by complementary strands. As a result, as can be seen in the diagram, instead of one, two DNA molecules appear, exactly identical to the original. In each resulting "daughter" DNA molecule, one chain, as you can see, is entirely derived from the original, and the other is newly synthesized.
The main thing that needs to be emphasized once again is that the potential for accurate reproduction lies in the double-stranded complementary structure of DNA itself, and the discovery of this, of course, is one of the main achievements of biology.
However, the problem of DNA reproduction (reduplication) is not limited to the statement of the potential ability of its structure to accurately reproduce its nucleotide sequence. The fact is that DNA itself is not at all a self-reproducing molecule. To carry out the synthesis process - DNA 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 simultaneous polymerization of free nucleotides on them according to the complementary principle. Thus, DNA, like a matrix, only sets the order of the arrangement of nucleotides in the synthesized chains, and the process itself is led by the protein. The work of the enzyme in the course of DNA reduplication is today one of the most interesting problems... Apparently, DNA polymerase seems to be actively crawling along the double-stranded DNA molecule from one end to the other, leaving behind itself a bifurcated reduplicated “tail”. The physical principles of such a work of this protein are not yet clear.
However, DNA and its individual functional regions, 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 the DNA strands is the so-called process of transcription, or "rewriting". In this process, a chemically related polymer, ribonucleic acid (RNA), is synthesized on the DNA strand, as on a template. An RNA molecule is a single strand, the monomers of which are four kinds of ribonucleotides, which are considered as a small modification of four kinds of DNA deoxyribonucleotides. The sequence of the arrangement of the four types of ribonucleotides in the resulting RNA strand exactly repeats the sequence of the arrangement of the corresponding deoxyribonucleotides of one of the two DNA strands. In this way, the nucleotide sequence of genes is copied in the form of RNA molecules, i.e. information recorded in the structure of a given gene is completely rewritten on 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 directly involved in communication with protein-synthesizing particles of the cell and take a "personal" part in the processes of creating protein molecules. In other words, they transfer information from where it is stored to where it is sold. Accordingly, these RNAs are designated as messenger or messenger RNAs, abbreviated as mRNA (or mRNA).
It was found that the messenger RNA strand is synthesized directly using the corresponding DNA region as a template. The synthesized mRNA strand in this case exactly copies one of the two DNA strands in its nucleotide sequence (assuming that uracil (U) in RNA corresponds to its derivative thymine (T) in DNA). This is based on 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 C in the RNA chain under construction, to each C chain of DNA - G in the RNA chain, to each T chain of DNA - A in the RNA chain and to each A chain of DNA - 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 (assuming T = Y) of the second DNA strand. Thus, there is a "rewriting" of information from DNA to RNA, ie. transcription. The "rewritten" combinations of nucleotides of the RNA chain already directly determine the arrangement of the corresponding encoded amino acids in the protein chain.
Here, as in the consideration of DNA reduplication, it is necessary to point out its enzymatic character as one of the most essential moments of the transcription process. DNA, which is the matrix in this process, completely determines the location of nucleotides in the synthesized mRNA chain, the entire specificity of the RNA formed, 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 strand complementary to one of the DNA strands. The DNA molecule, which serves as a template, is not consumed or changed, remaining unchanged and always ready for such a rewriting of an unlimited number of "copies" - mRNA. The flow of these mRNAs from DNA to ribosomes is the flow of information that provides programming of the protein-synthesizing apparatus of the cell, the entire set of its ribosomes.
Thus, the considered part of the scheme describes the flow of information from DNA in the form of mRNA molecules to intracellular particles that synthesize proteins. We now turn to a different kind of flow — the flow of the material from which the protein is to be made. 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 corresponding flow entering the protein synthesizing particle, and already 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 accomplished through the attachment of free amino acids to special RNA molecules of a relatively small size. These RNAs, serving for the attachment of free amino acids to them, will not be informational, but have a different adaptive function, the meaning of which will be seen further. Amino acids attach 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 adapters that attach only this type of amino acid to the adapter RNA molecules. In this form, visited on RNA, amino acids enter the protein synthesizing particles.
