Proteins form the basis of life. Their functions in the cell are very diverse. However, proteins "can't" reproduce. And all the information about the structure of proteins is contained in genes (DNA).

In higher organisms, proteins are synthesized in the cytoplasm of the cell, and DNA is hidden behind the shell of the nucleus. Therefore, DNA cannot directly serve as a template for protein synthesis. This role is performed by another nucleic acid - RNA.

RNA molecule is an unbranched polynucleotide with tertiary structure. It is formed by one polynucleotide chain, and although the complementary nucleotides included in it are also capable of forming hydrogen bonds between themselves, these bonds occur between the nucleotides of one chain. RNA chains are much shorter than DNA chains. If the content of DNA in a cell is relatively constant, then the content of RNA fluctuates greatly. The greatest amount of RNA in cells is observed during protein synthesis.

RNA plays a major role in the transmission and implementation of hereditary information. In accordance with the function and structural features, several classes of cellular RNA are distinguished.

There are three main classes of cellular RNA.

1. Informational (mRNA), or matrix (mRNA). Its molecules are the most diverse in terms of size, molecular weight (from 0.05x106 to 4x106) and stability. They make up about 2% of the total amount of RNA in the cell. All mRNAs are carriers of genetic information from the nucleus to the cytoplasm, to the site of protein synthesis. They serve as a matrix (working drawing) for the synthesis of a protein molecule, as they determine the amino acid sequence (primary structure) of a protein molecule.

1. Ribosomal RNA (rRNA). They make up 80–85% of the total RNA content in the cell. Ribosomal RNA consists of 3–5 thousand nucleotides. It is synthesized in the nucleoli of the nucleus. In complex with ribosomal proteins, rRNA forms ribosomes - organelles on which protein molecules are assembled. The main significance of rRNA is that it provides the initial binding of mRNA and ribosome and forms the active center of the ribosome, in which peptide bonds are formed between amino acids during the synthesis of the polypeptide chain.
2. Transfer RNAs (tRNAs). tRNA molecules usually contain 75-86 nucleotides. The molecular weight of tRNA molecules is about 25 thousand. tRNA molecules play the role of intermediaries in protein biosynthesis - they deliver amino acids to the site of protein synthesis, that is, to ribosomes. The cell contains more than 30 types of tRNA. Each type of tRNA has its own unique nucleotide sequence. However, all molecules have several intramolecular complementary regions, due to the presence of which all tRNAs have a tertiary structure resembling a clover leaf in shape.

Secondary structure of RNA - characteristic of tRNA, single-stranded, shaped like a "clover leaf". Includes:

• relatively short double helixes - stems,
• single-stranded sections - loops.

There are 4 stems (acceptor, anticodon, dihydrouridyl, pseudouridyl) and 3 loops.

"Pseudoknot" - an element of the secondary structure of RNA, schematically

The acceptor stem contains the 3'- and 5'-ends of the polynucleotide chain, the 5'-end ends with a guanilic acid residue, the 3'-end is a CCA triplet and serves to form an ester bond with AA.

The anticodon stem recognizes its codon on mRNA in ribosomes by the principle of complementarity.

The pseudouridyl stem serves to attach to the ribosome.

The dihydrouridyl stem serves to bind to the aminoacyl-tRNA synthetase.

RNA

The structure of RNA molecules is in many ways similar to the structure of DNA molecules. However, there are also a number of significant differences. In the RNA molecule, instead of deoxyribose, nucleotides contain ribose. The thymidyl nucleotide (T) is replaced by the uridyl nucleotide (U). The main difference from DNA is that the RNA molecule is a single strand. However, its nucleotides are capable of forming hydrogen bonds with each other (for example, in tRNA, rRNA molecules), but in this case we are talking about an intrastrand connection of complementary nucleotides.

RNA chains are much shorter than DNA.

Types of RNA

There are several types of RNA in the cell, which differ in the size of the molecules, structure, location in the cell, and functions.

Informational (matrix) RNA - mRNA- the most heterogeneous in size and structure. mRNA is an open polynucleotide chain. It is synthesized in the nucleus with the participation of the RNA polymerase enzyme according to the principle of complementarity with the DNA region responsible for encoding this protein. mRNA performs an essential function in the cell. It serves as templates for the synthesis of proteins, transferring information about their structure from DNA molecules. Each cell protein is encoded by its specific mRNA.

