Cytology (Greekκύτος - "receptacle", here: "cell" and λόγος - "teaching", "science") - section biology who studies living cells, them organelles, their structure, functioning, processes of cell reproduction, aging and death.

The terms are also used cell biology, cell biology (English Cell Biology).

The emergence and development of cytology

Drawing by Robert Hooke depicting a section of cork tissue under a microscope (from the book Micrographia, 1664)

The term "cell" was first used Robert Hooke in 1665, in describing his "investigations into the structure of cork with magnifying lenses". AT 1674 Anthony van Leeuwenhoek established that the substance inside the cell is organized in a certain way. He was the first to discover cell nuclei. At this level, the idea of ​​a cell lasted for more than 100 years.

The study of the cell accelerated in the 1830s when improved microscopes. In 1838-1839 botanist Matthias Schleiden and anatomist Theodor Schwann almost simultaneously put forward the idea of ​​the cellular structure of the body. T. Schwann proposed the term " cell theory and presented this theory to the scientific community. The emergence of cytology is closely connected with the creation cell theory- the broadest and most fundamental of all biological generalizations. According to the cellular theory, all plants and animals consist of similar units - cells, each of which has all the properties of a living thing.

The most important addition to the cell theory was the statement of the famous German naturalist Rudolf Virchow that each cell is formed as a result of the division of another cell.

In the 1870s, two methods of eukaryotic cell division were discovered, later called mitosis and meiosis. Already 10 years later, it was possible to establish the main genetic features of these types of division. It was found that prior to mitosis, chromosome doubling and their uniform distribution between daughter cells occur, so that the former number of chromosomes is retained in the daughter cells. Before meiosis, the number of chromosomes also doubles, but in the first (reduction) division, two-chromatid chromosomes diverge to the poles of the cell, so that cells with a haploid set are formed, the number of chromosomes in them is two times less than in the mother cell. It was found that the number, shape and size of chromosomes - karyotype- the same in all somatic cells of animals of a given species, and the number of chromosomes in gametes twice smaller. Subsequently, these cytological discoveries formed the basis chromosome theory of heredity.

Clinical Cytology

Clinical cytology is a section of laboratory diagnostics and is descriptive. In particular, an important section of clinical cytology is oncocytology, which is faced with the task of diagnosing neoplasms.

Ribonucleic acid (RNA) is one of the three main macromolecules(the other two are DNA and squirrels), which are contained in the cells of all living organisms.

Just like DNA (deoxyribonucleic acid), RNA is made up of a long chain in which each link is called nucleotide. Each nucleotide is made up of nitrogenous base, sugar ribose and phosphate group. The sequence of nucleotides allows RNA to encode genetic information. All cellular organisms use RNA ( mRNA) for programming protein synthesis.

Cellular RNAs are formed in a process called transcription, that is, the synthesis of RNA on a DNA template, carried out by special enzymes - RNA polymerases. Then messenger RNAs(mRNA) take part in a process called broadcast. Translation is a synthesis squirrel on the mRNA matrix with the participation ribosome. Other RNAs undergo chemical modifications after transcription, and after the formation of secondary and tertiary structures, they perform functions that depend on the type of RNA.

Single-stranded RNAs are characterized by a variety of spatial structures in which some of the nucleotides of the same chain are paired with each other. Some highly structured RNAs are involved in cell protein synthesis, for example, transfer RNAs serve to recognize codons and delivery of relevant amino acids to the site of protein synthesis ribosomal RNA serve as a structural and catalytic basis of the ribosome.

However, the functions of RNA in modern cells are not limited to their role in translation. So, small nuclear RNA take part in splicing eukaryotic messenger RNA and other processes.

In addition to the fact that RNA molecules are part of some enzymes (for example, telomerase), some RNAs have their own enzymatic activity: the ability to make breaks in other RNA molecules or, conversely, "glue" two RNA fragments. Such RNAs are called ribozymes.

