protein synthesis- one of the main processes of metabolism in the cell. This is matrix synthesis. Protein synthesis requires DNA, mRNA, tRNA, rRNA (ribosomes), amino acids, enzymes, magnesium ions, ATP energy. The main role in determining the structure of a protein belongs to DNA.

Information about the amino acid sequence in a protein molecule is encoded in the DNA molecule. The method of recording information is called coding. The genetic code is a system for recording information about the sequence of amino acids in proteins using the sequence of nucleotides in messenger RNA.

The composition of RNA includes nucleotides of 4 types: A, G, C, U. The composition of protein molecules includes 20 amino acids. Each of the 20 amino acids is coded for by a sequence of 3 nucleotides called a triplet or codon. From 4 nucleotides, 64 different combinations of 3 nucleotides each can be created (4 3 = 64).

Properties of the genetic code

1. Genetic code triplet:

2. Code degenerate. This means that each amino acid is encoded by more than one codon (from 2 to 6):

3. Code non-overlapping. This means that consecutive codons are sequentially arranged triplets of nucleotides:

4. Universal for all cells (human, animal, plant).

5. Specific. The same triplet cannot correspond to several amino acids.

6. Protein synthesis starts from the start (initial) codon OUT, which codes for the amino acid methionine.

7. Protein synthesis ends with one of three stop codons, non-coding amino acids: UAT, UAA, UTA.

Table of the genetic code

A section of DNA containing information about the structure of a particular protein is called a gene. The gene is not directly involved in protein synthesis. Messenger RNA (mRNA) is the mediator between a gene and a protein. DNA plays the role of a template for mRNA synthesis in the cell nucleus. The DNA molecule in the gene section unwinds. Information is written from one of its chains to mRNA in accordance with the principle of complementarity between the nitrogenous bases of nucleic acids. This process is called transcription. Transcription occurs in the cell nucleus with the participation of the RNA polymerase enzyme and using the energy of ATP (Fig. 37).

Rice. 37. Transcription.

Protein synthesis is carried out in the cytoplasm on ribosomes, where mRNA serves as a template (Fig. 38). The translation of a sequence of nucleotide triplets in an mRNA molecule into a specific amino acid sequence is called broadcast. The synthesized mRNA exits through the pores in the nuclear envelope into the cytoplasm of the cell, combines with ribosomes, forming polyribosomes (polysomes). Each ribosome consists of two subunits - large and small. mRNA attaches to the small subunit in the presence of magnesium ions (Fig. 39).

Rice. 38. Protein synthesis.

Rice. 39.The main structures involved in protein synthesis.

Transfer RNAs (tRNAs) are found in the cytoplasm. Each amino acid has its own tRNA. The tRNA molecule on one of the loops has a triplet of nucleotides (anticodon), which is complementary to the triplet of nucleotides on the mRNA (codon).

Amino acids located in the cytoplasm are activated (interact with ATP) and, with the help of the enzyme aminoacyl-tRNA synthetase, are attached to tRNA. The first (start) codon of mRNA - AUG - carries information about the amino acid methionine (Fig. 40). This codon is matched by a tRNA molecule containing a complementary anticodon and carrying the first amino acid methionine. This ensures the connection of the large and small subunits of the ribosome. The second mRNA codon adds a tRNA containing an anticodon complementary to this codon. tRNA contains a second amino acid. A peptide bond is formed between the first and second amino acids. The ribosome intermittently, triplet by triplet, moves along the mRNA. The first tRNA is released and released into the cytoplasm, where it can combine with its amino acid.

As the ribosome moves along the mRNA, amino acids corresponding to mRNA triplets and imported tRNAs are added to the polypeptide chain (Fig. 41).

“Reading” by the ribosome of the information contained in the mRNA occurs until it reaches one of the three stop codons (UAA, UGA, UAG). Polypeptide chain

Rice. 40. Protein synthesis.

BUT- binding aminoacyl - tRNA;

B- formation of a peptide bond between methionine and the 2nd amino acid;

AT- movement of the ribosome by one codon.

leaves the ribosome and acquires the structure characteristic of this protein.

The direct function of an individual gene is to encode the structure of a specific protein-enzyme that catalyzes one biochemical reaction that occurs under certain environmental conditions.

Gene (section of DNA) → mRNA → protein-enzyme → biochemical reaction → hereditary trait.

Rice. 41. Broadcast.

