Life in carbon form exists due to the presence of protein molecules. And protein biosynthesis in the cell is the only possibility for gene expression. But the implementation of this process requires the launch of a number of processes associated with the "unpacking" of genetic information, the search for the desired gene, its reading and reproduction. The term "transcription" in biology just refers to the process of transferring information from a gene to messenger RNA. This is the start of biosynthesis, that is, the direct implementation of genetic information.
Storage of genetic information
In the cells of living organisms, genetic information is localized in the nucleus, mitochondria, chloroplasts and plasmids. Mitochondria and chloroplasts contain a small amount of animal and plant DNA, while bacterial plasmids are the site of storage of genes responsible for rapid adaptation to environmental conditions.
In viral bodies, hereditary information is also stored in the form of RNA or DNA polymers. But the process of its implementation is also associated with the need for transcription. In biology, this process is of exceptional importance, since it is this process that leads to the realization of hereditary information, triggering protein biosynthesis.
In animal cells, hereditary information is represented by a DNA polymer, which is compactly packed inside the nucleus. Therefore, before protein synthesis or reading of any gene, some stages must go through: unwinding of condensed chromatin and “release” of the desired gene, its recognition by enzyme molecules, transcription.
In biology and biological chemistry, these stages have already been studied. They lead to the synthesis of a protein whose primary structure was encoded in the read gene.
Transcription scheme in eukaryotic cells
Although transcription in biology has not been studied enough, its sequence is traditionally presented in the form of a diagram. It consists of initiation, elongation and termination. This means that the whole process is divided into three components of its phenomena.
Initiation is a set of biological and biochemical processes that lead to the start of transcription. The essence of elongation is to continue building up the molecular chain. Termination is a set of processes that lead to the termination of RNA synthesis. By the way, in the context of protein biosynthesis, the process of transcription in biology is usually identified with the synthesis of messenger RNA. Based on it, a polypeptide chain will later be synthesized.
Initiation
Initiation is the least understood transcription mechanism in biology. What this is from the point of view of biochemistry is unknown. That is, the specific enzymes responsible for starting transcription are not recognized at all. Also unknown are the intracellular signals and methods of their transmission, which indicate the need for the synthesis of a new protein. For cytology and biochemistry, this is a fundamental task.
Elongation
It is not yet possible to separate the process of initiation and elongation in time due to the impossibility of conducting laboratory studies designed to confirm the presence of specific enzymes and trigger factors. Therefore, this boundary is very conditional. The essence of the elongation process is reduced to the elongation of a growing chain synthesized on the basis of a DNA template region.
It is believed that elongation begins after the first translocation of RNA polymerase and the beginning of the attachment of the first cadon to the starting site of RNA. In the course of elongation, kadons are read in the direction of the 3'-5' strand in the despiralized and divided into two strands DNA region. At the same time, the growing RNA chain is added with new nucleotides complementary to the DNA template region. In this case, the DNA is “embroidered” to a width of 12 nucleotides, that is, to 4 canons.
The RNA polymerase enzyme moves along the growing chain, and “behind” it, the reverse “crosslinking” of DNA into a double-stranded structure occurs with the restoration of hydrogen bonds between nucleotides. This partly answers the question of what process is called transcription in biology. It is elongation that is the main phase of transcription, because in its course the so-called mediator between the gene and protein synthesis is assembled.
Termination
The process of termination in transcription of eukaryotic cells is poorly understood. So far, scientists have reduced its essence to the termination of DNA reading at the 5 "end and the addition of a group of adenine bases to the 3" end of RNA. The latter process makes it possible to stabilize the chemical structure of the resulting RNA. There are two types of termination in bacterial cells. This is a Rho-dependent and Rho-independent process.
The first proceeds in the presence of the Rho protein and is reduced to a simple break of hydrogen bonds between the DNA template region and the synthesized RNA. The second, Rho-independent, occurs after the appearance of the stem-loop if there is a set of uracil bases behind it. This combination leads to the detachment of the RNA from the DNA template. Obviously, transcription termination is an enzymatic process, but its specific biocatalysts have not yet been found.
Viral transcription
Viral bodies do not have their own protein biosynthesis system, and therefore cannot multiply without exploiting the cells. But viruses have their own genetic material that needs to be realized, as well as built into the genes of infected cells. To do this, they have a number of enzymes (or exploit cell enzyme systems) that transcribe their nucleic acid. That is, this enzyme, based on the genetic information of the virus, synthesizes an analogue of messenger RNA. But it is not RNA at all, but a DNA polymer complementary to genes, for example, in humans.