Series genomes viruses consist of RNA, that is, in them it plays the role that DNA plays in higher organisms. Based on the diversity of RNA functions in the cell, a hypothesis was put forward, according to which RNA is the first molecule that was capable of self-reproduction in prebiological systems.

The time in which we live is marked by amazing changes, huge progress, when people receive answers to more and more new questions. Life is rapidly moving forward, and what until recently seemed impossible is beginning to come true. It is quite possible that what seems today to be a plot from the science fiction genre will soon also acquire the features of reality.

One of the most important discoveries in the second half of the twentieth century was the nucleic acids RNA and DNA, thanks to which man came closer to unraveling the mysteries of nature.

Nucleic acids

Nucleic acids are organic compounds with high molecular weight properties. They include hydrogen, carbon, nitrogen and phosphorus.

They were discovered in 1869 by F. Misher, who investigated pus. However, at that time his discovery was not given much importance. Only later, when these acids were found in all animal and plant cells, did the understanding of their enormous role come.

There are two types of nucleic acids: RNA and DNA (ribonucleic and deoxyribonucleic acids). This article is devoted to ribonucleic acid, but for a general understanding, we will also consider what DNA is.

What

DNA is made up of two strands that are connected according to the law of complementarity by hydrogen bonds between nitrogenous bases. Long chains are twisted into a spiral, one turn contains almost ten nucleotides. The diameter of the double helix is ​​two millimeters, the distance between nucleotides is about half a nanometer. The length of one molecule sometimes reaches several centimeters. The length of DNA in the nucleus of a human cell is almost two meters.

The structure of DNA contains all DNA that has replication, which means a process during which two completely identical daughter molecules are formed from one molecule.

As already noted, the chain is made up of nucleotides, which, in turn, consist of nitrogenous bases (adenine, guanine, thymine and cytosine) and a phosphorus acid residue. All nucleotides differ in nitrogenous bases. Hydrogen bonding does not occur between all bases; adenine, for example, can only combine with thymine or guanine. Thus, there are as many adenyl nucleotides in the body as thymidyl nucleotides, and the number of guanyl nucleotides is equal to cytidyl nucleotides (Chargaff's rule). It turns out that the sequence of one chain predetermines the sequence of another, and the chains seem to mirror each other. Such a pattern, where the nucleotides of two chains are arranged in an orderly manner, and are also connected selectively, is called the principle of complementarity. In addition to hydrogen compounds, the double helix also interacts hydrophobically.

The two chains are in opposite directions, that is, they are located in opposite directions. Therefore, opposite the three "-end of one is the five"-end of the other chain.

Outwardly, it resembles a spiral staircase, the railing of which is a sugar-phosphate backbone, and the steps are complementary nitrogen bases.

What is ribonucleic acid?

RNA is a nucleic acid with monomers called ribonucleotides.

In chemical properties, it is very similar to DNA, since both are polymers of nucleotides, which are a phosphorylated N-glycoside that is built on a pentose (five-carbon sugar) residue, with a phosphate group at the fifth carbon atom and a nitrogen base at the first carbon atom.

It is a single polynucleotide chain (except for viruses), which is much shorter than that of DNA.

One RNA monomer is the residues of the following substances:

• nitrogen bases;
• five-carbon monosaccharide;
• phosphorus acids.

RNAs have pyrimidine (uracil and cytosine) and purine (adenine, guanine) bases. Ribose is the monosaccharide of the RNA nucleotide.

Differences between RNA and DNA

Nucleic acids differ from each other in the following properties:

• its quantity in the cell depends on the physiological state, age and organ affiliation;
• DNA contains the carbohydrate deoxyribose, and RNA contains ribose;
• the nitrogenous base in DNA is thymine, and in RNA it is uracil;
• classes perform different functions, but are synthesized on the DNA matrix;
• DNA is made up of a double helix, while RNA is made up of a single strand;
• it is uncharacteristic of acting in DNA;
• RNA has more minor bases;
• chains vary greatly in length.