The central point of the protein biosynthesis process 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 included in messenger RNA. Although this diagram (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, the ribosome is a ribonucleoprotein: it consists of a special ribosomal RNA (this is the third class of RNA we know in addition to informational and adapter RNA) and structural ribosomal protein molecules. Together, this combination of several dozen macromolecules forms an ideally organized and reliable "machine" capable of reading the information contained in the mRNA chain and implementing it in the form of a ready-made protein molecule with a specific structure. Since the essence of the process lies in the fact that the linear arrangement of 20 types of amino acids in the protein chain is uniquely determined by the arrangement of four types of nucleotides in the chain of a chemically completely different polymer - nucleic acid (mRNA), then this process occurring in the ribosome is usually denoted by the term "translation", or "translation" - translation, as it were, from a 4-letter alphabet of nucleic acid chains into a 20-letter alphabet of protein (polypeptide) chains. As you can see, 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 RNAs, which perform the function of a translator.
The process of protein synthesis begins with the formation of amino acid compounds with molecules of adapter RNA, or tRNA. In this case, at first, the energetic "activation" of the amino acid occurs 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 increase in the chemical energy of the activated amino acid is stored in the form of energy chemical bond between amino acid and tRNA.
But at the same time the second task is 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 with the participation of 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 tRNA, each of the 20 types of amino acids is “assigned” only to a specific tRNA with a given characteristic nucleotide combination.
Some points of the process of protein biosynthesis, as far as we present them today, are shown schematically in Fig. 19.
Here, first of all, it is seen that the messenger RNA molecule is connected to the ribosome or, as they say, the ribosome is "programmed" by the messenger RNA. In every this moment directly in the ribosome itself there is only a relatively short segment of the mRNA chain. But it is this segment, with the participation of the ribosome, that can interact with the adapter RNA molecules. And here again the main role is played by the principle of complementarity, which was already examined 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 an adapter RNA (tRNA) is a necessary intermediate, or adapter, when each amino acid recognizes its triplet on mRNA.
Further, in the diagram (see Fig. 19), it is seen that in addition to the just considered tRNA molecule with an attached amino acid, there is one more tRNA molecule in the ribosome. But, unlike the tRNA molecule discussed above, this tRNA molecule is attached by its end to the end of the protein (polypeptide) chain in the process of synthesis. This situation reflects the dynamics of events occurring in the ribosome during the synthesis of a protein molecule. This dynamics 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 build, a tRNA attached to it and just entered the ribosome and bound to the triplet of a new tRNA molecule with the corresponding amino acid. Apparently, the very act of attaching a tRNA molecule to an mRNA triplet located in a given place of the ribosome leads to such a 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 arises in such a way that the end of the protein chain under construction, attached to 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, playing the role of a "donor", will be free, and the protein chain - transferred to the "acceptor" - the "left" (received) aminoacyl-tRNA, as a result, the protein chain will be lengthened by one amino acid and attached to the "left »TRNA. This is followed by a transfer of the "left" tRNA together with the associated triplet of mRNA nucleotides "to the right", then the old "donor" tRNA molecule will be displaced from here and will leave the ribosomes, in its place a new tRNA will appear with the protein chain being built, extended 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, followed by the movement of the mRNA chain one triplet to the right with all the ensuing consequences, etc. In this way, the chain of messenger RNA 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. Simultaneously and in conjunction with this, a sequential, amino acid by amino acid, build-up of the protein chain occurs. Accordingly, tRNA molecules with amino acids enter the ribosome, one after another, and tRNA molecules without amino acids leave. Finding themselves in solution outside the ribosome, free tRNA molecules again combine with amino acids and again carry them into the ribosome, thus, they themselves, in a cyclical manner, circulate without destruction or change.
CellularCORE
1. General characteristics of the interphase nucleus. Kernel functions
2.
3.
4.
1. General characteristics of the interphase nucleus
The core is the most important component cells, which are found in almost all cells of multicellular organisms. Most cells have one nucleus, but there are binucleated and multinucleated cells (for example, striated muscle fibers). Dual-core and multi-core are due to functional features or pathological condition 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 nucleus has a round or oval shape, ranging in size 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 displaced 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 transmission of genetic information;
2) Regulation of protein synthesis, metabolism and energy in the cell.
Thus, the nucleus is not only a receptacle for genetic material, but also a place where this material functions and reproduces. Therefore, disruption of any of these functions will lead to cell death. All this points to the leading role of nuclear structures in the 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 differ well from each other in the shape of the "hat". At the base of the stem is the nucleus. In some experiments, the cap was removed 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 of one species of alga was combined with a stalk without a nucleus of another species. The resulting chimeras always developed a cap typical of the species to which the nucleus belonged.