This completely violates the traditional principles of transcription in biology, which should be considered in the example of the HIV virus. Its enzyme reversetase from viral RNA is capable of synthesizing DNA complementary to human nucleic acid. The process of synthesizing complementary DNA from RNA is called reverse transcription. This is in biology the definition of the process responsible for embedding the hereditary information of the virus into the human genome.
We meet the concept of transcription when studying a foreign language. It helps us to correctly rewrite and pronounce unknown words. What is meant by this term in natural science? Transcription in biology is a key process in the reaction system of protein biosynthesis. It is he who allows the cell to provide itself with peptides that will perform building, protective, signaling, transport and other functions in it. Only the rewriting of information from the DNA locus to the informational ribonucleic acid molecule launches the protein-synthesizing apparatus of the cell, which provides biochemical translation reactions.
In this article, we will consider the stages of transcription and protein synthesis that occur in various organisms, and also determine the significance of these processes in molecular biology. In addition, we will give a definition of what transcription is. In biology, knowledge on the processes of interest to us can be obtained from its sections such as cytology, molecular biology, and biochemistry.
Features of matrix synthesis reactions
For those who are familiar with the basic types of chemical reactions studied in the course of general chemistry, the processes of matrix synthesis will be completely new. The reason for this is as follows: such reactions occurring in living organisms ensure copying of parent molecules using a special code. It was not discovered immediately, it is better to say that the very idea of \u200b\u200bthe existence of two different languages \u200b\u200bfor storing hereditary information made its way over two centuries: from the end of the 19th to the middle of the 20th. To better imagine what transcription and translation are in biology and why they relate to the reactions of matrix synthesis, let us turn to technical vocabulary for an analogy.
Everything is like in typography
Imagine that we need to print, for example, one hundred thousand copies of a popular newspaper. All the material that enters it is collected on the mother carrier. This first sample is called the matrix. Then it is replicated on printing presses - copies are made. Similar processes take place in a living cell, only DNA and mRNA molecules serve as templates in it, and messenger RNA and protein molecules serve as copies. Let's take a closer look at them and find out that transcription in biology is a reaction of matrix synthesis that occurs in the cell nucleus.
The genetic code is the key to the mystery of protein biosynthesis
In modern molecular biology, no one argues about which substance is the carrier of hereditary properties and stores data on all proteins of the body without exception. Of course, this is deoxyribonucleic acid. However, it is built from nucleotides, and the proteins, information about the composition of which is stored in it, are represented by amino acid molecules that have no chemical affinity with DNA monomers. In other words, we are dealing with two different languages. In one of them, the words are nucleotides, in the other, amino acids. What will act as a translator who will recode the information received as a result of transcription? Molecular biology believes that this role is performed by the genetic code.
Unique properties of the cellular code
This is what the code is, the table of which is presented below. Cytologists, geneticists, biochemists worked on its creation. In addition, knowledge from cryptography was used in the development of the code. Given its rules, it is possible to establish the primary structure of the synthesized protein, because translation in biology is the process of translating information about the structure of a peptide from the language of nucleotides and RNA into the language of amino acids of a protein molecule.
The idea of coding in living organisms was first voiced by G. A. Gamov. Further scientific developments led to the formulation of its basic rules. First, it was established that the structure of 20 amino acids is encrypted in 61 messenger RNA triplets, which led to the concept of code degeneracy. Next, we found out the composition of nonsense codons that play the role of starting and stopping the process of protein biosynthesis. Then there were statements about its collinearness and universality, which completed the coherent theory of the genetic code.
Where does transcription and translation take place?
In biology, several of its sections that study the structure and biochemical processes in the cell (cytology and molecular biology) determined the localization of matrix synthesis reactions. So, transcription occurs in the nucleus with the participation of the enzyme RNA polymerase. In its karyoplasm, an mRNA molecule is synthesized from free nucleotides according to the principle of complementarity, which writes off information about the structure of the peptide from one structural gene.
Then it exits the cell nucleus through the pores in the nuclear membrane and ends up in the cytoplasm of the cell. Here, the mRNA must combine with several ribosomes to form a polysome, a structure ready to meet transport ribonucleic acid molecules. Their task is to bring amino acids to the site of another reaction of matrix synthesis - translation. Let us consider the mechanisms of both reactions in detail.