History of study

The RNA cell was first discovered by a German biochemist R. Altman while studying yeast cells. In the middle of the twentieth century, the role of DNA in genetics was proven. Only then were RNA types, functions, and so on described. Up to 80-90% of the mass in the cell falls on rRNA, which together with proteins form the ribosome and participate in protein biosynthesis.

In the sixties of the last century, it was first suggested that there must be a certain species that carries the genetic information for protein synthesis. After that, it was scientifically established that there are such informational ribonucleic acids representing complementary copies of genes. They are also called messenger RNA.

The so-called transport acids are involved in decoding the information recorded in them.

Later, methods began to be developed for identifying the nucleotide sequence and establishing the structure of RNA in the acid space. So it was found that some of them, which were called ribozymes, can cleave polyribonucleotide chains. As a result, they began to assume that at the time when life was born on the planet, RNA acted without DNA and proteins. Moreover, all transformations were carried out with her participation.

The structure of the ribonucleic acid molecule

Almost all RNAs are single chains of polynucleotides, which, in turn, consist of monoribonucleotides - purine and pyrimidine bases.

Nucleotides are denoted by the initial letters of the bases:

• adenine (A), A;
• guanine (G), G;
• cytosine (C), C;
• uracil (U), U.

They are interconnected by three- and five-phosphodiester bonds.

A very different number of nucleotides (from several tens to tens of thousands) is included in the structure of RNA. They can form a secondary structure consisting mainly of short double-stranded strands that are formed by complementary bases.

Structure of a ribnucleic acid molecule

As already mentioned, the molecule has a single-stranded structure. RNA receives its secondary structure and shape as a result of the interaction of nucleotides with each other. It is a polymer whose monomer is a nucleotide consisting of a sugar, a phosphorus acid residue and a nitrogen base. Outwardly, the molecule is similar to one of the DNA chains. Nucleotides adenine and guanine, which are part of RNA, are purine. Cytosine and uracil are pyrimidine bases.

Synthesis process

In order for an RNA molecule to be synthesized, the template is a DNA molecule. True, the reverse process also happens, when new molecules of deoxyribonucleic acid are formed on the ribonucleic acid matrix. This occurs during the replication of certain types of viruses.

Other molecules of ribonucleic acid can also serve as a basis for biosynthesis. Its transcription, which occurs in the cell nucleus, involves many enzymes, but the most significant of them is RNA polymerase.

Kinds

Depending on the type of RNA, its functions also differ. There are several types:

• informational i-RNA;
• ribosomal r-RNA;
• transport t-RNA;
• minor;
• ribozymes;
• viral.

Information ribonucleic acid

Such molecules are also called matrix. They make up about two percent of the total in the cell. In eukaryotic cells, they are synthesized in the nuclei on DNA templates, then passing into the cytoplasm and binding to ribosomes. Further, they become templates for protein synthesis: they are joined by transfer RNAs that carry amino acids. This is how the process of information transformation takes place, which is realized in the unique structure of the protein. In some viral RNAs, it is also a chromosome.

Jacob and Mano are the discoverers of this species. Not having a rigid structure, its chain forms curved loops. Not working, i-RNA gathers into folds and folds into a ball, and unfolds in working condition.

The mRNA carries information about the sequence of amino acids in the protein that is being synthesized. Each amino acid is encoded in a specific place using genetic codes, which are characterized by:

• triplet - from four mononucleotides it is possible to build sixty-four codons (genetic code);
• non-crossing - information moves in one direction;
• continuity - the principle of operation is that one mRNA is one protein;
• universality - one or another type of amino acid is encoded in all living organisms in the same way;
• degeneracy - twenty amino acids are known, and sixty-one codons, that is, they are encoded by several genetic codes.

Ribosomal ribonucleic acid

Such molecules make up the vast majority of cellular RNA, namely eighty to ninety percent of the total. They combine with proteins and form ribosomes - these are organelles that perform protein synthesis.

Ribosomes are sixty-five percent rRNA and thirty-five percent protein. This polynucleotide chain easily bends along with the protein.