Questions for self-control

1. Where does protein synthesis take place in the cell?

2. Where is information about protein synthesis recorded?

3. What properties does the genetic code have?

4. What codon does protein synthesis begin with?

5. What codons end protein synthesis?

6. What is a gene?

7. How and where does transcription take place?

8. What are nucleotide triplets in an mRNA molecule called?

9. What is broadcast?

10. How is an amino acid attached to tRNA?

11. What is the name of a triplet of nucleotides in a tRNA molecule? 12. Which amino acid provides a large and

small subunit of the ribosome?

13. How is the formation of a polypeptide chain of a protein?

Key words of the topic “Protein synthesis”

nitrogenous bases alanine

amino acids

anticodon

protein

biochemical reaction

valine

gene

genetic code action

DNA

record information magnesium ions

mRNA

coding

codon

leucine

matrix

metabolism

methionine

hereditary trait nucleic acids peptide bond loop

polyribosome pore

intermediary sequence

principle of ribosome complementarity

rRNA

serine

synthesis

combination

way

structure

subunit

transcription

broadcast

triplet

tRNA

plot

phenylalanine

enzymes

chain

cytoplasm

ATP energy

Every field of science has its own "blue bird"; cyberneticians dream of "thinking" machines, physicists - of controlled thermonuclear reactions, chemists - of the synthesis of "living matter" - protein. Protein synthesis has long been the subject of science fiction novels, a symbol of the coming power of chemistry. This is explained by the enormous role that protein plays in the living world, and by the difficulties that inevitably confronted every daredevil who dared to “compose” an intricate protein mosaic from individual amino acids. And not even the protein itself, but only peptides.

The difference between proteins and peptides is not only terminological, although the molecular chains of both are composed of amino acid residues. At some stage, quantity turns into quality: the peptide chain - the primary structure - acquires the ability to coil into spirals and balls, forming secondary and tertiary structures, already characteristic of living matter. And then the peptide becomes a protein. There is no clear boundary here - a demarcation mark cannot be put on the polymer chain: hitherto - peptide, from here - protein. But it is known, for example, that adranocorticotropic hormone, consisting of 39 amino acid residues, is a polypeptide, and the hormone insulin, consisting of 51 residues in the form of two chains, is already a protein. The simplest, but still protein.

The method of combining amino acids into peptides was discovered at the beginning of the last century by the German chemist Emil Fischer. But for a long time after that, chemists could not seriously think not only about the synthesis of proteins or 39-membered peptides, but even much shorter chains.

Process of protein synthesis

In order to connect two amino acids together, many difficulties must be overcome. Each amino acid, like the two-faced Janus, has two chemical faces: a carboxylic acid group at one end and an amine basic group at the other. If the OH group is taken away from the carboxyl of one amino acid, and the hydrogen atom is taken away from the amine group of the other, then the two amino acid residues formed in this case can be connected to each other by a peptide bond, and as a result, the simplest of peptides, the dipeptide, will arise. And a water molecule will split off. By repeating this operation, one can increase the length of the peptide.

However, this seemingly simple operation is practically difficult to implement: amino acids are very reluctant to combine with each other. We have to activate them, chemically, and “heat up” one of the ends of the chain (most often carboxylic), and carry out the reaction, strictly observing the necessary conditions. But that's not all: the second difficulty is that not only residues of different amino acids, but also two molecules of the same acid can combine with each other. In this case, the structure of the synthesized peptide will already differ from the desired one. Moreover, each amino acid can have not two, but several "Achilles' heels" - side chemically active groups capable of attaching amino acid residues.

In order to prevent the reaction from deviating from the given path, it is necessary to camouflage these false targets - to “seal” all the reactive groups of the amino acid, except for one, for the duration of the reaction, by attaching the so-called protective groups to them. If this is not done, then the target will grow not only from both ends, but also sideways, and the amino acids will no longer be able to be connected in a given sequence. But this is precisely the meaning of any directed synthesis.

But, getting rid of one trouble in this way, chemists are faced with another: after the end of the synthesis, the protective groups must be removed. In Fischer's time, groups that were split off by hydrolysis were used as "protection". However, the hydrolysis reaction usually turned out to be too strong a "shock" for the resulting peptide: its difficult-to-build "construction" fell apart as soon as the "scaffolding" - protective groups - was removed from it. Only in 1932, Fischer's student M. Bergmann found a way out of this situation: he proposed protecting the amino group of an amino acid with a carbobenzoxy group, which could be removed without damaging the peptide chain.

