The process of synthesis and RNA from a DNA molecule. Protein synthesis in the cell - description, functions of the process. Biological significance of synthesis

22.06.2021

Synthesis of DNA, RNA and proteins

The topic of today's lecture is the synthesis of DNA, RNA and proteins. DNA synthesis is called replication or reduplication (doubling), RNA synthesis is called transcription (rewriting from DNA), protein synthesis carried out by a ribosome on messenger RNA is called translation, that is, we translate from the language of nucleotides to the language of amino acids.

We will try to give short review of all these processes, while at the same time dwelling in more detail on the molecular details, in order to give you an idea of the depth to which this subject has been studied.

DNA replication

The DNA molecule, consisting of two helices, doubles during cell division. DNA doubling is based on the fact that when the strands are unwoven, a complementary copy can be added to each strand, thus obtaining two strands of a DNA molecule that copies the original one.

One of the DNA parameters is also indicated here, this is the helix pitch, for each full turn there are 10 base pairs, note that one step is not between the nearest protrusions, but after one, since DNA has a minor groove and a large one. Proteins that recognize the nucleotide sequence interact with DNA through the major groove. The spiral pitch is 34 angstroms, and the diameter double helix– 20 angstroms.

DNA replication is carried out by the enzyme DNA polymerase. This enzyme is capable of extending DNA only at the 3΄– end. You remember that the DNA molecule is antiparallel, its different ends are called the 3΄ end and the 5΄ end. When new copies are synthesized on each strand, one new strand elongates in the 5΄ to 3΄ direction, and the other in the 3΄ to 5-end direction. However, DNA polymerase cannot extend the 5΄ end. Therefore, the synthesis of one strand of DNA, the one that grows in the direction “convenient” for the enzyme, occurs continuously (it is called the leading or leading strand), and the synthesis of the other strand is carried out in short fragments (they are called Okazaki fragments in honor of the scientist who described them). Then these fragments are stitched together, and such a thread is called lagging; in general, the replication of this thread is slower. The structure that forms during replication is called a replication fork.

If we look at the replicating DNA of a bacterium, and this can be observed in an electron microscope, we will see that it first forms an “eye”, then it expands, and eventually the entire circular DNA molecule is replicated. The replication process occurs with great accuracy, but not absolute. Bacterial DNA polymerase makes mistakes, that is, it inserts the wrong nucleotide that was in the template DNA molecule, with a frequency of approximately 10-6. In eukaryotes, enzymes work more accurately, since they are more complex; the level of errors during DNA replication in humans is estimated as 10-7 - 10 -8. The accuracy of replication may vary in different parts of the genome; there are areas with an increased frequency of mutations and there are more conservative areas where mutations occur rarely. And in this we should distinguish between two different processes: the process of the appearance of a DNA mutation and the process of fixation of the mutation. After all, if mutations are fatal, they will not appear in the next generations, and if the error is not fatal, it will take hold in the next generations, and we will be able to observe and study its manifestation. Another feature of DNA replication is that DNA polymerase cannot begin the synthesis process itself; it needs a “primer.” Typically, an RNA fragment is used as such a primer. If we are talking about the bacterial genome, then there is a special point called the origin of replication; at this point there is a sequence that is recognized by the enzyme that synthesizes RNA. It belongs to the class of RNA polymerases, and in this case is called primase. RNA polymerases do not require primers, and this enzyme synthesizes a short fragment of RNA - the very “primer” with which DNA synthesis begins.

Transcription

The next process is transcription. Let's look at it in more detail.

Transcription is the synthesis of RNA on DNA, that is, the synthesis of a complementary strand of RNA on a DNA molecule is carried out by the enzyme RNA polymerase. Bacteria, for example, Escherichia coli, have one RNA polymerase, and all bacterial enzymes are very similar to each other; in higher organisms (eukaryotes) there are several enzymes, they are called RNA polymerase I, RNA polymerase II, RNA polymerase III, they also have similarities with bacterial enzymes, but are more complex in structure, they contain more proteins. Each type of eukaryotic RNA polymerase has its own special functions, that is, it transcribes a specific set of genes. The DNA strand that serves as a template for RNA synthesis during transcription is called sense or template. The second strand of DNA is called non-coding (the RNA complementary to it does not encode proteins, it is “senseless”).

The transcription process can be divided into three stages. The first stage is the initiation of transcription - the beginning of the synthesis of the RNA strand, the first bond between nucleotides is formed. Then the thread grows, its lengthening occurs - elongation, and when the synthesis is completed, termination occurs, the release of the synthesized RNA. At the same time, RNA polymerase “gets off” the DNA and is ready for a new round of transcription. Bacterial RNA polymerase has been studied in great detail. It consists of several protein subunits: two α-subunits (these are small subunits), β- and β΄-subunits (large subunits) and an ω-subunit. Together they form the so-called minimal enzyme, or core enzyme. The σ subunit can attach to this core enzyme. The σ subunit is necessary for the initiation of RNA synthesis and the initiation of transcription. After initiation has taken place, the σ-subunit is disconnected from the complex, and further work (chain elongation) is carried out by the core enzyme. When attached to DNA, the σ subunit recognizes the site where transcription should begin. It's called a promoter. A promoter is a sequence of nucleotides indicating the beginning of RNA synthesis. Without the σ subunit, the core enzyme cannot recognize the promoter. The σ subunit together with the core enzyme is called a complete enzyme, or holoenzyme.

Having contacted DNA, namely the promoter recognized by the σ-subunit, the holoenzyme unwinds the double-stranded helix and begins RNA synthesis. The region of untwisted DNA is the transcription initiation point, the first nucleotide to which a ribonucleotide must be complementarily attached. Transcription is initiated, the σ subunit leaves, and the core enzyme continues elongation of the RNA chain. Then termination occurs, the core enzyme is released and becomes ready for a new cycle of synthesis.

How does transcription elongation occur?

The RNA is extended at the 3΄ end. With the addition of each nucleotide, the core enzyme takes a step along the DNA and shifts one nucleotide. Since everything in the world is relative, we can say that the core enzyme is motionless, and DNA is “dragged” through it. It is clear that the result will be the same. But we will talk about movement along the DNA molecule. The size of the protein complex that makes up the core enzyme is 150 Å. The dimensions of RNA polymerase are 150×115×110Ǻ. That is, it is such a nanomachine. The speed of RNA polymerase is up to 50 nucleotides per second. The complex of the core enzyme with DNA and RNA is called the elongation complex. It contains a DNA-RNA hybrid. That is, this is the region where DNA is paired with RNA, and the 3΄ end of the RNA is open for further growth. The size of this hybrid is 9 base pairs. The untwisted section of DNA occupies approximately 12 base pairs.