The ribosome consists of amino acid and peptide regions. They are located on contact surfaces.

Ribosomes move freely to the right places. They are not very specific and can not only read information from mRNA, but also form a matrix with them.

Transport ribonucleic acid

tRNAs are the most studied. They make up ten percent of cellular ribonucleic acid. These types of RNA bind to amino acids thanks to a special enzyme and are delivered to ribosomes. In this case, amino acids are carried by transport molecules. However, it happens that different codons code for an amino acid. Then several transport RNAs will carry them.

It curls up into a ball when inactive, and when functioning, has the appearance of a clover leaf.

It contains the following sections:

• an acceptor stem having an ACC nucleotide sequence;
• site for attachment to the ribosome;
• an anticodon encoding the amino acid that is attached to this tRNA.

Minor species of ribonucleic acid

Recently, RNA species have been replenished with a new class, the so-called small RNAs. They are most likely universal regulators that turn genes on or off in embryonic development, and also control processes within cells.

Ribozymes have also recently been identified, they are actively involved when the RNA acid is fermented, acting as a catalyst.

Viral types of acids

The virus is capable of containing either ribonucleic acid or deoxyribonucleic acid. Therefore, with the corresponding molecules, they are called RNA-containing. When such a virus enters a cell, reverse transcription occurs - new DNA appears on the basis of ribonucleic acid, which are integrated into cells, ensuring the existence and reproduction of the virus. In another case, the formation of complementary RNA occurs on the incoming RNA. Viruses are proteins, vital activity and reproduction goes on without DNA, but only on the basis of the information contained in the RNA of the virus.

replication

In order to improve the overall understanding, it is necessary to consider the process of replication, which results in two identical nucleic acid molecules. This is how cell division begins.

It involves DNA polymerases, DNA-dependent, RNA polymerases and DNA ligases.

The replication process consists of the following steps:

• despiralization - there is a sequential unwinding of the maternal DNA, capturing the entire molecule;
• breakage of hydrogen bonds, in which the chains diverge, and a replication fork appears;
• adjustment of dNTPs to the released bases of maternal chains;
• cleavage of pyrophosphates from dNTP molecules and the formation of phosphorodiester bonds due to the released energy;
• respiration.

After the formation of the daughter molecule, the nucleus, cytoplasm and the rest are divided. Thus, two daughter cells are formed that have completely received all the genetic information.

In addition, the primary structure of proteins that are synthesized in the cell is encoded. DNA takes an indirect part in this process, and not direct, which consists in the fact that it is on DNA that the synthesis of proteins, RNA involved in the formation, takes place. This process is called transcription.

Transcription

The synthesis of all molecules occurs during transcription, that is, the rewriting of genetic information from a specific DNA operon. The process is similar in some ways to replication, and in others it is very different.

Similarities are the following parts:

• the beginning comes from the despiralization of DNA;
• there is a break in the hydrogen bonds between the bases of the chains;
• NTFs are complementarily adjusted to them;
• hydrogen bonds are formed.

Differences from replication:

• during transcription, only the section of DNA corresponding to the transcripton is untwisted, while during replication, the entire molecule is untwisted;
• during transcription, tunable NTPs contain ribose, and instead of thymine, uracil;
• information is written off only from a certain area;
• after the formation of the molecule, the hydrogen bonds and the synthesized strand are broken, and the strand slips off the DNA.

For normal functioning, the primary structure of RNA should consist only of DNA sections written off from exons.

The newly formed RNA begins the process of maturation. Silent regions are excised, and informative regions are fused to form a polynucleotide chain. Further, each species has transformations inherent only to it.

In mRNA, attachment to the initial end occurs. Polyadenylate joins the final site.

Bases are modified in tRNA to form minor species.

In r-RNA, individual bases are also methylated.

Protect from destruction and improve the transport of proteins into the cytoplasm. RNA in the mature state are connected to them.