Features of the formation of i-RNA molecules
Transcription in biology is the rewriting of information about the structure of a peptide from a structural DNA gene to a ribonucleic acid molecule, which is called informational. As we said earlier, it occurs in the nucleus of the cell. First, the enzyme DNA restriction enzyme breaks the hydrogen bonds connecting the chains of deoxyribonucleic acid, and its helix unwinds. The enzyme RNA polymerase attaches to free polynucleotide regions. It activates the assembly of a copy - an i-RNA molecule, which, in addition to informative sections - exons, also contains empty nucleotide sequences - introns. They are ballast and need to be removed. This process in molecular biology is called processing or maturation. It completes the transcription. Biology briefly explains this as follows: only having lost unnecessary monomers, the nucleic acid will be able to leave the nucleus and be ready for further stages of protein biosynthesis.
Reverse transcription in viruses
Non-cellular life forms are strikingly different from prokaryotic and eukaryotic cells not only in their external and internal structure, but also in the reactions of matrix synthesis. In the seventies of the last century, science proved the existence of retroviruses - organisms whose genome consists of two RNA chains. Under the action of the enzyme - reversetase - such viral particles copy DNA molecules from ribonucleic acid sections, which are then introduced into the karyotype of the host cell. As you can see, the writing off of hereditary information in this case goes in the opposite direction: from RNA to DNA. This form of coding and reading is typical, for example, for pathogenic agents that cause various types of oncological diseases.
Ribosomes and their role in cellular metabolism
Reactions of plastic exchange, which include the biosynthesis of peptides, proceed in the cytoplasm of the cell. To obtain a ready-made protein molecule, it is not enough to copy the nucleotide sequence from a structural gene and transfer it to the cytoplasm. Structures are also needed that will read information and ensure the connection of amino acids into a single chain through peptide bonds. These are ribosomes, the structure and functions of which are given great attention by molecular biology. We have already found out where transcription occurs - this is the karyoplasm of the nucleus. The place of translation processes is the cellular cytoplasm. It is in it that the channels of the endoplasmic reticulum are located, on which protein-synthesizing organelles, ribosomes, sit in groups. However, their presence does not yet ensure the onset of plastic reactions. We need structures that will deliver protein monomer molecules - amino acids - to the polysome. They are called transport ribonucleic acids. What are they and what is their role in translation?
Amino acid carriers
Small molecules of transport RNA in their spatial configuration have a section consisting of a sequence of nucleotides - an anticodon. For the implementation of translational processes, it is necessary that an initiative complex arise. It should include the template triplet, ribosomes, and the complementary region of the transport molecule. As soon as such a complex is organized, this is a signal to start assembling the protein polymer. Both translation and transcription in biology are assimilation processes, always occurring with the absorption of energy. For their implementation, the cell prepares in advance, accumulating a large number of molecules of adenosine triphosphoric acid.
The synthesis of this energy substance occurs in mitochondria - the most important organelles of all eukaryotic cells without exception. It precedes the onset of matrix synthesis reactions, occupying a place in the presynthetic stage of the cell life cycle and after replication reactions. The splitting of ATP molecules accompanies transcriptional processes and translation reactions, the energy released in this case is used by the cell at all stages of the biosynthesis of organic substances.
Translation stages
At the beginning of the reactions leading to the formation of a polypeptide, 20 types of protein monomers bind to certain transport acid molecules. In parallel, the formation of a polysome occurs in the cell: ribosomes are attached to the matrix at the location of the start codon. The start of biosynthesis begins, and the ribosomes move along the mRNA triplets. Molecules that transport amino acids are suitable for them. If the codon in the polysome is complementary to the anticodon of transport acids, then the amino acid remains in the ribosome, and the resulting polypeptide bond connects it to the amino acids already there. As soon as the protein-synthesizing organelle reaches the stop triplet (usually UAG, UAA or UGA), translation stops. As a result, the ribosome, together with the protein particle, is separated from the mRNA.
How does a peptide get its native form?
The last stage of translation is the process of transition of the primary structure of the protein to the tertiary form, which has the form of a globule. Enzymes remove unnecessary amino acid residues in it, add monosaccharides or lipids, and additionally synthesize carboxyl and phosphate groups. All this occurs in the cavities of the endoplasmic reticulum, where the peptide enters after completion of biosynthesis. Next, the native protein molecule passes into the channels. They penetrate the cytoplasm and ensure that the peptide enters a certain area of the cytoplasm and is then used for the needs of the cell.
In this article, we found out that translation and transcription in biology are the main reactions of matrix synthesis that underlie the preservation and transmission of the organism's hereditary inclinations.