The general plan of the 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 membrane (karyolemma, karyoteca) consists of outer and inner nuclear membranes 7 nm thick. Between them is 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 envelopes. Nuclear shells are characterized by relatively low content cholesterol and high - phospholipids. The nuclear membrane is directly connected with the endoplasmic reticulum and the contents of the nucleus. On both sides, netlike structures adjoin it. The network-like structure lining the inner nuclear membrane looks like a thin shell and is called nuclear lamina. The nuclear lamina maintains 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 dotted 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 type of cell, the 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 envelope. 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 complex globular and fibrillar structures. The collection of membrane perforations and these structures is called a nuclear pore complex. The complex pore complex 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 is a means for constructing conceptual models of the side of the nucleus, the other is a means for constructing conceptual models of the side of the cytoplasm, 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 the peripheral granules, can converge in the center and create, as it were, a septum, a diaphragm, across the pore. In the center of the hole, a so-called central granule can often be seen.
Nuclear cytoplasmic transport
The process of translocation of a substrate through the nuclear pore (for the case of import) consists of several stages. At the first stage, the transported complex is anchored on the fibril facing the cytoplasm. Then the fibril bends and moves the complex to the entrance to the nuclear pore canal. 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 transferring substances from the nucleus to the cytoplasm. It is associated with the formation of outgrowths of the nuclear envelope, which can be separated from the nucleus in the form of vacuoles, and then their contents are poured out or thrown into the cytoplasm.
Thus, the exchange of substances between the nucleus and the cytoplasm is carried out in two main ways: through the pores and by lacing.
Functions of the nuclear shell:
1. Barrier.This function is to separate the contents of the nucleus from the cytoplasm. As a result, the processes of RNA / DNA synthesis from protein synthesis are spatially separated.
2. Transport.The nuclear envelope actively regulates the transport of macromolecules between the nucleus and the cytoplasm.
3. Organizer.One of the main functions of the nuclear shell is its participation in the creation of an intranuclear order.
3. Structure and function of chromatin and chromosomes
Hereditary material can be in the cell nucleus in two structural and functional states:
1. Chromatin.It is a decondensed, metabolically active state designed to support the processes of transcription and reduplication in the interphase.
2. Chromosomes.This is the most condensed, compact, metabolically inactive state, designed for the distribution and transport of genetic material to daughter cells.
Chromatin.In the nucleus of cells, zones of dense matter are identified, which are well stained with basic dyes. These structures are called "chromatin" (from the Greek. "Chromo"– color, paint). Chromatin of interphase nuclei is a decondensed chromosome. The degree of chromosome decondensation can vary. The complete decondensation zones are called euchromatin. With incomplete decondensation in the interphase nucleus, areas of condensed chromatin, called heterochromatin. The degree of chromatin decondensation in the interphase reflects the functional load of this structure. The more "diffuse" 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 condensed chromatin zones.Maximum condensation of condensed chromatin is achieved during mitotic cell division. During this period, the 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 a linear molecule consisting of tandemly (one after another) replicons of different sizes. The average size of a replicon is about 30 µm. Replicons are sections of DNA that are synthesized as independent units. Replicons have a starting point and a terminal point for DNA synthesis. RNA is 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. The synthesized histones migrate from the cytoplasm to the nucleus, where they bind to DNA regions.
Structurally, chromatin is a filamentous complex molecule of deoxyribonucleoprotein (DNP), which consists of DNA associated with histones. The chromatin strand is a double helix of DNA that surrounds the histone rod. It consists of repeating units called nucleosomes. The number of nucleosomes is enormous.
Chromosomes(from the Greek. chromo and soma) are organelles of the cell nucleus, which 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 invisible second shoulder are called acrocentric.
In the area of the primary constriction, there is a centromere, which is a lamellar structure in the form of a disk. The bundles of microtubules of the mitotic spindle are attached to the centromere, going in the direction of the centrioles. These bundles of microtubules are involved 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 small plot, satellite. Secondary constrictions are called nucleolar organizers. This is where the DNA responsible for the synthesis of r-RNA is located. The shoulders of the chromosomes end in telomeres, terminal sites. The telomeric ends of chromosomes are unable to connect with other chromosomes or their fragments. In contrast, the torn ends of chromosomes can attach to the same torn ends of other chromosomes.