Ribosomal RNA-rRNA. These are single-stranded nucleic acids, which, in combination with proteins, form ribosomes - organelles on which protein synthesis occurs. Information about the structure of rRNA is encoded in DNA regions located in the region of the secondary constriction of chromosomes. rRNA accounts for 80% of all cell RNA, since cells contain a large number of ribosomes. rRNAs have a complex secondary and tertiary structure, forming loops in complementary regions, which leads to self-organization of these molecules into a complex body. The composition of ribosomes includes 3 types of rRNA - in prokaryotes and 4 types of rRNA - in eukaryotes.

Transport (transfer) RNA - tRNA. A tRNA molecule consists of an average of 80 nucleotides. The content of tRNA in the cell is about 15% of all RNA. The function of tRNA is the transfer of amino acids to the site of protein synthesis and participation in the translation process. The number of different types of tRNA in a cell is small (about 40). All of them have a similar spatial organization. Due to intrastrand hydrogen bonds, the tRNA molecule acquires a characteristic secondary structure called clover leaf.

The 3D model of tRNA looks somewhat different. Four loops are distinguished in tRNA: an acceptor loop (serves as a site for attaching an amino acid), an anticodon loop (recognizes a codon in mRNA during translation), and two side loops.

Nucleic acids are macromolecular compounds with a strictly defined linear sequence of monomers. The structure of DNA and RNA is a way of "recording information", which ensures the formation of two information flows in the body. One of the streams reproduces information contained in DNA molecules. The duplication of DNA molecules is called "replication". As a result of this process and subsequent division, daughter cells inherit the parent cell's genome, which contains a complete set of genes, or "instructions" for the structure of RNA and all the proteins of the body.

The second flow of information is realized in the course of the cell's vital activity. In this case, "reading" occurs, or transcription, genes in the form of mRNA polynucleotide sequences and their use as templates for the synthesis of the corresponding proteins. In the latter case, the transfer (broadcast) information contained in mRNA into the "language" of amino acids. This flow of information from DNA through RNA to protein is called "the central dogma of biology". It is characteristic of all living organisms, with the exception of some RNA-containing viruses.

Correction of errors that occur in the structure of DNA under the influence of factors of the external and internal environment, carries out another matrix synthesis - repair. It is a variant of limited replication and restores the original structure of DNA using a section of an intact DNA strand as a template. During the reproduction of RNA-containing viruses in the cells of eukaryotic organisms, new DNA molecules can be synthesized using a process in which RNA serves as a template for the synthesis of complementary DNA, which can be included in the genome of higher organisms. (reverse transcription).

Deoxyribonucleic acid molecules are long chains containing hundreds of nucleotides. Each nucleotide consists of a purine or pyrimidine base, a 2-deoxyribose molecule, and phosphoric acid. Purine bases are adenine or guanine, pyrimidine bases are cytosine or thymine.

Fig.1

The polynucleotide has a sugar-phosphate backbone, and purine and pyrimidine bases are attached to the sugar residue at position 1 (the carbon atom of the aldehyde group). Since deoxyribose does not have a hydroxyl group at position 2, phosphoric acid links the C-3 carbon of one sugar residue to the C-5 carbon of the next sugar residue.

Purine bases A - adenine, G - guanine.

Pyrimidine bases T - thymine, C - cytosine.

DNA exists in the nuclei of cells in the form of paired strands twisted into a double helix (Fig. 2). Each purine or pyrimidine base is directed inside the helix towards its axis and is hydrogen bonded to another purine or pyrimidine base located on the other thread. Purine bases always form a hydrogen bond with pyrimidine bases and vice versa. In this case, due to the donor-acceptor nature of the groups forming a hydrogen bond, adenine always forms a hydrogen bond with thymine, and guanine with cytosine. Therefore, the number of adenine residues is always equal to the number of thymine residues, and the number of guanine residues is always equal to the number of cytosine residues.

Rice. 2. Structure of DNA*. a - double helix; b - base pairing between strands

Fig.3

Bases in DNA are linked by hydrogen bonds. The hydrogen bonds of the adenine-cytosine pair are not as stable as the bonds of the adenine-thymine pair. In order for adenine and cytosine to form hydrogen bonds, it is necessary that the amino group of adenine, located in position 6, undergoes a tautomeric transition * to an imino group, as shown in the diagram below. But this adenine conformation is again not stable.