RNA polymerase binds to DNA upstream of the untwisted region. This region is called the forward DNA duplex and is 10 base pairs in size. The polymerase is also bound to a longer piece of DNA called the back duplex DNA. The size of messenger RNAs that synthesize RNA polymerases in bacteria can reach 1000 nucleotides or more. In eukaryotic cells, the size of synthesized DNA can reach 100,000 or even several million nucleotides. True, it is not known whether they exist in such sizes in cells, or whether they can be processed during the synthesis process.

The elongation complex is quite stable, because he has a lot of work to do. That is, it will not “fall off” with DNA on its own. It is capable of moving through DNA at speeds of up to 50 nucleotides per second. This process is called movement (or translocation). The interaction of DNA with RNA polymerase (core enzyme) does not depend on the sequence of this DNA, unlike the σ subunit. And the core enzyme, upon passing certain termination signals, completes DNA synthesis.

Let's look at it in more detail molecular structure core enzyme. As mentioned above, the core enzyme consists of α- and β-subunits. They are connected in such a way that they form a kind of “mouth” or “claw”. The α-subunits are located at the base of this “claw” and perform a structural function. They apparently do not interact with DNA and RNA. The ω subunit is a small protein that also performs a structural function. The bulk of the work comes from the β- and β΄-subunits. In the figure, the β΄ subunit is shown at the top and the β subunit at the bottom.

Inside the “mouth,” called the main channel, is the active site of the enzyme. This is where nucleotides combine and a new bond is formed during RNA synthesis. The main channel in RNA polymerase is where DNA resides during elongation. This structure also has a so-called secondary channel on the side, through which nucleotides are supplied for RNA synthesis.

The distribution of charges on the surface of RNA polymerase ensures its functions. The distribution is very logical. The nucleic acid molecule is negatively charged. Therefore, the cavity of the main channel, where negatively charged DNA should be held, is lined with positive charges. The surface of RNA polymerase is made with negatively charged amino acids to prevent DNA from sticking to it.

RNA polymerase works like a molecular machine, and it has various parts, each of which performs a different function. For example, the part of the β΄ subunit hanging over the “mouth” holds the anterior DNA duplex. This part is called the "flap". After binding to the DNA, the flap moves down a distance of 30 angstroms and clamps the DNA so that it cannot fall out during transcription.

Inside the “mouth” there is the active center of RNA polymerase, that is, the place where the complementary interaction of the ribonucleotide triphosphate entering through the side channel with the DNA template directly occurs. If the newly arrived nucleotide is complementary to the matrix, then it is enzymatically attached to the free 3" end of the RNA. By nature, the reaction of the formation of a new bond in RNA is a nucleophilic substitution reaction. Two magnesium ions are involved in it. One ion is constantly located in the active center, and the second a magnesium ion enters with the nucleotide and after the formation of a new bond between the ribonucleotides leaves, then a new nucleotide arrives with its new magnesium ion.

Upon exit from RNA polymerase, the DNA-RNA hybrid must be unwoven. This involves a structure called a "spike".

Translocation, that is, the movement of RNA polymerase along a DNA strand, involves an α-helical structure protruding from the bottom up from the β-subunit.

How did they find out which part of the enzyme plays which role? Molecular biologists proceed as follows. They remove part of the protein sequence and see what function is missing. It was shown that if you throw away a fragment of the clamp (when it was thrown away, they did not yet know that it was holding DNA), then the DNA would not hold. The same result is obtained if the DNA of the upstream duplex is removed. The remaining part - the RNA-DNA hybrid and the rear duplex - turns out to be weakly associated with RNA polymerase.

It is known that magnesium coordinates the bond between the phosphates of the growing DNA molecule and the phosphates of newly entering nucleotides. In this case, a sequence of reactions occurs called nucleophilic substitution reactions. It is known how the connections within this complex change. The new nucleotide arrives bound to yet another magnesium ion. The new nucleotide thus interacts with the growing DNA strand. At the end of the reaction, a second magnesium ion is removed from the active site of the enzyme.

RNA polymerase is a representative of molecular machines. In addition to the fact that the shutter is lowered at the beginning of DNA synthesis, the conformation of other parts of RNA synthase changes; cyclic changes occur in it during the growth of the RNA chain, which are not as strong as at the beginning of chain synthesis. At the beginning, the shutter is lowered by 30 Ǻ, and with each step of the enzyme, the DNA is extended by one nucleotide. An element of RNA polymerase F-helix (alpha-helical structure running from the beta subunit upward into the main channel) is involved in the movement along DNA. In this case, the F-helix bends, moves along with the RNA-DNA complex, is freed from them and straightens again. The F-helix moves 3.4 Å in one step. This is exactly the step of RNA polymerase.

Changes in the conformation of various parts of RNA polymerase occur due to changes in potential energy, which is associated with electrostatic and hydrophobic interactions. We can draw the following analogy. If we take a tray with a pile of apples, then after we shake this tray, the apples will scatter in an even layer on the tray. At the same time, their potential energy associated with the action of gravity will change. If the RNA synthase molecule is “shocked” (and it is “shocked”, like all other molecules in the cell, by Brownian motion), then it will begin to take on a conformation with a lower potential energy. That is, the source of motion of a molecular machine is the energy of thermal motion of its individual components, and the design of the machine is such that this motion leads to the desired result. In this case, the molecular machine consumes energy, which is mainly used to change the state of certain bonds.

Now let's look at the initiation of transcription. As already mentioned, initiation is carried out with the participation of the σ-subunit. It interacts with a DNA structure called a promoter. It has the same structure in E. coli. Ten nucleotides before the initiation point there is a TATA box. This is not necessarily the sequence, but it is the “ideal” sequence for interaction with the σ-subunit, that is, the one with which transcription is initiated most efficiently. Substitution of individual nucleotides in this sequence reduces the efficiency of transcription initiation. About 35 more nucleotides before this is a structure called “-35”. This sequence is also recognized by the σ subunit. This structure (a combination of sequences “–10” and “–35”) was called a classical promoter, because she was described first. But it turned out that the structure of the promoter may be different. This variant includes the same TATA box, but does not have the “-35” sequence, but there are two additional nucleotides, and this is enough for the σ subunit to recognize the promoter.

This structure is called an extended promoter. The σ-subunit of RNA polymerase sits on the promoter in DNA and interacts with parts of the promoter using different parts of the protein molecule. It is recognized by the σ-subunit through the major groove of DNA. After the σ-subunit in the core enzyme binds to the promoter, the DNA in this area begins to melt (DNA strands unravel). In the last lecture it was discussed that in pairs A-T communications breaks between nucleotides more easily than in G-C pair, since the latter contains 3 hydrogen bonds, and the first - two. The promoter contains A-T pairs, so it melts quite easily. And then RNA synthesis begins, the growing RNA chain pushes out the σ-subunit and other changes occur that cause the dissociation of the σ-subunit from the core enzyme.

