Irk decryption. RNA - description, functions and history of discovery

To maintain life in a living organism, many processes occur. We can observe some of them - breathing, eating, getting rid of waste products, receiving information through the senses and forgetting this information. But most of the chemical processes are hidden from view.

Reference. Classification

In scientific terms, metabolism is metabolism.

Metabolism is usually divided into two stages:
during catabolism, complex organic molecules break down into simpler ones to produce energy; (energy is wasted)
In the processes of anabolism, energy is spent on the synthesis of complex biomolecules from simple molecules. (energy is stored)

Biomolecules, as seen above, are divided into small molecules and large ones.

Small:
Lipids (fats), phospholipids, glycolipids, sterols, glycerolipids,
Vitamins
Hormones, neurotransmitters
Metabolites
Large:
Monomers, oligomers and polymers.

Monomers Oligomers Biopolymers

Amino acids Oligopeptides Polypeptides, proteins
Monosaccharides Oligosaccharides Polysaccharides (starch, cellulose)
Nucleotides Oligonucleotides Polynucleotides, (DNA, RNA)

The biopolymers column contains polynucleotides. This is where ribonucleic acid is located - the object of the article.

Ribonucleic acids. Structure, purpose.

The figure shows an RNA molecule.
Nucleic acids DNA and RNA are present in the cells of all living organisms and perform the functions of storing, transmitting and implementing hereditary information.

Similarities and differences between RNA and DNA

As you can see, there is an external resemblance to the known structure of the DNA molecule (deoxyribonucleic acid).
However, RNA can have either a double-stranded or single-stranded structure.
Nucleotides (pentagons and hexagons in the figure)
In addition, the RNA strand consists of four nucleotides (or nitrogenous bases, which are the same thing): adenine, uracil, guanine and cytosine.
The DNA strand consists of another set of nucleotides: adenine, guanine, thymine and cytosine.
Chemical structure of RNA polynucleotide:

As you can see, there are characteristic nucleotides uracil (for RNA) and thymine (for DNA).
All 5 nucleotides in the picture:


The hexagons in the pictures are benzene rings, into which, instead of carbon, other elements are embedded, in this case, nitrogen.

Benzene. For reference.

The chemical formula of benzene is C6H6. Those. Each corner of a hexagon contains a carbon atom. The 3 additional inner lines in the hexagon indicate the presence of double covalent bonds between these carbon atoms. Carbon is an element of group 4 of the periodic table, therefore, it has 4 electrons that can form a covalent bond. In the figure there is one bond with a hydrogen electron, a second with a carbon electron on the left, and 2 more with 2 carbon electrons on the right. However, physically there is a single electron cloud covering all 6 carbon atoms of benzene.

Compounding nitrogenous bases

Complementary nucleotides link to each other (hybridize) using hydrogen bonds. Adenine is complementary to uracil, and guanine is complementary to cytosine. The longer the complementary regions on a given RNA, the stronger the structure they form will be; on the contrary, short sections will be unstable. This determines the function of a particular RNA.
The figure shows a fragment of the complementary region of RNA. Nitrogen bases are colored blue

RNA structure

The linkage of many groups of nucleotides is formed by RNA hairpins (primary structure):


Many pins in the tape interlock to form a double helix. When expanded, this structure resembles a tree (Secondary structure):


The spirals also interact with each other (tertiary structure). You can see how different spirals are connected to each other:


Other RNAs fold similarly. Resembles a set of ribbons (quaternary structure).

Conclusion

To calculate the conformations that RNA will adopt based on their primary sequence, there are

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

An RNA molecule is an unbranched polynucleotide with a tertiary structure. The joining of nucleotides into one chain occurs as a result of a condensation reaction between the phosphoric acid residue of one nucleotide and the 3" ribose carbon of the second nucleotide.

Unlike DNA, RNA is formed not by 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 inter-chain compounds of complementary nucleotides. Two hydrogen bonds are formed between A- and U-nucleotides, and three hydrogen bonds are formed between G- and C-nucleotides. RNA chains are much shorter than DNA chains.

