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How does transcription occur in a cell? What is transcription in biology and how does it occur. Transcription process diagram

Initiation of transcription

Transcription elongation

The moment at which RNA polymerase transitions from transcription initiation to elongation is not precisely determined. Three major biochemical events characterize this transition in the case of Escherichia coli RNA polymerase: the release of the sigma factor, the first translocation of the enzyme molecule along the template, and the strong stabilization of the transcription complex, which, in addition to the RNA polymerase, includes the growing RNA chain and the transcribed DNA. The same phenomena are also characteristic of eukaryotic RNA polymerases. The transition from initiation to elongation is accompanied by the rupture of bonds between the enzyme, promoter, transcription initiation factors, and in some cases, by the transition of RNA polymerase to a state of elongation competence (for example, phosphorylation of the CTD domain in RNA polymerase II). The elongation phase ends after the growing transcript is released and the enzyme dissociates from the template (termination).

Elongation is carried out with the help of basic elongation factors, which are necessary so that the process does not stop prematurely.

Recently, evidence has emerged showing that regulatory factors may also regulate elongation. During the elongation process, RNA polymerase pauses at certain parts of the gene. This is especially clearly seen at low concentrations of substrates. In some areas of the matrix there are long delays in the advancement of RNA polymerase, the so-called. pauses are observed even at optimal substrate concentrations. The duration of these pauses can be controlled by elongation factors.

Termination

Bacteria have two transcription termination mechanisms:

  • a rho-dependent mechanism in which the Rho (rho) protein destabilizes the hydrogen bonds between the DNA template and the mRNA, releasing the RNA molecule.
  • rho-independent, in which transcription stops when the newly synthesized RNA molecule forms a stem-loop, followed by several uracils (...UUUU), which leads to the detachment of the RNA molecule from the DNA template.

Transcription termination in eukaryotes is less studied. It ends with cutting the RNA, after which the enzyme adds several adenines (...AAAA) to its 3" end, the number of which determines the stability of a given transcript.

Transcription factories

There is a number of experimental data indicating that transcription occurs in the so-called transcription factories: huge, according to some estimates, up to 10 Da complexes that contain about 8 RNA polymerases II and components for subsequent processing and splicing, as well as correction of the newly synthesized transcript. In the cell nucleus, there is a constant exchange between pools of soluble and activated RNA polymerase. Active RNA polymerase is involved in such a complex, which in turn is a structural unit that organizes chromatin compaction. Recent data indicate that transcription factories exist even in the absence of transcription, they are fixed in the cell (it is not yet clear whether they interact with the nuclear matrix of the cell or not) and represent an independent nuclear subcompartment. The transcription factory complex containing RNA polymerase I, II or III was analyzed by mass spectrometry.

Reverse transcription

Reverse transcription scheme

Some viruses (such as HIV, which causes AIDS), have the ability to transcribe RNA into DNA. HIV has an RNA genome that is integrated into DNA. As a result, the DNA of the virus can be combined with the genome of the host cell. The main enzyme responsible for synthesizing DNA from RNA is called reversease. One of the functions of reversetase is to create complementary DNA (cDNA) from the viral genome. The associated enzyme ribonuclease H cleaves RNA, and reversease synthesizes cDNA from the DNA double helix. The cDNA is integrated into the host cell genome by integrase. The result is the synthesis of viral proteins by the host cell, which form new viruses. In the case of HIV, apoptosis (cell death) of T-lymphocytes is also programmed. In other cases, the cell may remain a spreader of viruses.

Some eukaryotic cells contain the enzyme telomerase, which also exhibits reverse transcription activity. With its help, repeating sequences in DNA are synthesized. Telomerase is often activated in cancer cells to indefinitely duplicate the genome without losing the protein-coding DNA sequence.