Now let's give an example of how functions are studied different parts squirrel. If you cut off a small piece of protein and see how the functions of the protein have changed, you can understand what the functions of the cut piece were. In our case we did it differently. We took two DNA polymerases, one was taken from Escherichia coli, and the other from a heat-loving bacterium (thermophilic), which grows at 800 C (in laboratory conditions they are grown in a flask, which is in a thermostat in almost boiling water, in natural conditions they live in hot springs, there are those that can live at 98°C), therefore the optimum operation of its RNA polymerase and σ-subunit is 80°C, (in the figure the σ-subunit of a thermophilic bacterium is shown in red, and that of E. coli is shown in yellow), and in the intestinal rods work most effectively at human body temperature (since they live in the intestines). Its σ-subunit has only four parts, they cut the protein and stitched this σ-subunit with a piece from the σ-subunit of a thermophilic bacterium. And then different pieces from the thermophilic bacterium were inserted, replacing different fragments of the σ-subunit with them. Then we looked at whether the resulting hybrid protein was active at 200 C or not. The thermophilic bacterium does not work at this temperature, it is too cold for it, and E. coli is active. The figure shows that at a given temperature, only that combination works in which the σ-subunit has the first and second parts from Escherichia coli, and the third and fourth from thermophilic bacteria. Thus, it is concluded that the operating temperature of the σ-subunit is determined by the first and second components.

In fact, it is not the protein that is cut, but the DNA, then pieces of DNA from different bacteria are stitched together and then introduced into the bacterium, where, when this part of the DNA is activated, a hybrid protein is synthesized. This technology refers to genetic engineering and was developed in the 70s.

Another feature of transcription is that the core enzyme of the bacterial cell is the same, but the σ-subunits can be different. Escherichia coli has only 7 σ subunits, and they recognize different promoters. Why is this necessary? If a cell urgently needs to switch protein synthesis from one group of genes to another, it can use different σ subunits. For example, there are heat shock genes, if E. coli is heated to a state where it becomes very difficult for it to live, it turns on an emergency system of resistance to heat shock, resistance to the destruction that has occurred in the cell. This system includes a set of genes that should not normally work; these genes have their own special promoter in front of them. And then another σ-subunit, not the main one, is synthesized and activates these genes. That is, a change in subunit is a change in the gene program. This is a way to regulate the functioning of genes.

Broadcast

Let's move on to translation - protein synthesis. It is carried out by ribosomes. The ribosome consists of two subparticles: large and small.

Each subparticle consists of several dozen proteins, each of which has already been studied; it is known how each protein is folded into the subparticle. When studying proteins, the method of electrophoresis is used, that is, in an electric field in a special gel or a special carrier, protein molecules are separated depending on their charge and molecular weight, that is, under the influence of the field they begin to move and can move away from each other at different distances. Another method for separating proteins is chromatography; as a result of this method, spots are obtained on the carrier, each of which corresponds to a separate protein.

The proteins in the ribosome are held together by a scaffold made of ribosomal RNA. The formation of a ribosome begins when the ribosomal RNA folds and proteins begin to adhere to it in a certain order. The figure shows ribosomal RNA. In it, self-complementary sections of the RNA strand pair to form hairpins (secondary structure), and then the RNA folds (tertiary RNA structure), forming a framework of subparticles.

Another type of RNA involved in protein synthesis is transfer RNA (tRNA). tRNA molecules are relatively small (compared to ribosomal or messenger RNA). All tRNAs have a common secondary structure. Due to the pairing of complementary sections of the tRNA molecule, three “stems” are formed with loops at the ends and one “stem” formed by the 5" and 3" ends of the tRNA molecule (sometimes an additional fifth loop is formed). The image of this structure is similar to a cross or clover leaf. The “head” on this sheet is represented by an anticodon loop; here is the anticodon - those three nucleotides that complementarily interact with the codon in the mRNA. The stem opposite the anticodon loop, formed by the ends of the molecule, is called the acceptor stem - the corresponding amino acid is added here. Recognize suitable friend Between tRNA and amino acids there are special enzymes called aminoacyl-tRNA synthetases. Each amino acid has its own aminoacyl-tRNA synthetase.

The ribosome contains messenger RNA (mRNA). The anticodon of the transfer RNA, on which the amino acid residue hangs, is complementarily associated with the codon (three nucleotides) of the mRNA. The figure shows this structure (tRNA together with an amino acid called aminocil-tRNA).

The process of translation, as well as the process of transcription, is associated with movement along the nucleic acid molecule; the difference is that the ribosome steps three nucleotides, while RNA polymerase moves one.

Aminocil t-RNA enters the ribosome, complementarily binding to the codon of the mRNA, then a reaction occurs in which amino acid residues bind to each other, and the t-RNA is removed.

The “dictionary” for translating from the language of nucleotides to the language of amino acids is called the genetic code. There are 20 amino acids, 4 nucleotides, the number of combinations of 4 by 2 = 16, and 20 amino acids, so the encoding is not two, but three-letter, each three is called a codon. Each amino acid is encoded by three nucleotides in mRNA (which in turn is encoded by DNA).

In the table in the figure, the side columns encode the left and right letter of the codon, the top line – the middle one. For example, the AUG codon codes for the amino acid methionine. The number of combinations of 4 by 3 = 64, that is, some amino acids are encoded by several codons. Three codons do not code for any amino acid; they are called stop codons. When they get caught in the mRNA, the ribosome stops working and the finished polypeptide chain is thrown out.

The genetic code table was compiled in the 60s. The beginning was made by Nirenberg and Mattei. They tried to carry out in vitro experiments on cell extracts to which artificial RNA templates were added. At that time, it was believed that single nucleotide codons (UUU or AAA) did not code for amino acids. Nirenberg and Mattei used polyU-RNA (that is, consisting only of uracils) as a control in their experiments, but it was in this test tube that the reaction took place. It became clear that the UUU codon encodes the amino acid phenylalanine. A table of the genetic code was then compiled.

The genetic code is universal. It is the same for all microorganisms. There are slight differences in the genetic code of mitochondria.

The genetic code is a table of codons corresponding to amino acids. When journalists write that the human genetic code has recently been deciphered, this is a gross terminological error. The human genetic code was deciphered at the same time as all other living beings - in the 60s of the 20th century. The human genome, that is, the complete sequence of nucleotides of all DNA molecules, has recently been deciphered.

The lecture uses images of RNA polymerase provided by Andrey Kulbachinsky (Institute of Molecular Genetics, Russian Academy of Sciences).

Bibliography

To prepare this work, materials from the site http://bio.fizteh.ru were used

Questions inside the paragraph: How are the proteins needed by the cell and the body formed?

To maintain body weight and growth of any organism, it requires intake from external environment proteins, fats and carbohydrates, vitamins, mineral salts, water. Metabolism begins with the intake of nutrients and gases. For example, at the initial stage of metabolism, processes of protein breakdown take place under the action of enzymes in special organs (stomach, small intestine) or structures (in single-celled organisms) into amino acids with the release of energy. The next stage of exchange is chemical reactions synthesis of own proteins, enzymes, hormones and other protein structures using energy.