Information about the structure of an RNA molecule is contained in DNA molecules. The sequence of nucleotides in RNA is complementary to the codogenic strand of DNA, but the adenyl nucleotide of DNA is complementary to the uridyl nucleotide of RNA. While the DNA content in a cell is relatively constant, the RNA content fluctuates greatly. The largest 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.

Messenger RNAs. 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 protein molecules, because determine the amino acid sequence of the primary structure of the protein molecule. mRNA accounts for up to 5% of the total RNA content in the cell.

Transfer RNAs. 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 a 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 RNAs. 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 ensures the initial binding of mRNA and the ribosome and forms the active center of the ribosome, in which the formation of peptide bonds between amino acids occurs during the synthesis of the polypeptide chain.

The functions of RNA vary depending on the type of ribonucleic acid.

1) Messenger RNA (i-RNA).

2) Ribosomal RNA (r-RNA).

3) Transfer RNA (tRNA).

4) Minor (small) RNAs. These are RNA molecules, most often with a small molecular weight, located in various parts of the cell (membrane, cytoplasm, organelles, nucleus, etc.). Their role is not fully understood. It has been proven that they can help the maturation of ribosomal RNA, participate in the transfer of proteins across the cell membrane, promote the reduplication of DNA molecules, etc.

5) Ribozymes. A recently identified type of RNA that takes an active part in cellular enzymatic processes as an enzyme (catalyst).

6) Viral RNA. Any virus can contain only one type of nucleic acid: either DNA or RNA. Accordingly, viruses containing an RNA molecule are called RNA-containing viruses. When a virus of this type enters a cell, the process of reverse transcription (the formation of new DNA based on RNA) can occur, and the newly formed DNA of the virus is integrated into the genome of the cell and ensures the existence and reproduction of the pathogen. The second scenario is the formation of complementary RNA on the matrix of the incoming viral RNA. In this case, the formation of new viral proteins, the vital activity and reproduction of the virus occurs without the participation of deoxyribonucleic acid only on the basis of genetic information recorded on the viral RNA. Ribonucleic acids. RNA, structure, structures, types, role. Genetic code. Mechanisms of transmission of genetic information. Replication. Transcription

Ribosomal RNA.

rRNA accounts for 90% of the total RNA in a cell and is characterized by metabolic stability. In prokaryotes, there are three different types of rRNA with sedimentation coefficients of 23S, 16S and 5S; Eukaryotes have four types: -28S, 18S,5S and 5,8S.

RNAs of this type are localized in ribosomes and participate in specific interactions with ribosomal proteins.

Ribosomal RNAs have the form of a secondary structure in the form of double-stranded regions connected by a curved single strand. Ribosomal proteins are associated predominantly with single-stranded regions of the molecule.

rRNA is characterized by the presence of modified bases, but in significantly smaller quantities than in tRNA. rRNA contains mainly methylated nucleotides, with methyl groups attached either to the base or to the 2/-OH group of ribose.

Transfer RNA.

tRNA molecules are a single chain consisting of 70-90 nucleotides, with a molecular weight of 23000-28000 and a sedimentation constant of 4S. In cellular RNA, transfer RNA makes up 10-20%. tRNA molecules have the ability to covalently bind to a specific amino acid and connect through a system of hydrogen bonds with one of the nucleotide triplets of the mRNA molecule. Thus, tRNAs implement a code correspondence between an amino acid and the corresponding mRNA codon. To perform the adapter function, tRNAs must have a well-defined secondary and tertiary structure.