First, establish the sequence of steps in protein biosynthesis, starting with transcription. The entire sequence of processes occurring during the synthesis of protein molecules can be combined into 2 stages:
Transcription.
Broadcast.
Structural units of hereditary information are genes - sections of the DNA molecule that encode the synthesis of a particular protein. In terms of chemical organization, the material of heredity and variability of pro- and eukaryotes is not fundamentally different. The genetic material in them is presented in the DNA molecule, the principle of recording hereditary information and the genetic code is also common. The same amino acids in pro- and eukaryotes are encrypted by the same codons.
The genome of modern prokaryotic cells is characterized by a relatively small size, the DNA of Escherichia coli has the form of a ring, about 1 mm long. It contains 4 x 10 6 base pairs, forming about 4000 genes. In 1961, F. Jacob and J. Monod discovered the cistronic, or continuous organization of prokaryotic genes, which consist entirely of coding nucleotide sequences, and they are entirely realized during protein synthesis. The hereditary material of the DNA molecule of prokaryotes is located directly in the cytoplasm of the cell, where the tRNA and enzymes necessary for gene expression are also located. Expression is the functional activity of genes, or gene expression. Therefore, mRNA synthesized with DNA is able to immediately act as a template in the process of translation of protein synthesis.
The eukaryotic genome contains much more hereditary material. In humans, the total length of DNA in the diploid set of chromosomes is about 174 cm. It contains 3 x 10 9 base pairs and includes up to 100,000 genes. In 1977, a discontinuity was discovered in the structure of most eukaryotic genes, which was called the "mosaic" gene. It has coding nucleotide sequences exonic And intron plots. Only exon information is used for protein synthesis. The number of introns varies in different genes. It has been established that the chicken ovalbumin gene includes 7 introns, and the mammalian procollagen gene - 50. The functions of silent DNA - introns have not been completely elucidated. It is assumed that they provide: 1) the structural organization of chromatin; 2) some of them are obviously involved in the regulation of gene expression; 3) introns can be considered as a store of information for variability; 4) they can play a protective role, taking on the action of mutagens.
Transcription
The process of rewriting information in the cell nucleus from a portion of a DNA molecule to an mRNA molecule (mRNA) is called transcription(lat. Transcriptio - rewriting). The primary product of the gene, mRNA, is synthesized. This is the first step in protein synthesis. On the corresponding section of DNA, the RNA polymerase enzyme recognizes the sign of the start of transcription - preview The starting point is considered to be the first DNA nucleotide, which is included by the enzyme in the RNA transcript. As a rule, coding regions begin with the codon AUG, sometimes GUG is used in bacteria. When RNA polymerase binds to the promoter, the DNA double helix is locally untwisted and one of the strands is copied according to the principle of complementarity. mRNA is synthesized, its assembly speed reaches 50 nucleotides per second. As the RNA polymerase moves, the mRNA chain grows, and when the enzyme reaches the end of the copying site - terminator, the mRNA moves away from the template. The DNA double helix behind the enzyme is repaired.
Transcription of prokaryotes takes place in the cytoplasm. Due to the fact that DNA consists entirely of coding nucleotide sequences, therefore, the synthesized mRNA immediately acts as a template for translation (see above).
Transcription of mRNA in eukaryotes occurs in the nucleus. It begins with the synthesis of large molecules - precursors (pro-mRNA), called immature, or nuclear RNA. The primary product of the gene - pro-mRNA is an exact copy of the transcribed DNA region, includes exons and introns. The process of formation of mature RNA molecules from precursors is called processing. mRNA maturation occurs by splicing are cuttings by enzymes restrictase introns and connection of sites with transcribed exon sequences by ligase enzymes. (Fig.). Mature mRNA is much shorter than pro-mRNA precursor molecules, the size of introns in them varies from 100 to 1000 nucleotides or more. Introns account for about 80% of all immature mRNA.
It has now been shown that it is possible alternative splicing, in which nucleotide sequences can be deleted from one primary transcript in its different regions and several mature mRNAs will be formed. This type of splicing is characteristic of the immunoglobulin gene system in mammals, which makes it possible to form different types of antibodies based on a single mRNA transcript.
Upon completion of processing, the mature mRNA is selected before leaving the nucleus. It has been established that only 5% of mature mRNA enters the cytoplasm, and the rest is cleaved in the nucleus.