What is the structure of a nucleotide? A nucleotide is a monomer of nucleic acids that consists of three components: a nitrogenous base, a five-carbon sugar, and a phosphoric acid residue.

Remember which nitrogenous bases are called complementary? Complementarity is a universal principle of correspondence that underlies the structure and functioning of DNA in a cell. There are four nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). The principle of complementarity is ensured by hydrogen bonds between nitrogenous bases. So thymine corresponds to adenine, since it forms the same number of hydrogen bonds as adenine. By the same principle, guanine is complementary to cytosine. A=T, G=C.

Page 62. Questions and tasks after §

1. What conditions are necessary for protein synthesis?

Each cell has its own set of proteins. Protein synthesis requires the following:

Building material – amino acids;

Information about the order of amino acids in the polypeptide chain;

Amino acid carrier molecules;

Cell organelles in which protein molecules are assembled - ribosomes;

Energy.

2. What process is called transcription?

The process of synthesizing an RNA molecule whose nucleotide sequence exactly matches the nucleotide sequence of the DNA template. This is the first stage of protein biosynthesis. Does this process take place in the cell nucleus?

3. What is the genetic code?

This is a system for recording information about the primary structure of a protein, represented by a certain combination of nucleotides and the sequence of their location in DNA and m-RNA molecules. For example, the triplet HCC encodes the amino acid alanine, GHC – glycine, etc. There are 64 types of triplets. Three triplets out of 64 do not encode any amino acids, but determine the cessation of protein synthesis, which is why they are called stop codons. The finished m-RNA is sent into the cell cytoplasm from the nucleus through the pores.

4. What role do tRNA molecules play in protein biosynthesis?

tRNA is transfer RNA. It transports amino acids. Amino acids are found in the cytoplasm. Each amino acid must be delivered to the place it should occupy in the protein molecule. The delivery function is performed by transfer RNAs. Each tRNA can carry only one of the 20 amino acids used in protein synthesis. Transfer RNAs recognize “their” amino acids and attach them to the free end of the molecule. This occurs with the participation of enzymes and ATP energy.

5. How is information about the sequence of amino acids in a protein contained in m-RNA implemented into the protein?

This process is implemented in a stage called translation. The ribosome has a functional center where only two triplets of m-RNA can simultaneously be present. it is in it that the information is translated (from the Latin “translatio” - transfer), recorded on m-RNA, into the “language” of amino acids, as a result of which the protein molecule grows. tRNAs are sequentially attached according to the principle of complementarity to their mRNA triplets. The position of each tRNA is determined by an anticodon, a triplet located at the front of the molecule. amino acids are connected by peptide bonds in the order written on the synthesis matrix - messenger RNA. the polypeptide chain increases as m-RNA moves through the ribosome. When the mRNA stop codon enters the ribosome, the assembly of the peptide chain ends. Subsequently, the protein acquires the necessary structure: it spirals or twists into a globule and is sent to the desired compartment of the cell to perform its functions.

6. Is it correct to say that the reactions of plastic and energy metabolism in a cell are inextricably linked and occur continuously? Explain your answer.

Yes. All transformations of substances in the cell are divided into two metabolic processes - plastic and energetic. Energy metabolism is a set of reactions that break down complex organic compounds into simpler ones. This process supplies the body with many intermediate products for the synthesis of its own substances, and proceeds with the release of energy contained in complex substances and its accumulation in compounds characteristic of the cell ATP. Plastic metabolism is a set of reactions for the synthesis of complex substances from simpler ones using the energy of ATP, which was formed in energy metabolism. The energy released during energy metabolism reactions goes to plastic metabolism reactions - to the synthesis of substances. Conversely, energy metabolism occurs when active participation many enzymes synthesized by the cell in plastic metabolism reactions.

7. Explain the meaning of the terms “triplet”, “anticodon”, “translation”.

A triplet or codon is a section of m-RNA consisting of three sequentially located nucleotides. Anticodon is a tRNA triplet that is complementary to the mRNA. Translation is the ribosomal synthesis of protein from amino acids using m-RNA, which occurs in the cytoplasm of cells.

We will try to give a brief overview of all these processes, while going into more detail in molecular detail, so that you get an idea of the depth to which this subject has been studied.

DNA replication

One of the DNA parameters is also indicated here, this is the helix pitch, for each full turn there are 10 base pairs, note that one step is not between the nearest protrusions, but after one, since DNA has a minor groove and a large one. Proteins that recognize the nucleotide sequence interact with DNA through the major groove. The helix pitch is 34 angstroms, and the diameter of the double helix is 20 angstroms.

DNA replication is carried out by the enzyme DNA polymerase. This enzyme is capable of extending DNA only at the 3΄ end. You remember that the DNA molecule is antiparallel, its different ends are called the 3΄ end and the 5΄ end. As new copies are synthesized on each strand, one new strand elongates in the 5΄ to 3΄ direction, and the other in the 3΄ to 5-end direction. However, DNA polymerase cannot extend the 5΄ end. Therefore, the synthesis of one strand of DNA, the one that grows in the direction “convenient” for the enzyme, occurs continuously (it is called the leading or leading strand), and the synthesis of the other strand is carried out in short fragments (they are called Okazaki fragments in honor of the scientist who described them). Then these fragments are stitched together, and such a thread is called lagging; in general, the replication of this thread is slower. The structure that forms during replication is called a replication fork.

If we look at the replicating DNA of a bacterium, and this can be observed in an electron microscope, we will see that it first forms an “eye”, then it expands, and eventually the entire circular DNA molecule is replicated. The replication process occurs with great accuracy, but not absolute. Bacterial DNA polymerase makes mistakes, that is, it inserts the wrong nucleotide that was in the template DNA molecule, with approximately a frequency of 10 -6. In eukaryotes, enzymes work more accurately, since they are more complex; the level of errors during DNA replication in humans is estimated as 10 -7 - 10 -8. The accuracy of replication may vary in different parts of the genome; there are areas with an increased frequency of mutations and there are more conservative areas where mutations occur rarely. And in this we should distinguish between two different processes: the process of the appearance of a DNA mutation and the process of fixation of the mutation. After all, if mutations are fatal, they will not appear in the next generations, and if the error is not fatal, it will take hold in the next generations, and we will be able to observe and study its manifestation. Another feature of DNA replication is that DNA polymerase cannot begin the synthesis process itself; it needs a “primer.” Typically, an RNA fragment is used as such a primer. If we are talking about the bacterial genome, then there is a special point called the origin of replication; at this point there is a sequence that is recognized by the enzyme that synthesizes RNA. It belongs to the class of RNA polymerases, and in this case is called primase. RNA polymerases do not need primers, and this enzyme synthesizes a short fragment of RNA - the very “primer” with which DNA synthesis begins.

Transcription

The next process is transcription. Let's look at it in more detail.