Each tRNA molecule has a constant secondary structure, has the shape of a two-dimensional cloverleaf and consists of helical regions formed by nucleotides of the same chain, and single-stranded loops located between them. The number of helical regions reaches half of the molecule. Unpaired sequences form characteristic structural elements (branches) that have typical branches:

A) acceptor stem, at the 3/-OH end of which in most cases there is a CCA triplet. The corresponding amino acid is added to the carboxyl group of the terminal adenosine using a specific enzyme;

B) pseudouridine or T C-loop, consists of seven nucleotides with the obligatory sequence 5 / -T CG-3 /, which contains pseudouridine; it is assumed that the T C loop is used to bind tRNA to the ribosome;

B) an additional loop - different in size and composition in different tRNAs;

D) the anticodon loop consists of seven nucleotides and contains a group of three bases (anticodon), which is complementary to the triplet (codon) in the mRNA molecule;

D) dihydrouridyl loop (D-loop), consisting of 8-12 nucleotides and containing from one to four dihydrouridyl residues; the D-loop is believed to be used to bind tRNA to a specific enzyme (aminoacyl-tRNA synthetase).

The tertiary packing of tRNA molecules is very compact and L-shaped. The corner of such a structure is formed by a dihydrouridine residue and a T C loop, the long leg forms an acceptor stem and a T C loop, and the short leg forms a D loop and an anticodon loop.

Polyvalent cations (Mg 2+ , polyamines), as well as hydrogen bonds between the bases and the phosphodiester backbone, participate in the stabilization of the tertiary structure of tRNA.

The complex spatial arrangement of the tRNA molecule is due to multiple highly specific interactions with both proteins and other nucleic acids (rRNA).

Transfer RNA differs from other types of RNA in its high content of minor bases - on average 10-12 bases per molecule, but the total number of them and tRNA increases as organisms move up the evolutionary ladder. Various methylated purine (adenine, guanine) and pyrimidine (5-methylcytosine and ribosylthymine) bases, sulfur-containing bases (6-thiouracil) were identified in tRNA, but the most common (6-thiouracil), but the most common minor component is pseudouridine. The role of unusual nucleotides in tRNA molecules is not yet clear, but it is believed that the lower the level of tRNA mitigation, the less active and specific it is.

The localization of modified nucleotides is strictly fixed. The presence of minor bases in tRNA makes the molecules resistant to the action of nucleases and, in addition, they are involved in maintaining a certain structure, since such bases are not capable of normal pairing and prevent the formation of a double helix. Thus, the presence of modified bases in tRNA determines not only its structure, but also many special functions of the tRNA molecule.

Most eukaryotic cells contain a set of different tRNAs. For each amino acid there is at least one specific tRNA. tRNAs that bind the same amino acid are called isoacceptor. Each type of cell in the body differs in its ratio of isoacceptor tRNAs.

Matrix (information)

Messenger RNA contains genetic information about the amino acid sequence for essential enzymes and other proteins, i.e. serves as a template for the biosynthesis of polypeptide chains. The share of mRNA in the cell accounts for 5% of the total amount of RNA. Unlike rRNA and tRNA, mRNA is heterogeneous in size, its molecular weight ranges from 25 10 3 to 1 10 6; mRNA is characterized by a wide range of sedimentation constants (6-25S). The presence of variable-length mRNA chains in a cell reflects the diversity of molecular weights of the proteins whose synthesis they provide.

In its nucleotide composition, mRNA corresponds to DNA from the same cell, i.e. is complementary to one of the DNA strands. The nucleotide sequence (primary structure) of mRNA contains information not only about the structure of the protein, but also about the secondary structure of the mRNA molecules themselves. The secondary structure of mRNA is formed due to mutually complementary sequences, the content of which is similar in RNA of different origins and ranges from 40 to 50%. A significant number of paired regions can be formed in the 3/ and 5/ regions of the mRNA.

Analysis of the 5/-ends of the 18s rRNA regions showed that they contain mutually complementary sequences.

The tertiary structure of mRNA is formed mainly due to hydrogen bonds, hydrophobic interactions, geometric and steric restrictions, and electrical forces.