Protein synthesis from amino acids

Over the years, a number of so-called soft methods have been proposed for "crosslinking" amino acids to each other. However, all of them were in fact only variations on the theme of Fisher's method. Variations in which sometimes it was even difficult to catch the original melody. But the principle itself remained the same. Yet the difficulties associated with protecting vulnerable groups remained the same. Overcoming these difficulties had to be paid for by increasing the number of reaction stages: one elementary act - the combination of two amino acids - was divided into four stages. And each extra stage is an inevitable loss.

Even if we assume that each stage comes with a useful yield of 80% (and this is a good yield), then after four stages these 80% "melt" to 40%. And this is with the synthesis of only a dipeptide! What if there are 8 amino acids? And if 51, as in insulin? Add to this the difficulties associated with the existence of two optical “mirror” forms of amino acid molecules, of which only one is needed in the reaction, add on the problems of separating the resulting peptides from by-products, especially in cases where they are equally soluble. What happens in total: Road to nowhere?

And yet these difficulties did not stop chemists. The pursuit of the "blue bird" continued. In 1954, the first biologically active polypeptide hormones, vasopressin and oxytocin, were synthesized. They had eight amino acids. In 1963, a 39-mer ACTH polypeptide, adrenocorticotropic hormone, was synthesized. Finally, chemists in the United States, Germany and China synthesized the first protein - the hormone insulin.

How is it, the reader will say, that the difficult road, it turns out, did not lead to anywhere or anywhere, but to the realization of the dream of many generations of chemists! This is a milestone event! Indeed, this is a landmark event. But let's evaluate it soberly, renouncing sensationalism, exclamation marks and excessive emotions.

Nobody argues: the synthesis of insulin is a huge victory for chemists. This is a colossal, titanic work, worthy of all admiration. But at the same time, the ego is, in essence, the ceiling of the old polypeptide chemistry. This is a victory on the verge of defeat.

Protein synthesis and insulin

There are 51 amino acids in insulin. To connect them in the right sequence, chemists needed to carry out 223 reactions. When, three years after the beginning of the first of them, the last was completed, the yield of the product was less than one hundredth of a percent. Three years, 223 stages, a hundredth of a percent - you must admit that the victory is purely symbolic. It is very difficult to talk about the practical application of this method: the costs associated with its implementation are too high. But in the final analysis, we are not talking about the synthesis of precious relics of the glory of organic chemistry, but about the release of a vital drug that is needed by thousands of people around the world. So the classical method of polypeptide synthesis has exhausted itself on the very first, simplest protein. So, the "blue bird" again slipped out of the hands of chemists?

A new method for protein synthesis

Approximately a year and a half before the world learned about the synthesis of insulin, another message flashed in the press, which at first did not attract much attention: the American scientist R. Maryfield proposed a new method for the synthesis of peptides. Since the author himself at first did not give the method a proper assessment, and there were many flaws in it, it looked in the first approximation even worse than the existing ones. However, already in early 1964, when Maryfield succeeded in using his method to complete the synthesis of a 9-membered hormone with a useful yield of 70%, scientists were amazed: 70% after all stages is 9% useful yield at each stage of synthesis.

The main idea of ​​the new method is that the growing chains of peptides, which were previously left to the mercy of chaotic movement in the solution, were now tied at one end to a solid carrier - they were, as it were, forced to anchor in the solution. Maryfield took a solid resin and “attached” the first amino acid assembled into a peptide to its active groups by the carbonyl end. The reactions took place inside individual resin particles. In the "labyrinths" of its molecules, the first short shoots of the future peptide first appeared. Then the second amino acid was introduced into the vessel, its carbonyl ends were linked with the free amino ends of the “attached” amino acid, and another “floor” of the future “building” of the peptide grew in the particles. So, stage by stage, the entire peptide polymer was gradually built up.

The new method had undoubted advantages: first of all, it solved the problem of separating unnecessary products after the addition of each amino acid - these products were easily washed off, and the peptide remained attached to the resin granules. At the same time, the problem of solubility of growing peptides, one of the main scourges of the old method, was excluded; earlier, they often precipitated, practically ceasing to participate in the growth process. The peptides “removed” after the completion of the synthesis from the solid support were obtained almost all of the same size and structure, in any case, the scatter in the structure was less than with the classical method. And accordingly more useful output. Thanks to this method, peptide synthesis - a painstaking, time-consuming synthesis - is easily automated.

Maryfield built a simple machine that itself, according to a given program, did all the necessary operations - supplying reagents, mixing, draining, washing, measuring a dose, adding a new portion, and so on. If according to the old method, it took 2-3 days to add one amino acid, then Maryfield connected 5 amino acids in a day on his machine. The difference is 15 times.