The transcription process can be divided into three stages. First stage - initiation transcription - the beginning of the synthesis of the RNA strand, the first bond between nucleotides is formed. Then comes the thread building, its lengthening - elongation, and when the synthesis is complete, there is termination, release of synthesized RNA. At the same time, RNA polymerase “gets off” the DNA and is ready for a new round of transcription. Bacterial RNA polymerase has been studied in great detail. It consists of several protein subunits: two α-subunits (these are small subunits), β- and β΄-subunits (large subunits) and an ω-subunit. Together they form the so-called minimal enzyme, or core enzyme. The σ subunit can attach to this core enzyme. The σ subunit is necessary for the initiation of RNA synthesis and the initiation of transcription. After initiation has taken place, the σ-subunit is disconnected from the complex, and further work (chain elongation) is carried out by the core enzyme. When attached to DNA, the σ subunit recognizes the site where transcription should begin. It's called a promoter. A promoter is a sequence of nucleotides indicating the beginning of RNA synthesis. Without the σ subunit, the core enzyme cannot recognize the promoter. The σ subunit together with the core enzyme is called a complete enzyme, or holoenzyme.

How it happens transcription elongation?

RNA polymerase binds to DNA upstream of the untwisted region. This region is called the forward DNA duplex and is 10 base pairs in size. The polymerase is also bound to a longer piece of DNA called the back duplex DNA. The size of messenger RNAs that synthesize RNA polymerases in bacteria can reach 1000 nucleotides or more. In eukaryotic cells, the size of synthesized RNA can reach 100,000 or even several million nucleotides. True, it is not known whether they exist in such sizes in cells, or whether they can be processed during the synthesis process.

Let us examine in more detail the molecular structure of the core enzyme. As mentioned above, the core enzyme consists of α- and β-subunits. They are connected in such a way that they form a kind of “mouth” or “claw”. The α-subunits are located at the base of this “claw” and perform a structural function. They apparently do not interact with DNA and RNA. The ω subunit is a small protein that also has a structural function. The bulk of the work comes from the β- and β΄-subunits. In the figure, the β΄ subunit is shown at the top and the β subunit at the bottom.

Upon exit from RNA polymerase, the DNA-RNA hybrid must be unwoven. This involves a structure called a "spike".

Translocation, that is, the movement of RNA polymerase along a DNA strand, involves an α-helical structure protruding from the bottom up from the β-subunit.

Now let's stop at transcription initiation. As already mentioned, initiation is carried out with the participation of the σ-subunit. It interacts with a DNA structure called a promoter. It has the same structure in E. coli. Ten nucleotides before the initiation point there is a TATA box. This is not necessarily the sequence, but it is the “ideal” sequence for interaction with the σ-subunit, that is, the one with which transcription is initiated most efficiently. Substitution of individual nucleotides in this sequence reduces the efficiency of transcription initiation. About 35 more nucleotides before this is a structure called “-35”. This sequence is also recognized by the σ subunit. This structure (a combination of sequences “-10” and “-35”) was called a classical promoter, because she was described first. But it turned out that the structure of the promoter may be different. This variant includes the same TATA box, but does not have the “-35” sequence, but there are two additional nucleotides, and this is enough for the σ subunit to recognize the promoter.

This structure is called an extended promoter. The σ-subunit of RNA polymerase sits on the promoter in DNA and interacts with parts of the promoter using different parts of the protein molecule. It is recognized by the σ-subunit through the major groove of DNA. After the σ-subunit in the core enzyme binds to the promoter, the DNA in this area begins to melt (DNA strands unravel). At the last lecture, it was discussed that in an A-T pair, the bonds between nucleotides are broken more easily than in a G-C pair, since the latter contains 3 hydrogen bonds, and the first - two. The promoter contains A-T pairs, so it melts quite easily. And then RNA synthesis begins, the growing RNA chain pushes out the σ-subunit and other changes occur that cause the dissociation of the σ-subunit from the core enzyme.

Now let's give an example of how the functions of different parts of a protein are studied. If you cut off a small piece of protein and see how the functions of the protein have changed, you can understand what the functions of the cut piece were. In our case we did it differently. We took two DNA polymerases, one was taken from Escherichia coli, and the other from a heat-loving bacterium (thermophilic), which grows at 800 C (in laboratory conditions they are grown in a flask, which is in a thermostat in almost boiling water, in natural conditions they live in hot springs, there are those that can live at 98°C), therefore the optimum operation of its RNA polymerase and σ-subunit is 80°C, (in the figure the σ-subunit of a thermophilic bacterium is shown in red, and that of E. coli is shown in yellow), and in the intestinal rods work most effectively at human body temperature (since they live in the intestines). Its σ-subunit has only four parts, they cut the protein and stitched this σ-subunit with a piece from the σ-subunit of a thermophilic bacterium. And then different pieces from the thermophilic bacterium were inserted, replacing different fragments of the σ-subunit with them. Then we looked at whether the resulting hybrid protein was active at 200 C or not. The thermophilic bacterium does not work at this temperature, it is too cold for it, and E. coli is active. The figure shows that at a given temperature, only that combination works in which the σ-subunit has the first and second parts from Escherichia coli, and the third and fourth from thermophilic bacteria. Thus, it is concluded that the operating temperature of the σ-subunit is determined by the first and second components.

Broadcast

Let's move on to translation - protein synthesis. It is carried out by ribosomes. The ribosome consists of two subparticles: large and small.

Another type of RNA involved in protein synthesis is transfer RNA (tRNA). tRNA molecules are relatively small (compared to ribosomal or messenger RNA). All tRNAs have a common secondary structure. Due to the pairing of complementary sections of the tRNA molecule, three “stems” are formed with loops at the ends and one “stem” formed by the 5" and 3" ends of the tRNA molecule (sometimes an additional fifth loop is formed). The image of this structure is similar to a cross or clover leaf. The “head” on this sheet is represented by an anticodon loop; here is the anticodon - those three nucleotides that complementarily interact with the codon in the mRNA. The stem opposite the anticodon loop, formed by the ends of the molecule, is called the acceptor stem - the corresponding amino acid is added here. Special enzymes called aminoacyl-tRNA synthetases recognize matching tRNAs and amino acids. Each amino acid has its own aminoacyl-tRNA synthetase.

Aminocil t-RNA enters the ribosome, complementarily binding to the codon of the mRNA, then a reaction occurs in which amino acid residues bind to each other, and the t-RNA is removed.

In the table in the figure, the side columns encode the left and right letter of the codon, the top line - the middle one. For example, the AUG codon codes for the amino acid methionine. The number of combinations of 4 by 3 = 64, that is, some amino acids are encoded by several codons. Three codons do not code for any amino acid; they are called stop codons. When they get caught in the mRNA, the ribosome stops working and the finished polypeptide chain is thrown out.

The genetic code is universal. It is the same for all microorganisms. There are slight differences in the genetic code of mitochondria.