Messenger RNA is a metabolically active and relatively unstable, short-lived form. Thus, the mRNA of microorganisms is characterized by rapid renewal, and its lifespan is several minutes. However, for organisms whose cells contain true membrane-bound nuclei, the lifespan of mRNA can reach many hours and even several days.

The stability of mRNA can be determined by various modifications of its molecule. Thus, it was found that the 5/-terminal sequence of mRNA of viruses and eukaryotes is methylated, or “blocked”. The first nucleotide in the 5/-terminal cap structure is 7-methylguanine, which is linked to the next nucleotide by a 5/-5/-pyrophosphate bond. The second nucleotide is methylated at the C-2/-ribose residue, and the third nucleotide may not have a methyl group.

Another ability of mRNA is that at the 3/-ends of many mRNA molecules of eukaryotic cells there are relatively long sequences of adenyl nucleotides, which are attached to the mRNA molecules with the help of special enzymes after completion of synthesis. The reaction takes place in the cell nucleus and cytoplasm.

At the 3/- and 5/- ends of the mRNA, modified sequences account for about 25% of the total length of the molecule. It is believed that 5/-caps and 3/-poly-A sequences are necessary either to stabilize the mRNA, protecting it from the action of nucleases, or to regulate the translation process.

RNA interference

Several types of RNA have been found in living cells that can reduce the degree of gene expression when complementary to the mRNA or the gene itself. MicroRNAs (21-22 nucleotides in length) are found in eukaryotes and exert their effects through the mechanism of RNA interference. In this case, a complex of microRNA and enzymes can lead to methylation of nucleotides in the DNA of the gene promoter, which serves as a signal to reduce gene activity. When using another type of regulation, the mRNA complementary to the microRNA is degraded. However, there are also miRNAs that increase rather than decrease gene expression. Small interfering RNAs (siRNAs, 20–25 nucleotides) are often produced by the cleavage of viral RNAs, but endogenous cellular siRNAs also exist. Small interfering RNAs also act through RNA interference by mechanisms similar to microRNAs. In animals, so-called Piwi-interacting RNA (piRNA, 29-30 nucleotides) has been found, acting in germ cells against transposition and playing a role in the formation of gametes. In addition, piRNAs can be epigenetically inherited on the maternal line, passing on their ability to inhibit transposon expression to offspring.

Antisense RNAs are widespread in bacteria, many of them suppress gene expression, but some activate expression. Antisense RNAs act by attaching to mRNA, which leads to the formation of double-stranded RNA molecules, which are degraded by enzymes. High molecular weight, mRNA-like RNA molecules have been found in eukaryotes. These molecules also regulate gene expression.

In addition to the role of individual molecules in gene regulation, regulatory elements can be formed in the 5" and 3" untranslated regions of mRNA. These elements can act independently to prevent translation initiation, or they can bind proteins such as ferritin or small molecules such as biotin.

Many RNAs are involved in modifying other RNAs. Introns are excised from pre-mRNA by spliceosomes, which, in addition to proteins, contain several small nuclear RNAs (snRNAs). In addition, introns can catalyze their own excision. The RNA synthesized as a result of transcription can also be chemically modified. In eukaryotes, chemical modifications of RNA nucleotides, for example, their methylation, are performed by small nuclear RNAs (snRNAs, 60-300 nucleotides). This type of RNA is localized in the nucleolus and Cajal bodies. After association of snRNAs with enzymes, snRNAs bind to the target RNA by forming base pairs between the two molecules, and the enzymes modify the nucleotides of the target RNA. Ribosomal and transfer RNAs contain many such modifications, the specific position of which is often conserved during evolution. SnRNAs and snRNAs themselves can also be modified. Guide RNAs carry out the process of RNA editing in the kinetoplast, a special region of the mitochondria of kinetoplastid protists (for example, trypanosomes).

Genomes made of RNA

Like DNA, RNA can store information about biological processes. RNA can be used as the genome of viruses and virus-like particles. RNA genomes can be divided into those that do not have a DNA intermediate step and those that are copied into a DNA copy and back into RNA (retroviruses) to reproduce.