Notes


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See what “Transcription (biology)” is in other dictionaries:

    - (from Latin transcriptio, lit. rewriting), biosynthesis of RNA molecules, resp. DNA sections; the first stage of genetic implementation. information in living cells. It is carried out by the enzyme DNA-dependent RNA polymerase, to the paradise of most studied... ... Biological encyclopedic dictionary

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    The science of life that includes all knowledge about the nature, structure, function and behavior of living things. Biology deals not only with the great variety of forms of different organisms, but also with their evolution, development and with those relationships that... ... Collier's Encyclopedia

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    I Transcription (from Latin transcriptio rewriting) written reproduction of words and texts, taking into account their pronunciation using a certain graphic system. T. can be scientific and practical. Scientific T. is used in linguistic...

    - (from Latin transcriptio, letters rewriting), RNA biosynthesis on a DNA matrix; the first stage of genetic implementation. information, in the course of cutting the nucleotide sequence of DNA is read in the form of a nucleotide sequence of RNA (see Genetic code) ... Chemical encyclopedia

    Pre mRNA with stem loop. Nitrogen atoms in the bases are highlighted in blue, oxygen atoms in the phosphate backbone of the molecule in red. Ribonucleic acids (RNA) are nucleic acids, polymers of nucleotides that contain an orthophosphoric acid residue ... Wikipedia

    A science that aims to understand the nature of life phenomena by studying biological objects and systems at a level approaching the molecular level, and in some cases reaching this limit. The ultimate goal is... ... Great Soviet Encyclopedia

    Reverse transcription is the process of producing double-stranded DNA from a single-stranded RNA template. This process is called reverse transcription, since the transfer of genetic information occurs in the “reverse”, relatively ... ... Wikipedia

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DNA, the carrier of all genetic information in a cell, does not directly participate in the synthesis of proteins. In animal and plant cells, DNA molecules are contained in the chromosomes of the nucleus and are separated by the nuclear membrane from the cytoplasm, where protein synthesis occurs. An information-carrying messenger is sent from the nucleus to the ribosomes, the site of protein assembly, and is able to pass through the pores of the nuclear membrane. Such an intermediary is messenger RNA (i-RNA). According to the principle of complementarity, it is read from DNA with the participation of an enzyme called RNA polymerase. The process of reading (or rather, copying), or synthesizing RNA, carried out by RNA polymerase, is called transcription (Latin transcriptio - rewriting). Messenger RNA is a single-stranded molecule, and transcription occurs from one strand of a double-stranded DNA molecule. If the transcribed DNA strand contains nucleotide G, then RNA polymerase includes C in the RNA; if it is T, it includes A; if it is A, it includes y (RNA does not include T) (Fig. 46). The length of each mRNA molecule is hundreds of times shorter than DNA. Messenger RNA is not a copy of the entire DNA molecule, but only part of it - one gene or a group of adjacent genes that carry information about the structure of proteins necessary to perform one function. In prokaryotes, such a group of genes is called an operon. You will read about how genes are combined into an operon and how transcription is controlled in the section on protein biosynthesis. At the beginning of each operon there is a kind of landing pad for RNA polymerase, called a promoter. This is a specific sequence of DNA nucleotides that the enzyme recognizes due to chemical affinity. Only by attaching to the promoter is RNA polymerase able to begin the synthesis of mRNA. Having reached the end of the operon, the enzyme encounters a signal (in the form of a specific nucleotide sequence) indicating the end of reading. The finished mRNA leaves the DNA and goes to the site of protein synthesis. In the described transcription process, four stages can be distinguished:

1) Binding of RNA polymerase to the promoter;

2) Initiation - the beginning of synthesis. It consists in the formation of the first phosphodiester bond between ATP or GTP and the second nucleotide of the synthesized mRNA molecule;

3) elongation - the growth of an RNA chain, i.e. the sequential addition of nucleotides to each other in the order in which complementary nucleotides appear in the transcribed DNA strand. The elongation rate reaches 50 nucleotides per second;

4) termination - completion of mRNA synthesis.

Transcription in biology is a multi-stage process of reading information from DNA, which is a component of Nucleic acid is the carrier of genetic information in the body, so it is important to correctly decipher it and transfer it to other cellular structures for further assembly of peptides.