The lecture uses images of RNA polymerase provided by Andrey Kulbachinsky (Institute of Molecular Genetics, Russian Academy of Sciences).

Tissue nucleotide metabolism

The breakdown products of nucleoproteins and nucleic acids - nucleotides and nucleosides - undergo various transformations in organs and tissues.

Nucleotides - both purine and pyrimidine - are involved in the synthesis of nucleic acids in cell nuclei. DNA synthesis is carried out by enzymes - DNA polymerases, for which deoxyribonucleoside triphosphates serve as substrates.

DNA synthesis is accompanied by the release of pyrophosphate molecules in an amount corresponding to the number of nucleoside triphosphate molecules that entered into the reaction. The DNA (sample) and the newly synthesized polynucleotide together form double-stranded DNA. The scheme of this process can be presented as follows:

DNA biosynthesis scheme

The letter “d” before the symbol of nucleoside triphosphate or mononucleotides in the synthesized DNA molecule means that nucleotides participate in biosynthesis, in which pentose is represented by deoxyribose, i.e. deoxyribonucleotides. The formation of deoxyribonucleotides occurs as a result of a complex process of ribonucleotide reduction under the action of a heat-insensitive protein - thioredoxin.

The reduced form of thioredoxin is formed under the action of reductase (an enzyme of flavoprotene nature), the coenzyme of which is reduced nicotinamide adenine nucleotide phosphate (NADP) according to the scheme:

The resulting reduced form of tporedoxin is involved in the formation of deoxynucleotide diphosphates (dNDPs) by transferring reducing equivalents to nucleotide diphosphates (NDPs) that accept nucleotides:

The newly formed DNA and the DNA that served as a template can join at their ends under the influence of the DNA ligase enzyme and form a cyclic DNA structure.

Rice. 6. Cycle tricarboxylic acids(according to Lehninger)

RNA synthesis is carried out with the participation of polynucleotide phosphorylase, an enzyme that causes a reversible reaction of combining nucleoside phosphates in the presence of magnesium ions and the original RNA:

Scheme of RNA biosynthesis

The resulting polymer contains 3′-5′-phosphodiester bonds, which are cleaved by ribonuclease. The reaction is reversible and can be directed from right to left (towards the decomposition of the polymer) with increasing concentration of inorganic phosphate. The original RNA in this case does not play the role of a template, according to which the polynucleotide is synthesized. Most likely, the free OH group located in the terminal nucleotide of RNA is necessary for the attachment of subsequent nucleotides to it, regardless of the bases they contain.

Apparently, in an intact cell, polynucleotide phosphorylase is responsible not for the formation of a polymer, but for the cleavage of RNA. As for high-polymer RNA with a specific nucleotide sequence, its formation is carried out by RNA polymerase, the action of which is similar to the enzyme that synthesizes DNA. RNA polymerase is active in the presence of a DNA template, synthesizes RNA from nucleoside triphosphates and assembles them in a sequence predetermined by the DNA structure:

Scheme of polymer RNA synthesis

Simple organic molecules, such as amino acids or nucleotides, are associated with the formation of large polymers. Two amino acids are connected by a peptide bond, two nucleotides by a phosphodiester bond. Successive repetition of these reactions leads to the formation of linear polymers called polypeptides and polynucleotides, respectively. Polypeptides or proteins and polynucleotides in the form of DNA and RNA are considered the most important components. The universal “building blocks” that make up proteins are only 20 amino acids, and DNA and RNA molecules are built from only four types of polynucleotides. The cell contains both types of polynucleotides - DNA and RNA; During evolution, they specialized and work together, each performing its own function. The structure of polynucleotides is well suited for storing and transmitting information. The chemical differences between the two types of polynucleotides make them suited to different tasks. For example, DNA is a repository of genetic information, since its molecule is more stable than an RNA molecule. This is partly due to the fact that if RNA has two hydroxyl groups, this polynucleotide is more susceptible to hydrolysis.

Consequently, all information about the structure and functioning of any living organism is contained in encoded form in its genetic material, the basis of which is DNA. DNA is a long double-stranded polymer molecule. All the characteristics of the organism are “recorded” in this giant double-stranded molecule. The sequence of monomeric units (deoxyribonucleotides) in one of its chains corresponds (complementary) to the sequence of deoxyribonucleotides in the other. The principle of complementarity ensures the identity of the original and newly synthesized DNA molecules formed during duplication (replications).

The mechanism of complementary matrix copying occupies a central place in the processes of transferring information into biological systems. The genetic information of each cell is encoded in the sequence of bases of its polynucleotides, and this information

passed from generation to generation thanks to complementary™ base pairing.

Individual genetic elements with a strictly specific nucleotide sequence encoding functional proteins or RNA are genes. Genes are located in the cell nucleus, on chromosomes. Some genes have only 800 nucleotide pairs, others have about a million. A person has 80-90 thousand genes. Some genes, called structural, encode proteins, others only RNA molecules. The information contained in the genes that encode proteins is deciphered through two sequential processes: synthesis of RNA, called transcriptions and protein synthesis - broadcasts . First, on a certain section of DNA, as on a matrix, mRNA (messenger RNA) is synthesized - in animal cells this process is carried out in the nucleus. Then, having transferred information from the nucleus to the cytoplasm, in the course of the coordinated work of a multicomponent system with the participation of tRNA (transfer RNA), mRNA, enzymes and various protein factors, the synthesis of a protein molecule is carried out. All these processes provide correct translation genetic information encrypted in DNA from the language of nucleotides to the language of amino acids. The amino acid sequence of a protein molecule uniquely determines its structure and functions. Nucleotides as subunits of DNA and RNA also act as energy carriers.

The structure of DNA (Fig. 5) is a linear polymer. Its monomer units (nucleotides) consist of a nitrogenous base, a five-carbon sugar (pentose) and a phosphate group. The phosphate group is attached to the 5" carbon atom of the monosaccharide residue, the organic base is attached to the 1" carbon atom. Each nucleotide is given a name corresponding to the name of the unique base it contains. There are two types of bases in DNA - purine (adenine - A and guanine - C) and pyrimidine (cytosine - C, thymine - T, uracil - U).

Nucleotides exist in two optical isomers - L and D. Without exception, all living organisms use only D forms to build their nucleotides. The presence of even a small amount of L-form nucleotides inhibits or completely blocks the work of DNA synthesis enzymes.

In DNA, the monosaccharide is represented by 2"-deoxyribose, containing one hydroxyl group; in RNA, by ribose, which has two hydroxyl groups. Nucleotides are connected to each other by phosphodiester bonds, while the phosphate group of the 5" carbon atom of one nucleotide is connected to 3 '-OH by the deoxyribose group of an adjacent nucleotide. At one end of the polynucleotide chain there is a 3'-OH group, at the other there is a 5'-phosphate group.