Many viruses, such as the influenza virus, contain a genome consisting entirely of RNA at all stages. RNA is contained within a typically protein shell and is replicated using RNA-dependent RNA polymerases encoded within it. Viral genomes consisting of RNA are divided into:

“minus strand RNA”, which serves only as a genome, and a molecule complementary to it is used as mRNA;

double-stranded viruses.

Viroids are another group of pathogens that contain an RNA genome and no protein. They are replicated by RNA polymerases of the host organism.

Retroviruses and retrotransposons

Other viruses have an RNA genome during only one phase of their life cycle. The virions of so-called retroviruses contain RNA molecules, which, when they enter the host cells, serve as a template for the synthesis of a DNA copy. In turn, the DNA template is read by the RNA gene. In addition to viruses, reverse transcription is also used in a class of mobile genome elements - retrotransposons.

Ribonucleic acid is a copolymer of purine and pyrimidine ribonucleotides connected to each other, as in DNA, by phosphodiester bridges (Fig. 37.6). Although these two types of nucleic acids have much in common, they differ from each other in a number of ways.

1. In RNA, the carbohydrate residue to which purine or pyrimidine bases and phosphate groups are attached is ribose, and not 2-deoxyribose (as in DNA).

2. The pyrimidine components of RNA are different from those of DNA. RNA, like DNA, contains the nucleotides adenine, guanine and cytosine. At the same time, RNA (with the exception of some special cases, which we will discuss below) does not contain thymine; its place in the RNA molecule is occupied by uracil.

3. RNA is a single-stranded molecule (unlike DNA, which has a double-stranded structure), however, if there are sections in the RNA chain with a complementary sequence (opposite polarity), a single RNA chain is capable of folding to form so-called “hairpins,” structures that have double-stranded characteristics ( Fig. 37.7).

Rice. 37.6. A fragment of a ribonucleic acid (RNA) molecule in which the purine and pyrimidine bases - adenine (A), uracil (U), cytosine (C) and guanine ( - are held by a phosphodiester backbone connecting ribosyl residues linked by an N-glycosidic bond to the corresponding nucleic bases Please note: the RNA chain has a certain directionality, which is indicated by the 5- and 3-terminal phosphate residues.

4. Since the RNA molecule is a single strand complementary to only one of the DNA strands, the guanine content in it is not necessarily equal to the cytosine content, and the adenine content is not necessarily equal to the uracil content.

5. RNA can be hydrolyzed with alkali to 2, 3-cyclic diesters of mononucleotides; the intermediate product of hydrolysis is 2,U,5-triester, which is not formed during alkaline hydrolysis of DNA due to the absence of 2-hydroxyl groups in the latter; The alkaline lability of RNA (compared to DNA) is a useful property for both diagnostic and analytical purposes.

The information contained in single-stranded RNA is implemented in the form of a specific sequence of purine and pyrimidine bases (i.e., in the primary structure) of the polymer chain. This sequence is complementary to the coding strand of the gene from which the RNA is “read”. Due to complementarity, the RNA molecule is able to specifically bind (hybridize) to the coding strand, but does not hybridize to the non-coding DNA strand. The RNA sequence (with the exception of the replacement of T with U) is identical to the sequence of the non-coding strand of the gene (Fig. 37.8).

Biological functions of RNA

Several types of RNA are known. Almost all of them are directly involved in the process of protein biosynthesis. Cytoplasmic RNA molecules that serve as templates for protein synthesis are called messenger RNA (mRNA). Another type of cytoplasmic RNA, ribosomal RNA (rRNA), acts as structural components of ribosomes (organelles that play an important role in protein synthesis). Adapter molecules of transfer RNA (tRNA) are involved in the translation (translation) of mRNA information into the amino acid sequence in proteins.