The sizes of chromosomes in different organisms vary widely. So, the length of chromosomes can range from 0.2 to 50 microns. The smallest chromosomes are found in some protozoa, fungi. The longest are found in some Orthoptera insects, in amphibians and in Liliaceae. The length of human chromosomes is in the range of 1.5-10 microns.
The number of chromosomes in different objects also varies considerably, but it is characteristic for each type 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 snake fern, 308 chromosomes in the mulberry tree. The smallest number of chromosomes (2 per diploid set) is observed in the malaria plasmodium, horse roundworm. In humans, the number of chromosomes is 46,chimpanzee, cockroach and pepper– 48, fruit fly fruit fly - 8, house fly - 12, carp - 104, spruce and pine - 24, pigeon - 80.
Karyotype (from the Greek. Karion - kernel, kernel of the nut, operators - sample, shape) - a set of characteristics of the chromosome set (number, size, shape of chromosomes), characteristic of a particular species.
Individuals of different sexes (especially in animals) of the same species can differ in the number of chromosomes (the difference is usually 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 several chromosomes.Consequently, the structure of the karyotype can be a taxonomic trait.
In the second half of the 20th century, the practice of chromosome analysis began to be introduced methods of differential staining of chromosomes. It is believed that the ability of individual sections of chromosomes to stain is associated with their chemical differences.
4. The nucleolus. Karyoplasm. Nuclear protein matrix
The nucleolus (nucleola) is an essential component of the cell nucleus of eukaryotic organisms. However, there are some exceptions. So the 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 would surround its contents. The size of the nucleolus reflects the degree of its functional activity, which varies widely in different cells. The nucleolus is a chromosome derivative. The nucleolus contains 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. So 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 content of RNA, the nucleoli are well stained with basic dyes. DNA in the nucleolus forms large loops, which are 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 - nucleolonemes with a thickness of about 0.2 microns. The fibrillar component of the nucleoli is the ribonucleoprotein strands of ribosome precursors, and the granules are the maturing ribosome subunits. The function of the nucleolus is the formation of ribosomal RNA (rRNA) and ribosomes, on which the synthesis of polypeptide chains in the cytoplasm takes place. The mechanism of ribosome formation is as follows: an rRNA precursor is formed on the DNA of the nucleolar organizer, which is dressed in protein in the nucleolus zone. In the nucleolus zone, ribosome subunits are assembled. In actively functioning nucleoli, 1500-3000 ribosomes are synthesized per minute. Ribosomes from the nucleolus through pores in the nuclear envelope enter the membranes of the endoplasmic reticulum. 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 during shifts in the chromosomal balance of the cell. Usually, the nuclei 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, that over-replication of the nucleolar organizer zone occurs, numerous copies leave the chromosomes and become additionally working nucleoli. This process is necessary for the accumulation of a huge amount of ribosomes per egg. This ensures the development of the embryo in the early stages, even in the absence of synthesis of new ribosomes. The supernumerary nucleoli disappear after the maturation of the egg cell.
The fate of the nucleolus during cell division. As the synthesis of r-RNA decays in prophase, the nucleolus loosens and the ready ribosomes exit into the karyoplasm, and then into the cytoplasm. During the condensation of chromosomes, 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 carried by chromosomes into daughter cells. In the early telophase, as chromosomes decondensate, matrix components are released. Its fibrillar part begins to assemble into small numerous associates - prenucleoli, which can unite with each other. As RNA synthesis resumes, the prenucleoli are transformed into normally functioning nucleoli.
Karyoplasm(from the Greek.< карион > – nut, kernel of a nut), or nuclear juice, surrounds chromatin and nucleoli in the form of a structureless semi-liquid mass. Nuclear juice contains proteins and various RNAs.
Nuclear protein matrix (nuclear skeleton) - a framework intranuclear system, which serves to maintain the general structure of the interphase nucleus of the unification of all nuclear components. It is an insoluble material that remains in the core after biochemical extractions. It has no clear morphological structure and is 98% protein.