Broadcast
Translation (lat. Translatio - transfer, transfer) - translation of information contained in the nucleotide sequence of the mRNA molecule into the amino acid sequence of the polypeptide chain (Fig. 10). This is the second stage of protein synthesis. The transfer of mature mRNA through the pores of the nuclear envelope produces special proteins that form a complex with the RNA molecule. In addition to mRNA transport, these proteins protect mRNA from the damaging effects of cytoplasmic enzymes. In the process of translation, tRNAs play a central role; they ensure the exact correspondence of the amino acid to the code of the mRNA triplet. The process of translation-decoding occurs in ribosomes and is carried out in the direction from 5 to 3. The complex of mRNA and ribosomes is called a polysome.
Translation can be divided into three phases: initiation, elongation, and termination.
Initiation.
At this stage, the entire complex involved in the synthesis of the protein molecule is assembled. There is a union of two ribosome subunits at a certain site of mRNA, the first aminoacyl - tRNA is attached to it, and this sets the frame for reading information. Any mRNA molecule contains a site that is complementary to the rRNA of the small subunit of the ribosome and specifically controlled by it. Next to it is the initiating start codon AUG, which encodes the amino acid methionine.
Elongation
- it includes all reactions from the moment of formation of the first peptide bond to the addition of the last amino acid. The ribosome has two sites for the binding of two tRNA molecules. The first t-RNA with the amino acid methionine is located in one section, peptidyl (P), and the synthesis of any protein molecule begins from it. The second t-RNA molecule enters the second site of the ribosome - aminoacyl (A) and attaches to its codon. A peptide bond is formed between methionine and the second amino acid. The second tRNA moves along with its mRNA codon to the peptidyl center. The movement of t-RNA with the polypeptide chain from the aminoacyl center to the peptidyl center is accompanied by the advancement of the ribosome along the mRNA by a step corresponding to one codon. The tRNA that delivered the methionine returns to the cytoplasm, and the amnoacyl center is released. It receives a new t-RNA with an amino acid encrypted by the next codon. A peptide bond is formed between the third and second amino acids, and the third tRNA, together with the mRNA codon, moves to the peptidyl center. The process of elongation, elongation of the protein chain. It continues until one of the three codons that do not code for amino acids enters the ribosome. This is a terminator codon and there is no corresponding tRNA for it, so none of the tRNAs can take a place in the aminoacyl center.
Termination
- completion of polypeptide synthesis. It is associated with the recognition by a specific ribosomal protein of one of the termination codons (UAA, UAG, UGA) when it enters the aminoacyl center. A special termination factor is attached to the ribosome, which promotes the separation of ribosome subunits and the release of the synthesized protein molecule. Water is attached to the last amino acid of the peptide and its carboxyl end is separated from the tRNA.
The assembly of the peptide chain is carried out at a high speed. In bacteria at a temperature of 37°C, it is expressed in the addition of 12 to 17 amino acids per second to the polypeptide. In eukaryotic cells, two amino acids are added to a polypeptide in one second.
The synthesized polypeptide chain then enters the Golgi complex, where the construction of the protein molecule is completed (second, third, fourth structures appear in succession). Here there is a complexation of protein molecules with fats and carbohydrates.
The whole process of protein biosynthesis is presented in the form of a scheme: DNA ® pro mRNA ® mRNA ® polypeptide chain ® protein ® protein complexing and their transformation into functionally active molecules.
The stages of the implementation of hereditary information also proceed in a similar way: first, it is transcribed into the nucleotide sequence of mRNA, and then translated into the amino acid sequence of the polypeptide on ribosomes with the participation of tRNA.
Transcription of eukaryotes is carried out under the action of three nuclear RNA polymerases. RNA polymerase 1 is located in the nucleolus and is responsible for the transcription of rRNA genes. RNA polymerase 2 is found in the nuclear sap and is responsible for the synthesis of the mRNA precursor. RNA polymerase 3 is a small fraction in the nuclear sap that synthesizes small rRNAs and tRNAs. RNA polymerases specifically recognize the nucleotide sequence of the transcription promoter. Eukaryotic mRNA is first synthesized as a precursor (pro-mRNA), information from exons and introns is written off to it. The synthesized mRNA is larger than necessary for translation and is less stable.