What are the difficulties in protein synthesis

Maryfield's method, called solid-phase, or heterogeneous, was immediately adopted by chemists around the world. However, after a short time it became clear that the new method, along with major advantages, also has a number of serious drawbacks.

As the peptide chains grow, it may happen that in some of them, say, the third “floor” is missing - the third amino acid in a row: its molecule will not reach the junction, getting stuck somewhere along the road in the structural “wilds” solid polymer. And then, even if all the other amino acids, starting with the fourth, line up in the proper order, this will no longer save the situation. The resulting polypeptide in its composition and, consequently, in its properties will have nothing to do with the substance obtained. The same thing happens as when dialing a phone number; it is worth skipping one digit - and the fact that we have typed all the rest correctly will no longer help us. It is practically impossible to separate such false chains from the “real” ones, and the drug turns out to be clogged with impurities. In addition, it turns out that the synthesis cannot be carried out on any resin - it must be carefully selected, since the properties of the growing peptide depend to some extent on the properties of the resin. Therefore, all stages of protein synthesis must be approached as carefully as possible.

DNA protein synthesis, video

And in the end, we bring to your attention an educational video on how protein synthesis occurs in DNA molecules.

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:

1. Transcription.

2. 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 pro-mRNA gene 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 attachment 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 tRNA 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.

Protein synthesis in a cell

The main question of genetics is the question of protein synthesis. Summarizing data on the structure and synthesis of DNA and RNA, Crick in 1960. proposed a matrix theory of protein synthesis based on 3 provisions:

1. Complementarity of nitrogenous bases of DNA and RNA.

2. The linear sequence of the location of genes in a DNA molecule.

3. The transfer of hereditary information can only occur from nucleic acid to nucleic acid or to protein.

From protein to protein, the transfer of hereditary information is impossible. Thus, only nucleic acids can be a template for protein synthesis.

Protein synthesis requires:

1. DNA (genes) on which molecules are synthesized.

2. RNA - (i-RNA) or (m-RNA), r-RNA, t-RNA

In the process of protein synthesis, the stages are distinguished: transcription and translation.

Transcription- census (rewriting) of information about the nucleic structure from DNA to RNA (t-RNA, and RNA, r-RNA).

Reading of hereditary information begins with a certain section of DNA, which is called a promoter. The promoter is located before the gene and includes about 80 nucleotides.

On the outer chain of the DNA molecule, i-RNA (intermediate) is synthesized, which serves as a matrix for protein synthesis and is therefore called matrix. It is an exact copy of the sequence of nucleotides on the DNA chain.

There are regions in DNA that do not contain genetic information (introns). The sections of DNA that contain information are called exons.

There are special enzymes in the nucleus that cut out introns, and exon fragments are “spliced” together in a strict order into a common thread, this process is called “splicing”. During splicing, mature mRNA is formed, which contains the information necessary for protein synthesis. Mature mRNA (matrix RNA) passes through the pores of the nuclear membrane and enters the channels of the endoplasmic reticulum (cytoplasm) and here it combines with ribosomes.

Broadcast- the sequence of nucleotides in i-RNA is translated into a strictly ordered sequence of amino acids in the synthesized protein molecule.

The translation process includes 2 stages: the activation of amino acids and the direct synthesis of a protein molecule.

One mRNA molecule binds to 5-6 ribosomes to form polysomes. Protein synthesis occurs on the mRNA molecule, with ribosomes moving along it. During this period, amino acids in the cytoplasm are activated by special enzymes secreted by enzymes secreted by mitochondria, each of them with its own specific enzyme.

Almost instantly, amino acids bind to another type of RNA - a low molecular weight soluble RNA that acts as an amino acid carrier to the mRNA molecule and is called transport (t-RNA). tRNA carries amino acids to the ribosomes to a certain place, where by this time the mRNA molecule is located. Then the amino acids are linked together by peptide bonds and a protein molecule is formed. By the end of protein synthesis, the molecule is gradually shedding from mRNA.

On one mRNA molecule, 10-20 protein molecules are formed, and in some cases much more.

The most obscure question in protein synthesis is how tRNA finds the appropriate mRNA site to which the amino acid it brings must be attached.

The sequence of arrangement of nitrogenous bases in DNA, which determines the arrangement of amino acids in the synthesized protein, is the genetic code.

Since the same hereditary information is “recorded” in nucleic acids by four characters (nitrogenous bases), and in proteins by twenty (amino acids). The problem of the genetic code is reduced to establishing a correspondence between them. Geneticists, physicists, and chemists played an important role in deciphering the genetic code.