Definition of "transcription in biology"

Protein synthesis is the main vital process in any cell of the body. Without the creation of peptide molecules, it is impossible to maintain normal life functions, since these organic compounds are involved in all metabolic processes, are structural components of many tissues and organs, and play signaling, regulatory and protective roles in the body.

The process that begins protein biosynthesis is transcription. Biology briefly divides it into three stages:

  1. Initiation.
  2. Elongation (growth of RNA chain).
  3. Termination.

Transcription in biology is a whole cascade of step-by-step reactions, as a result of which RNA molecules are synthesized on a DNA matrix. Moreover, in this way not only informational ribonucleic acids are formed, but also transport, ribosomal, small nuclear and others.

Like any biochemical process, transcription depends on many factors. First of all, these are enzymes that differ between prokaryotes and eukaryotes. These specialized proteins help initiate and carry out transcription reactions accurately, which is important for high-quality protein output.

Transcription of prokaryotes

Since transcription in biology is the synthesis of RNA on a DNA template, the main enzyme in this process is DNA-dependent RNA polymerase. In bacteria there is only one type of such polymerases for all molecules

RNA polymerase, according to the principle of complementarity, completes the RNA chain using the DNA template strand. This enzyme contains two β-subunits, one α-subunit and one σ-subunit. The first two components perform the function of forming the enzyme body, and the remaining two are responsible for retaining the enzyme on the DNA molecule and recognizing the promoter part of deoxyribonucleic acid, respectively.

By the way, the sigma factor is one of the signs by which a particular gene is recognized. For example, the Latin letter σ with the subscript N means that this RNA polymerase recognizes genes that are turned on when there is a lack of nitrogen in the environment.

Transcription in eukaryotes

Unlike bacteria, transcription in animals and plants is somewhat more complex. Firstly, each cell contains not one, but three types of different RNA polymerases. Among them:

  1. RNA polymerase I. It is responsible for the transcription of ribosomal RNA genes (with the exception of 5S RNA ribosomal subunits).
  2. RNA polymerase II. Its task is to synthesize normal information (template) ribonucleic acids, which subsequently participate in translation.
  3. RNA polymerase III. The function of this type of polymerase is to synthesize 5S-ribosomal RNA.

Secondly, for promoter recognition in eukaryotic cells it is not enough to have only a polymerase. Special peptides called TF proteins also participate in the initiation of transcription. Only with their help can RNA polymerase land on DNA and begin the synthesis of a ribonucleic acid molecule.

Transcription meaning

The RNA molecule, which is formed on the DNA template, subsequently attaches to ribosomes, where information is read from it and protein is synthesized. The process of peptide formation is very important for the cell, because Without these organic compounds, normal life activity is impossible: they are primarily the basis for the most important enzymes of all biochemical reactions.

Transcription in biology is also a source of rRNA, which as well as tRNA, which are involved in the transfer of amino acids during translation to these non-membrane structures. SnRNAs (small nuclear ones) can also be synthesized, the function of which is to splice all RNA molecules.

Conclusion

Translation and transcription in biology play an extremely important role in the synthesis of protein molecules. These processes are the main component of the central dogma of molecular biology, which states that RNA is synthesized on the DNA matrix, and RNA, in turn, is the basis for the beginning of the formation of protein molecules.

Without transcription, it would be impossible to read the information that is encoded in deoxyribonucleic acid triplets. This once again proves the importance of the process at the biological level. Any cell, be it prokaryotic or eukaryotic, must constantly synthesize new and new protein molecules that are currently needed to maintain life. Therefore, transcription in biology is the main stage in the work of each individual cell of the body.