The whole process of protein biosynthesis can be represented in the form of a very simple scheme that must be well remembered (Fig. 1). The concept that genetic information is stored in the cell in the form of a DNA molecule and is realized through transcription into RNA and subsequent translation into protein is known as the Central Dogma of Molecular Biology.
DNA ---- ®RNA ----- ® protein.
transcription broadcast
As you can see, the functioning (expression) of genes from DNA to protein is realized due to two global molecular genetic mechanisms: transcription and translation.
So, the gene information in all cells is encoded as a sequence of nucleotides in DNA. The first stage in the realization of this information is the formation of RNA similar to DNA, which is called transcription.
I stage of protein biosynthesis - transcription.
Transcription begins with the detection of a special region of the gene in the DNA molecule, which indicates the place of the beginning of transcription - a promoter (Fig. 2) with the help of a special enzyme RNA polymerase. After attachment to the promoter, the RNA polymerase unwinds the adjacent turn of the DNA helix. Two strands diverge and on one of them the enzyme synthesizes mRNA. The assembly of ribonucleotides into a chain takes place in compliance with the rule of nucleotide complementarity. Due to the fact that RNA polymerase is capable of assembling a polynucleotide in only one direction, namely from the 5 'to the 3' end, only the DNA strand that faces the enzyme with its 3 'end can serve as a template. Such a chain is called matrix or antisense (Fig. 2). Another, antiparallel DNA strand is called codogenic or semantic, because the nucleotide sequence of this strand fully corresponds to the RNA sequence and is read in the same direction, i.e. 5 'to 3' end. Therefore, the genetic code is sometimes written by the RNA molecule, sometimes by the codogenic DNA.
Moving along the DNA strand, RNA polymerase performs sequential accurate rewriting of information until it encounters a STOP transcription terminator codon on its way. A person has three stop codons - TAG, TGA, TAA (or UAG, UGA, UAA).
Stage II of protein biosynthesis - translation.
The broadcast includes 3 phases: initiation, elongation and termination.
1 - Initiation - the phase of the beginning of polypeptide synthesis.
1) There is a union of ribosome subparticles (large and small) located separately in the cytoplasm. A ribosome is formed, in which the peptidyl and aminoacyl centers are distinguished.
2) The first aminoacyl t-RNA is attached to the ribosome.
Let's consider how these processes take place in the cell.
1) In the molecule of any mRNA, near the 5'-end there is a region complementary to the sequence of rRNA nucleotides of the small subunit of the ribosome. Next to this site is the AUG start codon encoding the amino acid methionine. A small subunit of the ribosome binds to mRNA. Then the small subparticle unites with the large subunit, the ribosome is formed. In the ribosome, two important sites are formed - the peptidyl center - the P-site and the aminoacyl center - the A-site. By the end of the initiation phase, the P-site is occupied by the aminoacyl t-RNA bound to the start amino acid, methionine, and the A-site is ready to accept the next start codon.
2) The tRNA molecules are transported to the ribosomes (see table, Fig. 6). The tRNA molecule consists of 75-95 nucleotides and is shaped like a maple leaf (Fig. 7). They have two active centers in their composition:
1) the acceptor end, to which the transported amino acid is attached by a covalent bond with the expenditure of energy 1 ATP. Aminoacyl t-RNA is formed.
2) an anticodon loop complementary to the mRNA codon.
2nd phase elongation - elongation of the polypeptide (Fig. 6, table).
Inside the large subunit of the ribosome, there are simultaneously about 30 mRNA nucleotides and only 2 informative triplet codons: one in the aminoacyl A-site, the other in the peptidyl P-site. The tRNA molecule with an amino acid first approaches the A-center of the ribosome. In the event that the t-RNA anticodon is complementary to the mRNA codon, there is a temporary attachment of the aminoacyl-tRNA to the mRNA codon. After that, the ribosome moves 1 codon along the mRNA, and the tRNA with the amino acid moves to the P-site. A new aminoacyl-tRNA with an amino acid arrives at the vacated A-site and stops there again if the tRNA anticodon is complementary to the mRNA codon. A peptide bond is formed between an amino acid and a polypeptide, and at the same time the bond between the amino acid and its tRNA, as well as between tRNA and mRNA, is destroyed. The tRNA freed from the amino acid is released from the ribosome into the cytoplasm. She's ready to pair with the next amino acid. The ribosome moves again by 1 triplet.