* (Tautomerisady is an isomerization process in which a proton moves from atom 1 to atom 3. In the case of adenine, the following structures are involved in this process:

Fig.4

Therefore, education couples A-T would be preferable to education couples A-C. A similar approach can be used to show why a guanine-cytosine pair is preferred over a guanine-thymine pair.

The strict base-pairing requirement is important because it provides a mechanism for exact strand-pair doubling. The DNA is duplicated before cell division to provide each of the daughter cells with a complete set of DNA molecules. This occurs by breaking hydrogen bonds between chains and then forming new hydrogen bonds with new nucleotide partners: adenine with thymine and guanine with cytosine (Fig. 5). The new nucleotides then form sugar-phosphate bonds with each other, creating a new chain. The result is an accurate reproduction of the original paired circuits. This is the molecular basis of heredity. Any mistake in the doubling process causes a mutation.

Fig.5. Double stranded DNA molecule

The process of DNA replication is more understandable than the process of synthesis of the mRNA molecule. The main secret in mRNA synthesis is the fact that only one strand of RNA is synthesized on each double-stranded DNA molecule. The only mRNA molecule produced is an exact copy of one of the DNA strands, but not of the other strand. The mRNA then exits the nucleus and attaches to the ribosome*.

* (Ribosomes are large complex particles in the cytoplasm. They are globular structures rich in protein and RNA and are the site of protein synthesis in the cell.)

The RNA molecule is similar to the DNA molecule, except that RNA contains ribose instead of deoxyribose and a uracil base instead of thymine (uracil is demethylated thymine). Like thymine, uracil always pairs with adenine.

Fig.6

The backbone of DNA and RNA structures is the same, i.e., phosphoric acid residues link position 3 of one sugar molecule to position 5 of another sugar molecule. An important consequence of the presence of a hydroxyl group at position 2 in the ribose residue of RNA is that it makes RNA much more susceptible to mild alkaline hydrolysis than DNA. The reason for this is the participation of the hydroxyl in position 2 in the alkaline hydrolytic cleavage of RNA.

Fig.7

As a result of the principle of complementary base pairing, the RNA molecule accurately reflects the sequence of bases in the DNA molecule. Thus, mRNA contains adenine residues where DNA contains thymine, cytosine residues where DNA contains guanine, guanine where DNA contains cytosine, and uracil residues where DNA contains adenine. But how does this translate into a specific sequence of amino acid residues in a protein molecule? This is the most interesting part of the puzzle.

The sequence of bases in mRNA must somehow control the sequence in which amino acids are connected to form a protein molecule.

Messenger RNA contains four types of bases, while protein usually contains twenty different types of amino acids. Therefore, a single base cannot control the position of a particular amino acid in a protein chain, since four bases could only control four amino acids. Similarly, combinations of two adjacent bases could control a maximum of sixteen amino acids, since only sixteen different combinations of adjacent bases are possible (see below). (The combination of AC differs from the combination of CA due to the orientation of the 3,5-diester bond of the phosphoric acid residue.)

In order to be able to exercise specific control over the sequence of twenty amino acids, a combination of at least three messenger RNA bases is required, which gives 64 possible combinations. These base triplets on messenger RNA (called codons)

Fig.8

act as specific landing sites for complementary triplets located on tRNA* molecules (anticodons). The specificity of the docking of a codon and an anticodon is due to the specificity of the formation of hydrogen bonds between adenine and uracil and between cytosine and guanine. Complementary triplets of tRNA bases are located in the so-called anticodon loop near the middle of the tRNA chain, and parts of the chain that are not included in this loop are added to form a DNA-type double helix. One end of the chain is always slightly longer than the other, and it is this free end that carries the amino acid. Before joining a specific tRNA molecule, the amino acid is activated by an enzymatic reaction with ATP, forming a bond between the amino acid and adenosine monophosphate (AA-AMP). Surprisingly, this free end of a tRNA always has the same terminal base sequence (CCA) no matter which amino acid is on the end. (The amino acid is attached to the terminal ribose via an ester bond.) It is clear that the binding triplet must somehow control the choice of amino acid attached to the end of the molecule, but how this is done remains a mystery.

* (During protein synthesis in the cell, tRNA is associated with mRNA, which is temporarily attached to the ribosome (see Fig. 3.3).)