Native DNA consists of two polymer chains that form a helix. Polynucleotide chains wound around each other are held together by hydrogen bonds formed between the complementary bases of opposite chains. In this case, adenine forms a pair only with thymine, guanine - with cytosine. Pair bases A-T stabilized by two hydrogen bonds, pair S-S- three. The length of double-stranded DNA is usually measured by the number of complementary nucleotide pairs. For example, the DNA of human chromosome 1 is a single double helix, 263 million base pairs long.

The sugar-phosphate composition of the molecule, consisting of phosphate groups and deoxyribose residues connected by 5"-3"-phosphodiester bonds, forms the “sidewalls of the spiral staircase”, and the A-T and C-C pairs are “its steps”. The chains of the DNA molecule are antiparallel: one of them has a direction of 3"-5", the other 5"->3". Nucleotides are counted in pairs because in a DNA molecule there are two chains and their nucleotides are connected in pairs by cross-links.

The carrier of genetic information must satisfy two requirements - reproduce (replicate) with high precision and determine (encode) synthesis of protein molecules. According to the principle of complementarity, each DNA strand can serve as a template for the formation of a new complementary strand. When a cell needs to divide, just before it does so, it copies a DNA molecule in its ribosomes. In this case, two DNA strands diverge and on each of them, as on a matrix, a daughter strand is assembled, exactly repeating the one that was connected to this strand in the parent cell. As a result, two identical daughter chromosomes appear, which, when divided, are distributed according to different cells. This is how the transmission of hereditary characteristics from parents to descendants occurs in all cellular organisms that have a nucleus. Consequently, after each round of replication, two daughter molecules are formed, each of which has the same nucleotide sequence as the original DNA molecule. The nucleotide sequence of a structural gene uniquely determines the amino acid sequence of the protein it encodes. Consequently, each DNA chain serves as a template for the synthesis of a new complementary chain, and the sequence of bases in the synthesized (growing) chain is determined by the sequence of complementary bases of the template chain.

DNA synthesis in pro- and eukaryotes is carried out with the participation of many different enzymes. The main role is played by DNA polymerase, which sequentially attaches units of the growing polynucleotide chain in accordance with the principle of complementarity and catalyzes the formation of phosphodiester bonds.

To separate DNA, special gels based on agarose (a polysaccharide isolated from seaweed) have been developed. A modification of gel electrophoresis in agarose gel, called pulse electrophoresis, allowing the separation of large DNA molecules.

The nucleotide sequences of many mammalian genes have been determined, including genes encoding hemoglobin, insulin, and cytochrome C. The volume of information about DNA is so large (many millions of nucleotides) that powerful computers are needed to store and analyze the available data.

To determine which genes are active in a given cell type (specific sequence identification), a method called DNA footprinting. DNA fragments are labeled with P, then digested with nucleases, separated on a gel and detected by autoradiography. If water solution DNA is heated to 100 °C and strongly alkalized (pH 13), then the complementary base pairs holding the two strands of the double helix together are destroyed and the DNA quickly dissociates into two strands. This process, called DNA denaturation, previously considered irreversible. But if the complementary DNA strands are kept at a temperature of 65 ° C, they easily pair, restoring the structure of the double helix - a process called renaturation.

The vast majority of genes contain encoded information about protein synthesis. Polypeptides are characterized by great versatility; they consist of amino acids with chemically diverse side chains and are capable of taking on different spatial forms that are saturated with reactive sites. The properties of polypeptides make them ideally suited to perform a variety of structural and functional tasks. Proteins are involved in almost all processes occurring in living systems; they serve as catalysts for biochemical reactions, carry out transport within and between cells, regulate the permeability of cell membranes, and are used to build various structural elements. Proteins are not only the main building material of a living organism, many of them are enzymes that control processes in the cell. Proteins are involved in the implementation motor functions, provide protection against infections and toxins, regulate the synthesis of other gene products.

All amino acids have similar chemical structure: Attached to the central carbon atom is a hydrogen atom, an amino group, a carboxyl group, and a side chain. There are 20 different side groups and, accordingly, 20 amino acids: for example, in the amino acid alanine, the side chain is the methyl group (Table 1).

A peptide bond is formed between the carboxyl group of one amino acid and the amino group of another. The first amino acid of a protein molecule has a free amino group (N-terminus), the last one has a free carboxyl group (C-terminus).

The length of protein molecules varies from 40 to 1000 amino acid residues; Depending on their sequence and amino acid composition, protein molecules take different shapes (configuration, conformation). Many functionally active proteins consist of two or more polypeptide chains, both identical and slightly different. Proteins that perform key functions are complex protein complexes consisting of many different polypeptide chains - subunits.

Using the genetic code, the polynucleotide sequence determines the sequence of amino acids in a protein; different nucleotide triplets code for specific amino acids.

Important " transmission link"When translating genetic information from the language of nucleotides into the language of amino acids - RNA (ribonucleic acids), which are synthesized on certain sections of DNA, as on templates, in accordance with their nucleotide sequence.

RNA molecules carry information and have a chemical identity that influences their behavior. The RNA molecule has two important properties: information encoded in its nucleotide sequence is transmitted in the process replication and the unique spatial structure determines the nature of interaction with other molecules and the response to external conditions. Both of these properties are informational And functional- are necessary prerequisites for the evolutionary process. The nucleotide sequence of an RNA molecule is similar hereditary information, or genotype body. The spatial layout is similar phenotype- a set of characteristics of an organism subject to the action of natural selection.

RNA (Fig. 5) is a linear polynucleotide molecule that differs from DNA in two parameters:

1. The monosaccharide in RNA is ribose, containing not one but two hydroxyl groups;

2. One of the four bases in RNA is uracil, which takes the place of thymine.

The existence of RNA in the form of a single strand is due to:

the absence in all cellular organisms of an enzyme to catalyze the reaction of RNA formation on an RNA matrix; only some viruses have such an enzyme, the genes of which are “written” in the form of double-stranded RNA; other organisms can synthesize RNA molecules only on a DNA template; due to the absence of a methyl group in uracil, the connection between adenine and uracil is unstable and “retention” of the second (complementary) strand for RNA is problematic. Due to its single-stranded nature, RNA, unlike DNA, does not twist into a spiral, but forms structures in the form of “hairpins” and “loops”. Base pairing in an RNA molecule occurs in the same way as in DNA, except that instead of an A-T pair, A-U is formed. Complementary bases, as in DNA, are connected to each other by hydrogen bonds.

There are three main types of RNA:

informational (mRNA);

ribosomal (rRNA);

transport (tRNA).

Correct transcription, i.e. its beginning and completion in the required websites(specific areas), provide specific nucleotide sequences in DNA, as well as protein factors. Transcription into DNA occurs in the cell nucleus. mRNA molecules carry information from the nucleus to the cytoplasm, where it is used in the translation of proteins whose amino acid sequences are encoded in the nucleotide sequences of the mRNA (i.e., ultimately, in DNA). mRNA is associated with ribosomes, in which amino acids are combined to form proteins. Ribosomes - nucleotide particles, which include high-polymer RNA and structural protein. The biochemical role of ribosomes is protein synthesis. It is on ribosomes that individual amino acids are combined into polypeptides, resulting in the formation of proteins.