TRANSCRIPTION in biology(syn. template RNA synthesis) - synthesis of ribonucleic acid on a deoxyribonucleic acid matrix. T., which occurs in living cells, represents the initial stage of the implementation of genetic characteristics contained in DNA (see Deoxyribonucleic acids). As a result of T., RNA is formed (see Ribonucleic acids) - an exact copy of one of the DNA strands according to the sequence of nitrogenous bases in the polynucleotide chain. T. is catalyzed by DNA-dependent RNA polymerases (see Polymerases) and ensures the synthesis of three types of RNA: messenger RNA (mRNA), which encodes the primary structure of the protein, that is, the sequence of amino acid residues in the olipeptide chain under construction (see Proteins, biosynthesis); ribosomal RNA (rRNA), which is part of ribosomes (see), and transport RNA (tRNA), involved in the process of protein synthesis as a component that “recodes” the information contained in mRNA.

T. in microorganisms has been studied more fully than in higher organisms (see Bacteria, genetics). The process of T., catalyzed by RNA polymerase, is divided into 4 stages: binding of RNA polymerase to DNA, the beginning - initiation - of the synthesis of the RNA chain, the actual process of synthesis of the polynucleotide chain - elongation and the completion of this synthesis - termination.

RNA polymerase has the greatest affinity for certain regions of the DNA template containing a specific nucleotide sequence (the so-called promoter regions). Binding of the enzyme to such a site is accompanied by partial local melting of the DNA strands and their divergence. At the initiation stage, the first nucleotide - usually adenosine (A) or guanosine (G) - is inserted into the RNA molecule. During elongation, RNA polymerase locally unwinds the DNA double helix and copies one of its strands in accordance with the principle of complementarity (see Replication). As RNA polymerase moves along the DNA, the growing RNA chain moves away from the template, and the double-stranded structure of DNA is restored after passage of the enzyme. Termination of RNA synthesis also occurs at specific DNA sites. In some cases, additional proteins are needed to recognize termination signals, one of which is the p-factor, which is a protein with ATPase activity, in other cases it may be modified nitrogenous bases. When RNA polymerase reaches the terminator site, the synthesized RNA strand is finally separated from the DNA template.

The functional transcription unit in microorganisms is the operon (see), which includes one promoter, one operator and a number of genes encoding polypeptide chains (see Gene). The development of the operon begins with the stage of binding of RNA polymerase to the promoter, a region located at the very beginning of the operon. Immediately after the promoter there is an operator - a section of DNA capable of binding to the repressor protein. If the operator is free, then T. occurs throughout the operon, but if the operator is associated with a repressor protein, T. is blocked. All well-studied repressors are proteins capable of undergoing allosteric changes (see Conformation). The structure of repressor proteins is encoded by regulatory genes located either immediately before the operon or at a considerable distance from it. The synthesis and activity of repressors are determined by the conditions of the extra- and intracellular environment (concentration of metabolites, ions, etc.).

Transcription of DNA in higher organisms is carried out in separate sections called T. units - transcriptons. The T. unit includes the DNA of the corresponding gene and adjacent sections. Concepts about the structure of T. units have received significant development in connection with the identification of functional non-equivalence of the sequence of eukaryotic gene regions. It turned out that inside the structural genes of higher organisms there are so-called. introns are DNA insertion sequences that are not directly related to the coding of a given protein. The number and size of introns of different genes vary greatly; in many cases, the total length of all introns significantly exceeds the length of the coding part of the genes (exon). Clarification of the role of introns is one of the urgent tasks of molecular genetics (see).

In the process of transcription, RNA is formed, which is a copy of the entire transcription unit. In cases where genes encode protein synthesis, the primary product of T. is called the nuclear precursor of mRNA (pro-mRNA); it is several times larger in size than mRNA. Pro-mRNA includes sequences transcribed in coding regions (exons), introns, and possibly adjacent DNA regions. In the cell nucleus, pro-mRNA is converted into mature mRNA, the so-called. processing, or maturation. In this case, specific enzymes interact with pro-mRNA and selectively remove redundant sequences, in particular those synthesized on introns. At this same stage, certain modifications of RNA are carried out, such as methylation, addition of specific groups, etc. Mature mRNA released into the cytoplasm nevertheless contains redundant regions that are not directly related to the coding of protein structure and are believed to be necessary for correct interaction of RNA with ribosomes, protein translation factors (see), etc.