Fig.9

Another mystery, which until recently remained unresolved, is the problem of choosing a single "correct" triplet from a given sequence of bases. For example, the ACG sequence contains two triplets - ACG and CGU. Only one of them can be a "correct" triplet, corresponding to a specific amino acid that must be included in the protein programmed by this mRNA. If, on the other hand, the synthesis begins with the choice of an "incorrect" triplet, then this will lead to a continuous sequence of "incorrect" triplets, and therefore all amino acids will be "incorrect", which will mean an "incorrect" protein.

However, such errors do not occur in nature, and in the late 1950s, F. Crick, J. Griffith, and L. Orgel offered a witty explanation for this phenomenon. Assuming that the amino acid code is based on a sequence of triplets (resembling three-letter characters), they assumed that the code is non-overlapping. This means that in the hypothetical regular polynucleotide UGAUGAUGA only one of the three triplets (UGA GAU or AUG) has any "meaning". The other two are "meaningless" because they don't match any complementary triplets on the tRNA, and therefore no tRNAs will pair with those triplets.

They reasoned further that if this was true, the triplets AAA, CCC, GGG, and UUU could not be true triplets, because the repetition of any of them could cause overlap, and hence the wrong start of protein synthesis. Thus, the number of valid combinations of three RNA bases decreases from 64 to 60. They further believed that of these 60 combinations, two-thirds must be meaningless to avoid overlap.

Fig. 10 Such overlapping triplet selection would result in different amino acid sequences in the protein

Then only one third of the 60 will be "true" triplets. This number (twenty) corresponds exactly to the number of different amino acids found in proteins. While this assumption matched perfectly with what was known at the time, it turned out to be wrong. Subsequent work has shown that there are more than 20 significant triplets. Modern data show that the choice of the "correct" triplet occurs as a result of preferential binding of tRNA to one of the ends of the mRNA, and not in the middle of its chain.

How to determine which triplet will serve as a code for a particular amino acid? The most direct way is to prepare synthetic polynucleotides with a known base sequence, use these molecules as mRNA in protein synthesis, and then determine the amino acid sequence in the protein.

For example, a polynucleotide containing only one type of base can be obtained from nucleotides (diphosphates) and an enzyme called polynucleotide phosphorylase isolated by Ochoa and Grünberg-Manago. If the base is uracil, the synthetic polynucleotide is called poly-U (UUUUUUU...). In the presence of a mixture of tRNA molecules, enzymes and other cell components, poly-U initiates the synthesis of a polypeptide containing amino acids of only one type - namely, polyphenylalanine. Thus, it is clear that the UUU triplet is the codon for phenylalanine.

This method can be extended (and has been done by Ochoa and Nirenberg) to mixed base codons. For example, the polymerization of uracil can be initiated by the dinucleotide AUUUUUU.... This polynucleotide causes the synthesis of polyphenylalanine with a single tyrosine residue at the end. Therefore, the codon for the amino acid tyrosine must be the AYU triplet. As a result of this work, codon tables were compiled for all twenty amino acids. It turned out that most amino acids have more than one codon.

Therefore, according to modern concepts, each enzyme is synthesized by a linear sequence of amino acid fusion reactions, starting at one end of the mRNA and ending at its other end, where the protein chain is completely released. As the next peptide bond is formed, the "used" tRNA moves away from the mRNA. This allows fresh tRNAs to supply amino acids and begin the synthesis of the second protein molecule without waiting for the end of the synthesis of the first molecule (Fig. 11).

Rice. 11. Depiction of protein synthesis sequence. In this figure, AA stands for an amino acid; ATP - adenosine triphosphate; AMP - adenosine monophosphate; AA - AMP - amino acid adenylate: tRNA - transport ribonucleic acid; mRNA - informational, or matrix, ribonucleic acid, a, b, etc. denote binding sites on the ribosome (tRNA), where the formation of a peptide bond occurs

Various types of DNA and RNA - nucleic acids - is one of the objects of study of molecular biology. One of the most promising and rapidly developing areas in this science in last years was the study of RNA.

Briefly about the structure of RNA

So, RNA, ribonucleic acid, is a biopolymer, the molecule of which is a chain formed by four types of nucleotides. Each nucleotide, in turn, consists of a nitrogenous base (adenine A, guanine G, uracil U or cytosine C) in combination with a ribose sugar and a phosphoric acid residue. Phosphate residues, connecting with the riboses of neighboring nucleotides, "sew" the constituent blocks of RNA into a macromolecule - a polynucleotide. This is how the primary structure of RNA is formed.