A significant portion of primary RNA transcripts produced in eukaryotic cells, including mammalian cells, undergo degradation in the nucleus and do not play any structural or informational role in the cytoplasm. In cultivated

Rice. 37.7. The secondary structure of an RNA molecule is of the “stem-loop” (“hairpin”) type, resulting from the intramolecular formation of hydrogen bonds between complementary pairs of nucleic bases.

In human cells, a class of small nuclear RNAs has been discovered that are not directly involved in protein synthesis, but can influence RNA processing and the overall “architecture” of the cell. The sizes of these relatively small molecules vary, the latter containing from 90 to 300 nucleotides (Table 37.3).

RNA is the main genetic material in some animal and plant viruses. Some RNA viruses never undergo reverse transcription of RNA into DNA. However, most known animal viruses, such as retroviruses, are characterized by reverse transcription of their RNA genome, directed by RNA-dependent DNA polymerase (reverse transcriptase) to form a double-stranded DNA copy. In many cases, the resulting double-stranded DNA transcript is integrated into the genome and subsequently ensures the expression of viral genes, as well as the production of new copies of viral RNA genomes.

Structural organization of RNA

In all eukaryotic and prokaryotic organisms, there are three main classes of RNA molecules: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Representatives of these classes differ from each other in size, function and stability.

Messenger RNA (mRNA) is the most heterogeneous class in terms of size and stability. All representatives of this class serve as carriers of information from the gene to the protein-synthesizing system of the cell. They act as templates for the synthesized polypeptide, i.e. they determine the amino acid sequence of the protein (Fig. 37.9).

Messenger RNAs, especially eukaryotic ones, have some unique structural features. The 5-end of the mRNA is capped with a 7-methylguanosine triphosphate attached to the 5-hydroxyl of the adjacent 2-0-methylribonucleoside via a triphosphate residue (Figure 37.10). mRNA molecules often contain internal 6-methyladenine residues and 2-0-methylated ribonucleotides. Although the meaning of “capping” is not yet fully understood, it can be assumed that the resulting structure of the 5-end of the mRNA is used for specific recognition in the translation system. Protein synthesis begins at the 5" (capped) end of the mRNA. The other end of most mRNA molecules (3-end) contains a polyadenylate chain of 20-250 nucleotides. The specific functions of this have not been fully established. It can be assumed that this structure is responsible for maintaining intracellular stability mRNA. Some mRNAs, including histones, do not contain poly (A). The presence of poly (A) in the structure of mRNA is used to separate it from other types of RNA by fractionating total RNA on columns with oligo (T) immobilized on a solid support such as cellulose. with the column occurs due to complementary interactions of the poly (A) “tail” with the immobilized oligo (T).

Rice. 37.8. Sequence of the gene and its RNA transcript. The coding and noncoding strands are shown and their polarity noted. An RNA transcript having polarity is complementary to the coding strand (with a polarity of 3 - 5) and is identical in sequence (except for T to U substitutions) and polarity of the non-coding DNA strand.

Rice. 37.9. Expression of DNA genetic information in the form of an mRNA transcript and subsequent translation with the participation of ribosomes with the formation of a specific protein molecule.

(see scan)

Rice. 37.10. The structure of the “cap” found at the 5-end of most eukaryotic messenger RNAs 7-methylguanosine triphosphate is attached to the 5-end of the mRNA. which usually contains a 2-O-methylpurine nucleotide.

In mammalian cells, including human cells, mature mRNA molecules found in the cytoplasm are not a complete copy of the transcribed region of the gene. The polyribonucleotide formed as a result of transcription is a precursor of cytoplasmic mRNA; before leaving the nucleus, it undergoes specific processing. Full-length transcription products found in the nuclei of mammalian cells form a fourth class of RNA molecules. Such nuclear RNAs are very heterogeneous and reach significant sizes. Heterogeneous nuclear RNA molecules can have a molecular weight of more than , while the molecular weight of mRNA usually does not exceed 2106. They are processed in the nucleus, and the resulting mature mRNAs enter the cytoplasm, where they serve as a template for protein biosynthesis.