In the process of maturation of the mRNA molecule, introns are cut out with the help of restriction enzymes, and exons are sewn together with the help of ligase enzymes. The maturation of mRNA is called processing, and the joining of exons is called splicing. Thus, mature mRNA contains only exons and is much shorter than its predecessor, pro-mRNA. Intron sizes vary from 100 to 10,000 nucleotides or more. Intons account for about 80% of all immature mRNA. At present, the possibility of alternative splicing has been proven, in which nucleotide sequences can be deleted from one primary transcript in its different regions and several mature mRNAs will be formed. This type of splicing is characteristic of the immunoglobulin gene system in mammals, which makes it possible to form different types of antibodies based on a single mRNA transcript. Upon completion of processing, the mature mRNA is selected before being released into the cytoplasm from the nucleus. It has been established that only 5% of the mature mRNA enters, and the rest is cleaved in the nucleus. The transformation of the primary transcriptons of eukaryotic genes, associated with their exon-intron organization, and in connection with the transition of mature mRNA from the nucleus to the cytoplasm, determines the features of the realization of the genetic information of eukaryotes. Therefore, the eukaryotic mosaic gene is not a cistronome gene, since not all of the DNA sequence is used for protein synthesis.
Transcription in biology is a multi-stage process of reading information from DNA, which is a component. 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 activity, because these organic compounds are involved in all metabolic processes, are structural components of many tissues and organs, play signaling, regulatory and protective roles in the body.
The process by which protein biosynthesis begins is transcription. Biology briefly divides it into three stages:
- Initiation.
- Elongation (growth of the RNA chain).
- Termination.
Transcription in biology is a whole cascade of step-by-step reactions, as a result of which RNA molecules are synthesized on the DNA template. Moreover, not only information ribonucleic acids are formed in this way, 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 to 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 template DNA chain. This enzyme has two β-subunits, one α-subunit and one σ-subunit. The first two components perform the function of forming the body of the enzyme, and the remaining two are responsible for retaining the enzyme on the DNA molecule and recognizing the promoter part of the deoxyribonucleic acid, respectively.
By the way, the sigma factor is one of the signs by which this or that gene is recognized. For example, the Latin letter σ with index 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 is somewhat more complicated in animals and plants. Firstly, in each cell there are not one, but as many as three types of different RNA polymerases. Among them:
- RNA polymerase I. It is responsible for the transcription of ribosomal RNA genes (with the exception of the 5S RNA subunits of the ribosome).
- RNA polymerase II. Its task is to synthesize normal informational (matrix) ribonucleic acids, which are further involved in translation.
- RNA polymerase III. The function of this type of polymerase is to synthesize as well as 5S-ribosomal RNA.
Secondly, for promoter recognition in eukaryotic cells, it is not enough to have only a polymerase. Transcription initiation also involves special peptides called TF proteins. Only with their help can RNA polymerase sit on DNA and begin the synthesis of a ribonucleic acid molecule.
Transcription meaning
The RNA molecule, which is formed on the DNA matrix, subsequently attaches to the ribosomes, where information is read from it and a 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, first of all, the basis for the most important enzymes of all biochemical reactions.
Transcription in biology is also a source of rRNAs, which are also tRNAs that are involved in the transfer of amino acids during translation to these non-membrane structures. snRNAs (small nuclear nuclei) 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 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 needed at the moment to maintain life. Therefore, transcription in biology is the main stage in the work of each individual cell of the body.
RNA biosynthesis - transcription - the process of reading genetic information from DNA, in which the DNA nucleotide sequence is encoded as an RNA nucleotide sequence. Used as energy and substrate - nucleoside-3-phosphate with ribose. It is based on complementarity principle- a conservative process - a new single-stranded RNA is synthesized during the entire interphase, starts in certain areas - promoters, ends in terminators, and the section between them - an operon (trancrypton) - contains one or more functionally related genes, sometimes contains genes that do not encode proteins. Transcription differences: 1) individual genes are transcribed. 2) no primer required. 3) ribose is included in RNA, not deoxyribose.
Transcription steps: 1) binding of RNA polymerase to DNA. 2) initiation - the formation of an RNA chain. 3) elongation or growth of the RNA chain. 4) termination.
Stage 1 - the site with which RNA polymerase binds is called a promoter (40 nucleotide pairs) - it has a site for recognition, attachment, initiation. RNA polymerase, recognizing the promoter, sits on it and a closed promoter complex is formed, in which DNA is spiralized and the complex can easily dissociate and pass into an open promoter complex - the bonds are strong, the nitrogenous base turns outward.
Stage 2 - initiation RNA synthesis consists in the formation of several links in the RNA chain, the synthesis begins on one DNA strand 3'-5' and goes in the direction 5'-3'. The stage ends with the separation of the b-subunit.