To decipher the genetic code, first of all, it was necessary to find out what is the minimum number of nucleotides that can determine (encode) the formation of one amino acid. If each of the 20 amino acids were encoded by one base, then DNA would have to have 20 different bases, but in fact there are only 4. Obviously, the combination of two nucleotides is also not enough to code for 20 amino acids. It can only code for 16 amino acids 4 2 = 16.

Then it was proposed that the code includes 3 nucleotides 4 3 = 64 combinations and, therefore, is able to encode more than enough amino acids to form any proteins. This combination of three nucleotides is called a triplet code.

The code has the following properties:

1. The genetic code is triplet(each amino acid is encoded by three nucleotides).

2. Degeneracy- one amino acid can be encoded by several triplets, the exception is tryptophan and methionine.

3. In codons for one amino acid, the first two nucleotides are the same, and the third one changes.

4.Non-overlapping– triplets do not overlap each other. One triplet cannot be part of another; each of them independently encodes its own amino acid. Therefore, any two amino acids can be nearby in the polypeptide chain and any combination of them is possible, i.e. in the base sequence ABCDEFGHI, the first three bases code for 1 amino acid (ABC-1), (DEF-2), etc.

5.Universal, those. in all organisms, the codons for certain amino acids are the same (from chamomile to humans). The universality of the code testifies to the unity of life on earth.

6. Kneeling- the coincidence of the arrangement of codons in mRNA with the order of amino acids in the synthesized polypeptide chain.

A codon is a triplet of nucleotides that codes for 1 amino acid.

7. Pointless It does not code for any amino acid. Protein synthesis at this site is interrupted.

In recent years, it has become clear that the universality of the genetic code is violated in mitochondria, four codons in mitochondria have changed their meaning, for example, the codon UGA - answers to tryptophan instead of "STOP" - the cessation of protein synthesis. AUA - corresponds to methionine - instead of "isoleucine".

The discovery of new codons in mitochondria may serve as evidence that the code evolved and that it did not immediately become so.

Let hereditary information from a gene to a protein molecule can be expressed schematically.

DNA - RNA - protein

The study of the chemical composition of cells showed that different tissues of the same organism contain a different set of protein molecules, although they have the same number of chromosomes and the same genetic hereditary information.

We note the following circumstance: despite the presence in each cell of all the genes of the whole organism, very few genes work in a single cell - from tenths to several percent of the total number. The rest of the areas are "silent", they are blocked by special proteins. This is understandable, why, for example, hemoglobin genes work in a nerve cell? Just as the cell dictates which genes to be silent and which to work, it must be assumed that the cell has some kind of perfect mechanism that regulates the activity of genes, which determines which genes should be active at a given moment and which should be in an inactive (repressive) state. Such a mechanism, according to the French scientists F. Jacobo and J. Monod, was called induction and repression.

Induction- stimulation of protein synthesis.

Repression- inhibition of protein synthesis.

Induction ensures the work of those genes that synthesize a protein or enzyme, and which is necessary at this stage of the cell's life.

In animals, cell membrane hormones play an important role in the process of gene regulation; in plants, environmental conditions and other highly specialized inductors.

Example: when thyroid hormone is added to the medium, a rapid transformation of tadpoles into frogs takes place.

Milk sugar (lactose) is necessary for the normal functioning of the E (Coli) bacterium. If the environment in which the bacteria are located does not contain lactose, these genes are in a repressive state (i.e. they do not function). The lactose introduced into the medium is an inductor, including the genes responsible for the synthesis of enzymes. After the removal of lactose from the medium, the synthesis of these enzymes stops. Thus, the role of a repressor can be played by a substance that is synthesized in the cell, and if its content exceeds the norm or it is used up.

Different types of genes are involved in protein or enzyme synthesis.

All genes are in the DNA molecule.

Their functions are not the same:

- structural - genes that affect the synthesis of an enzyme or protein are located in the DNA molecule sequentially one after another in the order of their influence on the course of the synthesis reaction, or you can also say structural genes - these are genes that carry information about the amino acid sequence.

- acceptor- genes do not carry hereditary information about the structure of the protein, they regulate the work of structural genes.

Before a group of structural genes is a common gene for them - operator, and in front of him promoter. In general, this functional group is called feathered.

The entire group of genes of one operon is included in the synthesis process and is switched off from it simultaneously. Turning on and off structural genes is the essence of the entire process of regulation.