Significance of deoxyribonucleic and ribonucleic acids

Nucleic acids are of great importance in the life of organisms. They store, transfer to the cytoplasm and inherit to daughter cells information about the proteins synthesized in each cell. They are present in all living organisms, the stability of these acids plays an important role for the normal functioning of both cells and the whole organism. Any changes in their structure will lead to cellular changes.

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. The composition of RNA, as well as the composition of DNA, includes the nucleotides of 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 with a complementary sequence (opposite polarity) in the RNA chain, a single RNA chain can fold 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 retained by a phosphodiester backbone connecting ribosyl residues linked by an N-glycosidic bond to the corresponding nucleic bases Note that the RNA strand has a specific directionality 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 content of guanine in it is not necessarily equal to the content of cytosine, and the content of adenine is not necessarily equal to the content of uracil.

5. RNA can be hydrolyzed with alkali to 2,3-cyclic diesters of mononucleotides; 2, Y, 5- triester acts as an intermediate hydrolysis product, 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 a single-stranded RNA is realized 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) with the coding strand, but not hybridize with 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 gene strand (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 act as templates for protein synthesis are called messenger RNA (mRNA). Another type of cytoplasmic RNA, ribosomal RNA (rRNA), plays the role of structural components of ribosomes (organelles that play an important role in protein synthesis). Transfer RNA (tRNA) adapter molecules are involved in the translation (translation) of mRNA information into the amino acid sequence in proteins.

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

Rice. 37.7. The secondary structure of an RNA molecule of the “loop with a stem” (“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 affect RNA processing and the overall “architecture” of the cell. The sizes of these relatively small molecules vary, the latter contain from 90 to 300 nucleotides (Table 37.3).

RNA is the main genetic material in some animal and plant viruses. Some RNA viruses never go through the 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 further ensures the expression of virus 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: information (matrix or messenger) RNA (mRNA), transport (tRNA) and ribosomal (rRNA). Representatives of these classes differ from each other in size, function and stability.

Informational (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, that is, 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" by 7-methylguanosine triphosphate attached to the 5-hydroxyl of the neighboring 2-0-methylribonucleoside through a triphosphate residue (Fig. 37.10). mRNA molecules often contain internal 6-methyladenine residues and 2-0-methylated ribonucleotides. Although the meaning of "capping" has not yet been fully elucidated, it can be assumed that the resulting structure of the mRNA 5-terminus is used for specific recognition in the translation system. Protein synthesis begins at the 5"-(capped) end of 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 finally 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 mRNA structure is used to separate 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 poly (A) - "tail" with immobilized oligo (T).

Rice. 37.8. The sequence of a gene and its RNA transcript. The coding and non-coding strands are shown, and their polarities are noted. An RNA transcript having polarity is complementary to the coding strand (with polarity 3-5) and 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 to form a specific protein molecule.

(see scan)

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

In mammalian cells, including human cells, mature mRNA molecules located 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. The unprocessed transcription products found in the nuclei of mammalian cells form the fourth class of RNA molecules. Such nuclear RNAs are very heterogeneous and reach considerable sizes. Molecules of heterogeneous nuclear RNA 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 matrix for protein biosynthesis.

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

Molecules of all types of tRNA 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 the 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 mRNA. The D-arm is so named because of the presence of dihydrouridine in it, the -arm is named after the sequence of T-pseudouridine-C. The extra arm is the most variable structure and serves as the basis for the classification of tRNAs. 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 is 13-21 base pairs long and often includes an unpaired loop.

Rice. 37.11. The structure of the aminoacyl-tRNA molecule, to the 3-CCA-terminus of which an amino acid is attached. The 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 nucleotide bases of the corresponding arms, is characteristic of all species. The acceptor arm contains seven base pairs, the - arm - five base pairs, the D arm - three (or four) base pairs.

tRNA molecules are very stable in prokaryotes and somewhat less so in eukaryotes. The reverse situation is typical for mRNA, which is rather unstable in prokaryotes, while in eukaryotic organisms it has significant stability.