Stage 3 - elongation- elongation of the RNA chain - occurs due to Core-rRNA polymerase. The DNA strand is despiralized on 18 pairs, and on 12 - a hybrid - a common hybrid of DNA and RNA. RNA polymerase moves along the DNA chain, and after the restoration of the DNA chain. In eukaryotes, when the RNA reaches 30 nucleotides, a protective CEP structure is formed at the 5' end.
Stage 4 - termination- occurs on terminators. In the chain there is a site rich in GC, and then from 4 to 8 consecutive A. After passing through the site, a hairpin is formed in the RNA product and the enzyme does not go further, the synthesis stops. An important role is played by the protein termination factor - rho and tower. While the synthesis was proceeding, pyrophosphate inhibited the ro protein, since the enzyme has stopped (hairpin) the synthesis of phosphoric acid has stopped. The Rho protein is activated and exhibits nucleoside phosphatase activity, which leads to the release of RNA, RNA polymerase, which subsequently combines with the subunit.
Processing - RNA maturation. Includes: 1) the formation of CEP at the 5'-end, is involved in attachment to the ribosome. 2) polyadenylation occurs at the 3'-end and a tail of one hundred to two hundred adenyl nucleotides is formed, it protects the '-end from the action of nucleases and helps to pass through nuclear pores and plays a role in attaching to the ribosome. 3) splicing - non-coding sequences are cut out - introns. This happens in two ways: a) is carried out by the spliceosome - it is a nucleoprotein containing a number of proteins and small nuclear RNA. In the beginning, introns are looped out, leaving only coding sequences - exons. Endonuclease enzymes are cut and ligases ligate the remaining exons. THAT. the introns are gone. Alternative splicing - on the same nucleic acid sequence, RNA forms several proteins. Self-splicing is the self-removal of introns. Splicing disorders: 1) systemic lupus erythematosus. 2) phenylketonuria. 3) hemoglobinopathy. Matrix RNA of prokaryotes is not processed, because they don't have introns. tRNA processing. The tRNA precursor is cleaved and the nucleotide 5'-3'Q P is cleaved off. The CCA sequence with an OH group is attached to the 3'-end, and a phosphorylated purine base is attached to the 5'-end. Duhydrouridine loop - ARSase. rRNA processing. The rRNA precursor, proribosomal 45S RNA, is synthesized in the nucleolus and exposed to ribonucleases to form 5.8S 18S 28S. They are 70% spiralized. rRNA plays a role in the formation of the ribosome and is involved in catalytic processes. The subunit is formed from rRNA in the nucleus. The small subunit is 30S, the large subunit is 50S and the ribosome 70S is formed in prokaryotes, in eukaryotes 40S + 60S = 80S. Ribosome formation occurs in the cytoplasm.
Ribosome sites for RNA binding: 1) in small subunits that have the Shine-Dalhorn mRNA sequence 5'GGAGG3' 3'CCUCC5'. Messenger RNA is attached to the small subunit. In eukaryotes, CEP-binding site for mRNA. tRNA binding site: a) P-site - peptidyl center for binding mRNA to the growing peptide chain - peptidyl-tRNA-binding. b) A-site - for binding tRNA with amino acid - aminoacyl site 2) In the large subunit, E-site with peptidyl transferase activity.
reverse transcription characteristic of retroviruses or viruses containing RNA - HIV infection virus, oncoviruses.
On the RNA chain, DNA synthesis occurs under the action of the enzyme reverse transcriptase or reversetase, or DNA RNA polymerase. Invading the host cell, DNA synthesis occurs, into which it is integrated into the host DNA and the transcription of its RNA and the synthesis of its own proteins begin.
Genetic code, its characteristics. The genetic code is the nucleotide sequence of the rRNA molecule that contains code words for each amino acid. It consists in a certain sequence of nucleotides in the DNA molecule.
Characteristic. 1) the genetic code is triplet - i.e. each a/k is encrypted with three nucleotides. 2) the genetic code for a / c is degenerate or redundant - the vast majority of a / c is encoded by several codons. A total of 64 triplets are formed, of which 61 triplets encode a certain a / c, and three triplets - AUG, UAA, UGA are nonsense codons, because they do not encode any of the 20 a / c, they perform the function of terminating the synthesis. 3) The genetic code is continuous, there are no punctuation marks, i.e. signals indicating the end of one triplet and the beginning of another. The code is linear, unidirectional, continuous. For example - ATSGUTSGATSTS. 4) the AUG triplet serves as the synthesis activation codon. 5) The genetic code is universal.
22. Broadcast - protein biosynthesis. Translation stages: 1) initiation. 2) elongation. 3) termination. Initiation- A/C is activated.