In most prokaryotes, all RNAs are transcribed using the same RNA polymerase. In eukaryotes, mRNA, rRNA, and tRNA are transcribed by different RNA polymerases.

From a genetic point of view, a gene is a specific nucleotide sequence transcribed into RNA. The majority of transcribed DNA sequences are structural genes, on which mRNA is synthesized. The final product of a structural gene is a protein. In prokaryotes, a structural gene is a continuous section of a DNA molecule. In eukaryotes, most structural genes consist of several discrete (individual) coding regions - exons, separated by non-coding regions - nitrons. Upon completion of transcription of a eukaryotic structural gene, introns are excised by enzymes from the primary transcription product, exons are stitched together end to end. (splicing) with the formation of mRNA. Typically, the length of exons is from 150 to 200 nucleotides, the length of introns varies from 40 to 10,000 nucleotides.

In an actively functioning cell, approximately 3-5% of total RNA is mRNA, 90% is rRNA, and 4% is tRNA. mRNA can come in dozens of different types of molecules; rRNA - two types. Larger rRNA forms with proteins ribonucleotide complex, called the large ribosomal subunit. The smaller rRNA is a complex called the small ribosomal subunit. During protein synthesis, subunits combine to form a ribosome. rRNA plays the role of the main catalyst in the process of protein synthesis; it makes up more than 60% of the mass of the ribosome. In evolutionary terms, rRNA is the main component of the ribosome.

In addition to thousands of ribosomes, a cell that actively synthesizes proteins contains up to 60 various types tRNA. tRNA is a linear, single-stranded molecule from 75 to 93 nucleotides in length, which has several mutually complementary regions that pair with each other. With the help of specific enzymes (aminoacyl-tRNA synthetases), the corresponding amino acid is attached to the 3" end of the tRNA. For each of the 20 amino acids that make up all proteins, there is at least one specific tRNA. At the other end of the tRNA molecules there is a sequence of three nucleotides called anticodon, she recognizes specific tub in mRNA and determines which amino acid will be attached to the growing polypeptide chain.

Translation (protein synthesis) is carried out with the participation of mRNA, various tRNAs “loaded” with the corresponding amino acids, ribosomes and many protein factors that ensure the initiation, elongation, and termination of the synthesis of the polypeptide chain.

A nucleotide sequence that encodes more than one protein is called operon. The operon is under the control of a single promoter, and its transcription produces one long mRNA molecule encoding several proteins.

The synthesis of mRNA and, accordingly, protein synthesis is strictly regulated, since the cell does not have enough resources for the simultaneous transcription and translation of all structural genes. Pro- and eukaryotes constantly synthesize only those mRNAs that are necessary to carry out basic cellular functions. The expression of the remaining structural genes is carried out under the strict control of regulatory systems that trigger transcription only when there is a need for certain proteins. Additional transcription factors that bind to the corresponding sections of DNA are responsible for turning transcription on and off.

In the synthesis of protein molecules, the primary stage in the formation of a protein polypeptide chain is the process of activation of amino acids with the help of adenosine triphosphate. The activation process occurs with the participation of enzymes, resulting in the formation of aminoacyl adenylates. Then, under the action of the enzyme aminoacyl-tRNA synthetase (each of the 20 amino acids has its own special enzyme), the “activated” amino acid is combined with tRNA. Next, the aminoacyl-tRNA complex is transferred to ribosomes, where polypeptide synthesis occurs. A peptide bond is formed between the carboxyl group of one amino acid and the amino group of another. The first amino acid of a protein molecule has a free amino group (N-terminus), the last one has a free carboxyl group (C-terminus).

The formed proteins are released from the ribosomes, and the ribosomes can then attach new aminoacyl-tRNA complexes and synthesize new protein molecules. Ribosomes are associated with mRNA, which determines the sequence of alternating amino acids in polypeptide chains. Thus, the integrity and functional activity of ribosomes in cells is one of the necessary conditions for the synthesis of protein molecules.

Test control for Chapter 3 Choose the correct answers:

1. Statement “DNA is a repository of genetic information because its molecules, unlike RNA, are more stable”:

A - true;

B - incorrect;

B - requires clarification.

2. The carrier of genetic information must meet the requirements:

A - replicate with high accuracy;

B - not subject to chemical hydrolysis;

B - determine the synthesis of protein molecules;

G - act as a carrier of energy;

D - form a closed ring-shaped structure.

3. To separate DNA molecules use:

A - salting out;

B - reverse osmosis;

B - pulse electrophoresis;

G - gel electrophoresis;

D - electrodialysis.

4. The difference between an RNA molecule and a DNA molecule:

A - the monosaccharide is deoxyribose;

B - ribose is a monosaccharide;

B - nitrogenous base - thymine;

G - nitrogenous base - uracil;

D - nitrogenous base - guanine.

5. DNA molecule synthesis is carried out:

A - DNA ligase;

B - DNA polymerase;

B - from L-form nucleotides;

G - from D-form nucleotides;

D - from a mixture of D and L-forms of nucleotides.

6. Splicing:

A - excision of exons from the precursor mRNA and covalent connection of introns with the formation of mature mRNA molecules;

B - excision of introns from the precursor mRNA and covalent joining of exons with the formation of mature mRNA molecules;

B - synthesis of mature tRNA molecules from end-to-end cross-linking of individual nucleotides;

D - excision of introns from the precursor mRNA and their covalent connection with the formation of mature mRNA molecules;

D - sequential covalent connection of exons and introns with the formation of mature mRNA molecules.

A - three adjacent nucleotides of mRNA encoding a specific amino acid;

B - three neighboring tRNA nucleotides, complementary to the nucleotides of a specific codon in the mRNA molecule;

B - three adjacent tRNA nucleotides encoding a specific amino acid;

G - three neighboring tRNA nucleotides encoding a specific sequence of amino acids;

D - three adjacent nucleotides of mRNA encoding a specific amino acid.

8. The unique spatial structure of the RNA molecule determines:

A - replication process;

B - genotype;

B - phenotype;

G - the nature of interaction with other molecules and external

conditions; D - localization of the RNA molecule.

9. Transcription processes are underway:

A - constantly at the same speed;

B - under the control of regulatory systems;

B - periodically as energy accumulates;

G - associated with the processes of formation of DNA molecules;

D - at a rate proportional to the formation of structural genes.

10. Operon:

A - a section of DNA containing several structural genes;

B - DNA section containing one structural gene;

B - nucleotide sequence encoding one protein;

G - nucleotide sequence encoding more than one

D is a long mRNA molecule that encodes several proteins.

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Synthesis of DNA, RNA and proteins

DNA replication

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DNA replication

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