The secondary structure - the formation of a double chain - is formed in some parts of the molecule in accordance with the principle of complementarity of nitrogenous bases: adenine pairs with uracil through a double, and guanine with cytosine - a triple hydrogen bond.

In the working form, the RNA molecule also forms a tertiary structure - a special spatial structure, conformation.

RNA synthesis

All types of RNA are synthesized using the enzyme RNA polymerase. It can be DNA- and RNA-dependent, that is, it can catalyze synthesis on both DNA and RNA templates.

The synthesis is based on the complementarity of the bases and the antiparallelism of the reading direction of the genetic code and proceeds in several stages.

First, RNA polymerase is recognized and bound to a special nucleotide sequence on DNA - the promoter, after which the DNA double helix unwinds in a small area and the assembly of the RNA molecule begins over one of the chains, called the template (the other DNA chain is called coding - it is its copy that is synthesized RNA). The asymmetry of the promoter determines which of the DNA strands will serve as a template, and thus allows RNA polymerase to initiate synthesis in the correct direction.

The next step is called elongation. The transcription complex, which includes RNA polymerase and an untwisted region with a DNA-RNA hybrid, begins to move. As this movement proceeds, the growing RNA strand gradually separates, and the DNA double helix unwinds in front of the complex and reassembles behind it.

The final stage of synthesis occurs when RNA polymerase reaches a specific region of the matrix called the terminator. Termination (end) of the process can be achieved in various ways.

The main types of RNA and their functions in the cell

They are the following:

• Matrix or informational (mRNA). Through it, transcription is carried out - the transfer of genetic information from DNA.
• Ribosomal (rRNA), which provides the process of translation - protein synthesis on the mRNA template.
• Transport (tRNA). Produces recognition and transport of amino acids to the ribosome, where protein synthesis occurs, and also takes part in translation.
• Small RNAs are an extensive class of small molecules that perform various functions during the processes of transcription, RNA maturation, and translation.
• RNA genomes are coding sequences that contain the genetic information of some viruses and viroids.

In the 1980s, the catalytic activity of RNA was discovered. Molecules with this property are called ribozymes. There are not so many natural ribozymes yet known, their catalytic ability is lower than that of proteins, but in the cell they perform extremely important functions. Currently, successful work is underway on the synthesis of ribozymes, which, among other things, have applied significance.

Let us dwell in more detail on the different types of RNA molecules.

Matrix (information) RNA

This molecule is synthesized over the untwisted section of DNA, thus copying the gene encoding a particular protein.

RNA of eukaryotic cells, before becoming, in turn, a matrix for protein synthesis, must mature, that is, go through a complex of various modifications - processing.

First of all, even at the stage of transcription, the molecule undergoes capping: a special structure of one or more modified nucleotides, the cap, is attached to its end. It plays an important role in many downstream processes and enhances mRNA stability. The so-called poly(A) tail, a sequence of adenine nucleotides, is attached to the other end of the primary transcript.

The pre-mRNA is then spliced. This is the removal of non-coding regions from the molecule - introns, which are abundant in eukaryotic DNA. Next, the mRNA editing procedure occurs, in which its composition is chemically modified, as well as methylation, after which the mature mRNA leaves the cell nucleus.

Ribosomal RNA

The basis of the ribosome, a complex that provides protein synthesis, is made up of two long rRNAs that form subparticles of the ribosome. They are synthesized together as a single pre-rRNA, which is then separated during processing. The large subunit also includes low molecular weight rRNA synthesized from a separate gene. Ribosomal RNAs have a densely packed tertiary structure that serves as a scaffold for proteins that are present in the ribosome and perform auxiliary functions.

In the non-working phase, the ribosome subunits are separated; at the initiation of the translational process, the rRNA of the small subunit combines with messenger RNA, after which the elements of the ribosome are completely combined. When the RNA of the small subunit interacts with the mRNA, the latter, as it were, stretches through the ribosome (which is equivalent to the movement of the ribosome along the mRNA). The ribosomal RNA of the large subunit is a ribozyme, that is, it has enzymatic properties. It catalyses the formation of peptide bonds between amino acids during protein synthesis.

It should be noted that the largest part of all RNA in the cell is ribosomal - 70-80%. DNA has a large number of genes encoding rRNA, which ensures its very intensive transcription.

Transfer RNA

This molecule is recognized by a certain amino acid with the help of a special enzyme and, connecting with it, transports the amino acid to the ribosome, where it serves as an intermediary in the process of translation - protein synthesis. The transfer is carried out by diffusion in the cytoplasm of the cell.