The initiating aatRNA will interact with the 1 a/c of the future protein only with the carboxyl group, and the 1 a/c can give only the NH 2 group for synthesis, i.e. protein synthesis starts at the N-terminus.
Assembly of the initiating complex on a small subparticle. Factors: 30S mRNA fomylmethionyl tRNA IF 123 Mg 2+ GTP is an energy source
The small subunit loaded with initiation factors finds the start codon AUG or GUG on the mRNA and sets the reading frame according to it; the start codon is placed in the P site. Formlmethionyl tRNA approaches it, which is accompanied by the release of IF 3 factor, then the large subunit joins and IF 1 and IF2 are released, hydrolysis of 1GTP occurs and a ribosome is formed. Elongation is the working cycle of the ribosome. Includes three steps: 1) binding of aatRNA to the A-site; P-site is occupied – elongation factors EF-TU, EF-TS and GTP are needed. Elongation factors in prokaryotes: EF-TU, EF-TS, EF-G. 3 )Translocation– first, the EF-G deacylated tRNA of the P-site leaves the ribosome, moving 1 triplet towards the 3’ end; the movement of the peptide from A to the P-site - GTP is used and the elongation factor - EF-G-translocase, A - the site is again free and the process is repeated. Termination– recognition of termination codons UAA, UGA, UAG with the help of releasing factors RF 1 2 3. When a terminal codon enters the A-site, tRNA does not attach to it, but one of the termination factors that blocks elongation is attached, which is accompanied by activation of the esterase activity of peptidyl transferase site E. Hydrolysis of ester bonds between the peptide and tRNA occurs, the ribosome leaves the peptide, tRNA and dissociates into subunits, which can then be used.
Structure formation occurs simultaneously with the help of chaperone proteins - heat shock proteins. The synthesis of one peptide bond consumes 1ATP for aminoacylation of tRNA (attachment of an amino acid), 1GTP for the connection of aatRNA with the A-site, and 1GTP for translocation. Energy consumption is about 4 macroergic bonds for the synthesis of one peptide bond.
23. Lactose operon. Replication is regulated by the concentration of the Dna protein and guanosine tetraphosphate. The main regulation of gene expression is carried out at the level of transcription (depending on the stage of cell development, all factors, the action of hormones and other regulatory components). In different tissue cells, only 5% of genes are expressed, 97% are silent - junk DNA - transcription regulators are chronomeres and a number of regulatory sequences. If the attachment of a regulatory protein to DNA causes transcription, then this is a positive (+) regulation, if transcription suppression is a negative (-) regulation. Positive regulation- the gene is turned off, the attachment of the regulator protein leads to the beginning of synthesis, as a result, the gene is turned on. THAT. a regulatory protein can be an inducer or an activator . Negative regulation- the gene is turned on, RNA synthesis is in progress, if a protein regulatory factor (an inhibitor or repressor of protein synthesis) is added, the gene is turned off. Many hormones and other factors influence the attachment of the regulator protein. E. coli lactose operon- negative regulation. The main elements of its work: in the DNA molecule - a regulator site, a promoter, a pro-operon and three structural genes: lag 1, lag 2, lag 3 and terminator. Lag 1 - carries out the synthesis of the enzyme lactase or beta-galactosidase. Lag 2 is a permiase enzyme involved in the transport of lactose across the membrane. Lag 3 is the enzyme transacylase. Regulator - mRNA synthesis on the ribosome, leads to the formation of a repressor protein, it attaches to the operator (because it has an affinity), sits on it, and since it the regions of the promoter and operon overlap - RNA polymerase cannot attach to the promoter and transcription is turned off. Glucose and galactose provide repressor and operator similarity. If there is no similarity, lactose interacts with the repressor, changing its transformation, and it does not sit on the operon, because loses resemblance to it. RNA polymerase sits on the promoter and transcription of messenger RNA begins. Lactose is an inducer, and the process is induction, a form of downregulation, so called because transcription is terminated by the addition of a repressor and its cleavage initiates synthesis. Positive regulation - TATA factor– has similarities to the TATA-box area. The TATA factor sits on the TATA box - a signal for RNA polymerase to recognize its promoter, sits on it and starts transcription of adjacent genes. In prokaryotes, negative regulation prevails; for eukaryotes, this is not beneficial. Enhancer sites (transcriptional enhancers) + regulatory protein leads to increased transcription. Sincers + regulatory protein à turns off transcription and changes the structure of chromosomes.