Transfer RNA (tRNA) molecules typically contain about 75 nucleotides. The molecular weight of such molecules is . tRNAs are also formed as a result of specific processing of corresponding precursor molecules (see Chapter 39). Transfer tRNAs act as intermediaries during the translation of mRNA. In any cell there are at least 20 types of tRNA molecules. Each type (sometimes several types) of tRNA corresponds to one of the 20 amino acids necessary for protein synthesis. Although each specific tRNA differs from the others in its nucleotide sequence, they all have common features. Due to several intrastrand complementary regions, all tRNAs have a secondary structure called “cloverleaf” (Fig. 37.11).

All types of tRNA molecules have four main arms. The acceptor arm consists of a “stem” of paired nucleotides and ends with the CCA sequence. It is through the Y-hydroxyl group of the adenosyl residue that binding to the carboxyl group of the amino acid occurs. The remaining arms also consist of “stems” formed by complementary base pairs and loops of unpaired bases (Fig. 37.7). The anticodon arm recognizes a nucleotide triplet or codon (see Chapter 40) in the mRNA. The D-arm is named so due to the presence of dihydrouridine in it, the -arm is named after the sequence T-pseudouridine-C. The accessory arm is the most variable structure and serves as the basis for tRNA classification. Class 1 tRNAs (75% of their total number) have an additional arm 3-5 base pairs long. The extra arm of class 2 tRNA molecules consists of 13-21 base pairs and often includes an unpaired loop.

Rice. 37.11. The structure of an aminoacyl-tRNA molecule with an amino acid attached to the 3-CCA end. Intramolecular hydrogen bonds and the location of the anticodon, TTC, and dihydrouracil arms are indicated. (From J. D. Watson. Molecular biology of the Gene 3rd, ed.. Copyright 1976, 1970, 1965 by W. A. ​​Benjamin, Inc., Menlo Park Calif.)

The secondary structure, determined by the system of complementary interactions of the nucleotide bases of the corresponding arms, is characteristic of all species. The acceptor arm contains seven base pairs, the -arm contains five base pairs, and the D arm contains three (or four) base pairs.

tRNA molecules are very stable in prokaryotes and somewhat less stable in eukaryotes. The opposite situation is typical for mRNA, which is quite unstable in prokaryotes, but has significant stability in eukaryotic organisms.

Ribosomal RNA. A ribosome is a cytoplasmic nucleoprotein structure designed for protein synthesis using an mRNA template. The ribosome provides a specific contact, as a result of which the nucleotide sequence read from a specific gene is translated into the amino acid sequence of the corresponding protein.

In table Figure 37.2 shows the components of mammalian ribosomes having a molecular weight of 4.210 6 and a sedimentation rate (Svedberg units). Mammalian ribosomes consist of two nucleoprotein subunits - a large one with

Table 37.2. Components of mammalian ribosomes

molecular weight (60S), and low molecular weight (40S). The 608 subunit contains 58-ribosomal RNA (rRNA), 5.8S-rRNA and 28S-rRNA, as well as more than 50 different polypeptides. The small, 408-subunit includes a single 18S-pRNA and about 30 polypeptide chains. All ribosomal RNAs, with the exception of 5S-RNA, have a common precursor, 45S-RNA, localized in the nucleolus (see Chapter 40). The 5S-RNA molecule has its own precursor. In the nucleolus, highly methylated ribosomal RNAs are packaged with ribosomal proteins. In the cytoplasm, ribosomes are quite stable and capable of carrying out a large number of translation cycles.

Small stable RNAs. A large number of discrete, highly conserved, small and stable RNA molecules have been found in eukaryotic cells. Most RNAs of this type are found in ribonucleoproteins and are localized in the nucleus, cytoplasm, or both compartments simultaneously. The sizes of these molecules vary from 90 to 300 nucleotides, their content is 100,000-1,000,000 copies per cell.