The newly synthesized tRNA molecules, like other types of RNA, are processed. Mature tRNA in its active form has a conformation resembling a cloverleaf. On the "petiole" of the leaf - the acceptor site - there is a CCA sequence with a hydroxyl group that binds to the amino acid. At the opposite end of the "leaf" is an anticodon loop that connects to a complementary codon on the mRNA. The D-loop serves to bind the transfer RNA to the enzyme when interacting with the amino acid, and the T-loop - to bind to the large subunit of the ribosome.

Small RNA

These types of RNA play an important role in cellular processes and are now being actively studied.

For example, small nuclear RNAs in eukaryotic cells are involved in mRNA splicing and possibly have catalytic properties along with spliceosome proteins. Small nucleolar RNAs are involved in the processing of ribosomal and transfer RNA.

Small interfering and microRNAs are the most important elements of the gene expression regulation system, which is necessary for the cell to control its own structure and vital activity. This system is an important part of the cell's immune antiviral response.

There is also a class of small RNAs that function in complex with Piwi proteins. These complexes play a huge role in the development of germline cells, in spermatogenesis, and in the suppression of transposable genetic elements.

RNA genome

The RNA molecule can be used as the genome by most viruses. Viral genomes are different - single- and double-stranded, circular or linear. Also, RNA genomes of viruses are often segmented and generally shorter than DNA-containing genomes.

There is a family of viruses whose genetic information, encoded in RNA, after infection of the cell by reverse transcription, is rewritten to DNA, which is then introduced into the genome of the victim cell. These are the so-called retroviruses. These include, in particular, the human immunodeficiency virus.

Significance of RNA research in modern science

If earlier the opinion about the secondary role of RNA prevailed, now it is clear that it is a necessary and most important element of intracellular life activity. Many processes of paramount importance are not complete without active participation RNA. The mechanisms of such processes remained unknown for a long time, but thanks to the study various kinds RNA and their functions are gradually being clarified in many details.

It is possible that RNA played a decisive role in the emergence and development of life at the dawn of the Earth's history. The results of recent studies speak in favor of this hypothesis, testifying to the extraordinary antiquity of many mechanisms of cell functioning with the participation of certain types of RNA. For example, the recently discovered riboswitches as part of mRNA (a system of protein-free regulation of gene activity at the transcription stage), according to many researchers, are echoes of an era when primitive life was built on the basis of RNA, without the participation of DNA and proteins. MicroRNAs are also considered to be a very ancient component of the regulatory system. The structural features of the catalytically active rRNA indicate its gradual evolution by adding new fragments to the ancient protoribosome.

A thorough study of which types of RNA and how are involved in certain processes is also extremely important for theoretical and applied fields of medicine.

RNA molecules are polymers, the monomers of which are ribonucleotides formed by the residues of three substances: five-carbon sugar - ribose; one of the nitrogenous bases - from purines - adenine or guanine, from pyrimidine - uracil or cytosine; phosphoric acid residue.

The RNA molecule is an unbranched polynucleotide having a tertiary structure. The connection of nucleotides in one chain is carried out as a result of a condensation reaction between the phosphoric acid residue of one nucleotide and the 3 "-carbon of the ribose of the second nucleotide.

Unlike DNA, RNA is made up of not two, but one polynucleotide chain. However, its nucleotides (adenyl, uridyl, guanyl, and cytidyl) are also capable of forming hydrogen bonds with each other, but these are intra-, rather than interstrand compounds of complementary nucleotides. Two hydrogen bonds are formed between the A and U nucleotides, and three hydrogen bonds between the G and C nucleotides. RNA chains are much shorter than DNA chains.

Information about the structure of the RNA molecule is embedded in the DNA molecules. The sequence of nucleotides in RNA is complementary to the codogenous strand of DNA, but the adenyl nucleotide of DNA is complementary to the uridyl nucleotide of RNA. If the content of DNA in a cell is relatively constant, then the content of RNA fluctuates greatly. The greatest amount of RNA in cells is observed during protein synthesis.

There are three main classes of nucleic acids: messenger RNA - mRNA (mRNA), transfer RNA - tRNA, ribosomal RNA - rRNA.