Disturbances in the T. process can significantly alter cell metabolism. Defects in enzymes involved in RNA synthesis can cause a decrease in the intensity of T. of a large number of genes and lead to significant disruption of the functioning of the cell, including its death.

Genetic defects in the structure of an individual T. unit cause disruption of the synthesis of this RNA (and its corresponding protein) and thus can be the basis of a monogenic hereditary pathology (see Hereditary diseases).

There is reverse T. - DNA synthesis on an RNA matrix, in which the transfer of information occurs not from DNA to RNA, as in the process of direct T., but in the opposite direction. Reverse T. was first established in RNA-containing oncogenic viruses after an RNA-dependent DNA polymerase, called reverse transcriptase, or revertase, was discovered in mature viral particles (see). With the participation of this enzyme, in a cell infected with viruses, DNA is synthesized on an RNA matrix, which can subsequently serve as a matrix for the formation of RNA of new viral particles. Viral DNA synthesized by reverse T. can be incorporated into the DNA of the host cell and thereby cause malignant transformation of cells. Reverse T. in vitro is usually used in genetic engineering studies (see) for the synthesis of structural zones of the corresponding genes on any RNA templates.

Bibliography: Ashmarin I.P., Molecular biology, p. 70, L., 1974; 3 e n g b u sh P. Molecular and Cellular Biology, trans. with German, vol. 1, p. 135, M., 1982; Kiselev L.L. RNA-guided DNA synthesis. (Reverse transcription), M., 1978, bibliogr.; Watson J. Molecular biology of the gene, trans. from English, p. 268, M., 1978.

S. A. Limborskaya.

Before proteins begin to be synthesized, information about their structure must be “extracted” from DNA and delivered to the site of protein synthesis. This is done by messenger or messenger RNAs. At the same time, the cell needs amino acid transporters - transfer RNAs and structural components of protein-synthesizing organelles - ribosomal RNA. All information about the structure of transport and ribosomal RNAs is also found in DNA.

Therefore, there is a process of rewriting or transcribing data from DNA to RNA. transcription– rewriting) – biosynthesis of RNA on a DNA template.

As in any matrix biosynthesis, 5 necessary elements are distinguished in transcription:

  • matrix - one of the DNA strands,
  • growing chain - RNA,
  • substrate for synthesis - ribonucleotides (UTP, GTP, CTP, ATP),
  • energy source – UTP, GTP, CTP, ATP.
  • RNA polymerase enzymes and protein transcription factors.

RNA biosynthesis occurs in a section of DNA called transcripton, it is limited at one end promoter(beginning), from the other - terminator(end).

Eukaryotic RNA polymerases have two large subunits and several small subunits.

Transcription stages

There are three stages of transcription: initiation, elongation and termination.

Initiation

The promoter contains the transcription start signal – TATA box. This is the name of a certain sequence of DNA nucleotides that binds the first initiation factor TATA factor. This TATA factor ensures the attachment of RNA polymerase to the DNA strand that will be used as a template for transcription (DNA template strand). Since the promoter is asymmetric ("TATA"), it binds RNA polymerase in only one orientation, which determines the direction of transcription from the 5" end to the 3" end (5" → 3"). To bind RNA polymerase to the promoter, another initiation factor is required - the σ factor (Greek σ - “sigma”), but immediately after the synthesis of the RNA seed fragment (8-10 ribonucleotides long), the σ factor is detached from the enzyme.

Other initiation factors unwind the DNA helix in front of RNA polymerase.

Transcription process diagram

Elongation

Protein elongation factors ensure the progression of RNA polymerase along DNA and unwind the DNA molecule over approximately 17 nucleotide pairs. RNA polymerase moves at a speed of 40-50 nucleotides per second in the direction 5"→3". The enzyme uses ATP, GTP, CTP, UTP simultaneously as a substrate and as an energy source.