Small nuclear ribonucleic particles (often called snurps) appear to play a significant role in the regulation of gene expression. Nucleoprotein particles of the U7 type appear to be involved in the formation of the 3-termini of histone mRNAs. Particles are probably required for polyadenylation, a - for the removal of introns and mRNA processing (see Chapter 39). Table 37.3. summarizes some characteristics of small stable RNAs.

Table 37.3. Some types of small stable RNAs found in mammalian cells

LITERATURE

Darnell J. et al. Molecular Cell Biology, Scientific American Books, 1986.

Hunt T. DNA Makes RNA Makes Protein, Elsevier, 1983. Lewin B. Genes, 2nd ed., Wiley, 1985.

Rich A. et al. The chemistry and biology of left-handed Z-DNA, Annu. Rev. Biochem., 1984, 53, 847.

Turner P. Controlling roles for snurps, Nature, 1985, 316, 105. Watson J. D. The Double Helix, Atheneum, 1968.

Watson J.D., Crick F.H.C. Molecular structure of nucleic acids. Nature, 1953, 171, 737.

Zieve G. W. Two groups of small stable RNAs, Cell, 1981, 25, 296.

RNA- a polymer whose monomers are ribonucleotides. Unlike DNA, RNA is formed not by two, but by one polynucleotide chain (with the exception that some RNA-containing viruses have double-stranded RNA). RNA nucleotides are capable of forming hydrogen bonds with each other. RNA chains are much shorter than DNA chains.

RNA monomer - nucleotide (ribonucleotide)- consists of residues of three substances: 1) a nitrogenous base, 2) a five-carbon monosaccharide (pentose) and 3) phosphoric acid. The nitrogenous bases of RNA also belong to the classes of pyrimidines and purines.

The pyrimidine bases of RNA are uracil, cytosine, and the purine bases are adenine and guanine. The RNA nucleotide monosaccharide is ribose.

Highlight three types of RNA: 1) informational(messenger) RNA - mRNA (mRNA), 2) transport RNA - tRNA, 3) ribosomal RNA - rRNA.

All types of RNA are unbranched polynucleotides, have a specific spatial conformation and take part in the processes of protein synthesis. Information about the structure of all types of RNA is stored in DNA. The process of synthesizing RNA on a DNA template is called transcription.

Transfer RNAs usually contain 76 (from 75 to 95) nucleotides; molecular weight - 25,000–30,000. tRNA accounts for about 10% of the total RNA content in the cell. Functions of tRNA: 1) transport of amino acids to the site of protein synthesis, to ribosomes, 2) translational intermediary. There are about 40 types of tRNA found in a cell, each of them has a unique nucleotide sequence. However, all tRNAs have several intramolecular complementary regions, due to which the tRNAs acquire a clover-leaf-like conformation. Any tRNA has a loop for contact with the ribosome (1), an anticodon loop (2), a loop for contact with the enzyme (3), an acceptor stem (4), and an anticodon (5). The amino acid is added to the 3" end of the acceptor stem. Anticodon- three nucleotides that “identify” the mRNA codon. It should be emphasized that a specific tRNA can transport a strictly defined amino acid corresponding to its anticodon. The specificity of the connection between amino acid and tRNA is achieved due to the properties of the enzyme aminoacyl-tRNA synthetase.

Ribosomal RNA contain 3000–5000 nucleotides; molecular weight - 1,000,000–1,500,000. rRNA accounts for 80–85% of the total RNA content in the cell. In complex with ribosomal proteins, rRNA forms ribosomes - organelles that carry out protein synthesis. In eukaryotic cells, rRNA synthesis occurs in the nucleoli. Functions of rRNA: 1) a necessary structural component of ribosomes and, thus, ensuring the functioning of ribosomes; 2) ensuring the interaction of the ribosome and tRNA; 3) initial binding of the ribosome and the initiator codon of the mRNA and determination of the reading frame, 4) formation of the active center of the ribosome.