Information RNA. The most diverse class in terms of size and stability. All of them are carriers of genetic information from the nucleus to the cytoplasm. Messenger RNAs serve as a template for the synthesis of a protein molecule, tk. determine the amino acid sequence of the primary structure of the protein molecule. The share of mRNA accounts for up to 5% of the total RNA content in the cell.

transport RNA. Transfer RNA molecules usually contain 75-86 nucleotides. The molecular weight of tRNA molecules is 25,000. tRNA molecules play the role of intermediaries in protein biosynthesis - they deliver amino acids to the site of protein synthesis, to ribosomes. The cell contains more than 30 types of tRNA. Each type of tRNA has its own unique nucleotide sequence. However, all molecules have several intramolecular complementary regions, due to the presence of which all tRNAs have a tertiary structure resembling a clover leaf in shape.

Ribosomal RNA. The share of ribosomal RNA (rRNA) accounts for 80-85% of the total RNA content in the cell. Ribosomal RNA consists of 3-5 thousand nucleotides. In complex with ribosomal proteins, rRNA forms ribosomes - organelles on which protein synthesis occurs. The main significance of rRNA is that it provides the initial binding of mRNA and ribosome and forms the active center of the ribosome, in which peptide bonds are formed between amino acids during the synthesis of the polypeptide chain.

RNA consists of nucleotides, which include sugar - ribose, phosphate and one of the nitrogenous bases (adenine, uracil, guanine, cytosine). Forms primary, secondary and tertiary structure similar to those of DNA. Information about the amino acid sequence of a protein is contained in information RNA (mRNA, mRNA). Three consecutive nucleotides (codon) correspond to one amino acid. In eukaryotic cells, the transcribed mRNA precursor or pre-mRNA is processed into mature mRNA. Processing involves the removal of non-coding protein sequences (introns). After that, mRNA is exported from the nucleus to the cytoplasm, where it is joined by ribosomes that translate mRNA with the help of tRNAs connected to amino acids. Transport (tRNA)- small, consisting of approximately 80 nucleotides, molecules with a conservative tertiary structure. They carry specific amino acids to the site of peptide bond synthesis in the ribosome. Each tRNA contains an amino acid attachment site and an anticodon for recognition and attachment to mRNA codons. The anticodon forms hydrogen bonds with the codon, which places the tRNA in a position that facilitates the formation of a peptide bond between the last amino acid of the formed peptide and the amino acid attached to the tRNA. Ribosomal RNA (rRNA) - catalytic component of ribosomes. Eukaryotic ribosomes contain four types of rRNA molecules: 18S, 5.8S, 28S, and 5S. Three of the four types of rRNA are synthesized in the nucleolus. In the cytoplasm, ribosomal RNAs combine with ribosomal proteins to form a nucleoprotein called a ribosome. The ribosome attaches to the mRNA and synthesizes the protein. rRNA accounts for up to 80% of the RNA found in the cytoplasm of a eukaryotic cell.

Functions: the ability to reproduce itself, the ability to keep its organization constant, the ability to acquire changes and reproduce them.

10. Structure and properties of the genetic code

genetic code - A certain set and order of amino acids in peptide chains. In the variety of proteins that exist in nature, about 20 different amino acids have been found. For their encryption, a sufficient number of combinations of nucleotides can provide only triplet code, in which each amino acid is encrypted by three adjacent nucleotides, four nucleotides form 4 3 = 64 triplets. Of the 64 possible DNA triplets, 61 encode different amino acids; the remaining 3 are called meaningless, or "nonsense triplets". They do not encode amino acids and act as punctuation marks when reading hereditary information. These include ATT, ACT, ATC.

Properties of the genetic code: degeneracy - obvious redundancy of the code, many amino acids are encrypted by several triplets. This property is very important, since the occurrence of changes in the structure of the DNA molecule by the type of replacement of one nucleotide in the polynucleotide chain may not change the meaning of the triplet. The resulting new combination of three nucleotides encodes the same amino acid. Specificity - each triplet can code for only one specific amino acid. Versatility - the complete correspondence of the code in different species of living organisms testifies to the unity of the origin of the entire diversity of living forms on Earth in the process of biological evolution. Continuity And non-overlapping codons during reading – the sequence of nucleotides is read triple by triplet without gaps, while neighboring triplets do not overlap each other, i.e. each individual nucleotide is part of only one triplet for a given reading frame. The proof of the non-overlapping of the genetic code is the replacement of only one amino acid in the peptide when replacing one nucleotide in DNA.