The function of switching on and off is performed by a special section of the DNA molecule - gene operator. The gene operator is the starting point of protein synthesis or, as they say, "reading" of genetic information. further in the same molecule at some distance is a gene - a regulator, under the control of which a protein called a repressor is produced.

From all of the above, it can be seen that protein synthesis is very difficult. The cell genetic system, using the mechanisms of repression and induction, can receive signals about the need to start and end the synthesis of a particular enzyme and carry out this process at a given rate.

The problem of regulating the action of genes in higher organisms is of great practical importance in animal husbandry and medicine. Establishment of the factors regulating protein synthesis would open up wide possibilities for controlling ontogeny, creating highly productive animals, as well as animals resistant to hereditary diseases.

Test questions:

1. Name the properties of genes.

2. What is a gene?

3. What is the biological significance of DNA, RNA.

4. Name the stages of protein synthesis

5. List the properties of the genetic code.

Life is the process of existence of protein molecules. This is how many scientists express it, who are convinced that protein is the basis of all living things. These judgments are absolutely correct, because these substances in the cell have the largest number of basic functions. All other organic compounds play the role of energy substrates, and energy is again needed for the synthesis of protein molecules.

Stage characterization of protein biosynthesis

The structure of a protein is encoded in nucleic or RNA) in the form of codons. This is hereditary information that is reproduced every time a cell needs a new protein substance. The beginning of biosynthesis is in the nucleus about the need to synthesize a new protein with already given properties.

In response to this, a region of nucleic acid is despiralized, where its structure is encoded. This place is duplicated by messenger RNA and transferred to ribosomes. They are responsible for building a polypeptide chain based on a matrix - messenger RNA. Briefly, all stages of biosynthesis are presented as follows:

• transcription (the stage of doubling a piece of DNA with an encoded protein structure);
• processing (stage of information RNA formation);
• translation (protein synthesis in a cell based on messenger RNA);
• post-translational modification ("maturation" of the polypeptide, formation of its bulk structure).

Nucleic acid transcription

All protein synthesis in a cell is carried out by ribosomes, and information about molecules is contained in nucleic or DNA). It is located in the genes: each gene is a specific protein. Genes contain information about the amino acid sequence of a new protein. In the case of DNA, the removal of the genetic code is carried out in this way:

• release of the nucleic acid site from histones begins, despiralization occurs;
• DNA polymerase doubles the section of DNA that stores the gene for the protein;
• the doubled section is a precursor of messenger RNA, which is processed by enzymes to remove non-coding inserts (on its basis, mRNA is synthesized).

Based on the messenger RNA, mRNA is synthesized. It is already a matrix, after which protein synthesis in the cell occurs on ribosomes (in the rough endoplasmic reticulum).

Ribosomal protein synthesis

Messenger RNA has two ends, which are arranged as 3`-5`. Reading and synthesis of proteins on ribosomes begins at the 5'end and continues to the intron, a region that does not encode any of the amino acids. It happens like this:

• messenger RNA is "strung" on the ribosome, attaches the first amino acid;
• the ribosome shifts along the messenger RNA by one codon;
• transfer RNA provides the desired (encoded by the given mRNA codon) alpha-amino acid;
• the amino acid joins the starting amino acid to form a dipeptide;
• then the mRNA is again shifted by one codon, an alpha-amino acid is brought up and attached to the growing peptide chain.

Once the ribosome reaches the intron (non-coding insert), the messenger RNA simply moves on. Then, as the messenger RNA advances, the ribosome again reaches the exon - the site whose nucleotide sequence corresponds to a specific amino acid.

From this point, the addition of protein monomers to the chain begins again. The process continues until the next intron appears or until the stop codon. The latter stops the synthesis of the polypeptide chain, after which it is considered completed and the stage of postsynthetic (post-translational) modification of the molecule begins.

Post-translational modification

After translation, protein synthesis occurs in smooth cisterns. The latter contains a small number of ribosomes. In some cells, they may be completely absent in the RES. Such areas are needed to form first a secondary, then a tertiary or, if programmed, a quaternary structure.

All protein synthesis in the cell occurs with the expenditure of a huge amount of ATP energy. Therefore, all other biological processes are needed to maintain protein biosynthesis. In addition, some of the energy is needed for the transfer of proteins in the cell by active transport.

Many of the proteins are transferred from one location in the cell to another for modification. In particular, post-translational protein synthesis occurs in the Golgi complex, where a carbohydrate or lipid domain is attached to a polypeptide of a certain structure.