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

In table. 37.2 shows the components of mammalian ribosomes with a molecular weight of 4.210 6 and sedimentation rate (Swedberg units). Mammalian ribosomes are composed of two nucleoprotein subunits, the large c

Table 37.2. Mammalian ribosome components

molecular weight (60S), and small, having a molecular weight (40S). The 608 subunit contains 58-ribosomal RNA (rRNA), 5,8S-pRNA and 28S-pRNA, 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, share a common precursor, 45S-RNA, located 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 RNA. 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 simultaneously in both compartments. 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 - from the English small nuclear ribonucleic particles) probably play an essential role in the regulation of gene expression. Nucleoprotein particles of the U7 type seem to be involved in the formation of the 3-terminals of histone mRNAs. Particles are probably required for polyadenylation, a for intron removal and mRNA processing (see Chapter 39). Tab. 37.3. summarizes some characteristics of small stable RNAs.

Table 37.3. Some types of small stable RNA 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.

The RNA molecule is also a polymer, the monomers of which are ribonucleotides, RNA is a single-stranded molecule. It is built in the same way as one of the DNA strands. RNA nucleotides are similar to DNA nucleotides, although they are not identical to them. There are also four of them, and they consist of residues of a nitrogenous base, pentose and phosphoric acid. The three nitrogenous bases are exactly the same as in DNA: BUT, G and C. However, instead of T DNA in RNA contains a pyrimidine base of similar structure, uracil ( At). The main difference between DNA and RNA is the nature of the carbohydrate: in DNA nuclotides, the monosaccharide is deoxyribose, and in RNA, it is ribose. The connection between nucleotides is carried out, as in DNA, through a sugar and a phosphoric acid residue. Unlike DNA, the content of which is constant in the cells of certain organisms, the content of RNA in them fluctuates. It is noticeably higher where intensive synthesis occurs.

In relation to the functions performed, several types of RNA are distinguished.

Transfer RNA (tRNA). tRNA molecules are the shortest: they consist of only 80-100 nucleotides. The molecular weight of such particles is 25-30 thousand. Transport RNAs are mainly contained in the cytoplasm of the cell. Their function is to transfer amino acids to ribosomes, to the site of protein synthesis. Of the total RNA content of cells, tRNA accounts for about 10%.

Ribosomal RNA (rRNA). These are large molecules: they include 3-5 thousand nucleotides, respectively, their molecular weight reaches 1-1.5 million. Ribosomal RNAs make up an essential part of the ribosome. Of the total RNA content in the cell, rRNA accounts for about 90%.

Messenger RNA (mRNA), or messenger RNA (mRNA), found in the nucleus and cytoplasm. Its function is to transfer information about the protein structure from DNA to the site of protein synthesis in ribosomes. The share of mRNA accounts for approximately 0.5-1% of the total RNA content of the cell. The size of mRNA varies widely - from 100 to 10,000 nucleotides.

All types of RNA are synthesized on DNA, which serves as a kind of template.

DNA is the carrier of hereditary information.

Each protein is represented by one or more polypeptide chains. The section of DNA that carries information about one polypeptide chain is called genome. The totality of DNA molecules in a cell acts as a carrier of genetic information. Genetic information is passed on from mother cells to daughter cells and from parents to children. The gene is the unit of genetic, or hereditary information.

DNA is the carrier of genetic information in the cell - does not take a direct part in the synthesis of proteins. In eukaryotic cells, DNA molecules are contained in the chromosomes of the nucleus and are separated by a nuclear membrane from the cytoplasm, where proteins are synthesized. To ribosomes - protein assembly sites - an information carrier is sent from the nucleus, capable of passing through the pores of the nuclear envelope. Messenger RNA (mRNA) is such an intermediary. According to the principle of complementarity, it is synthesized on DNA with the participation of an enzyme called RNA- polymerase.

Messenger RNA is a single-stranded molecule, and transcription comes from one strand of a double-stranded DNA molecule. It is not a copy of the entire DNA molecule, but only part of it - one gene in eukaryotes or a group of adjacent genes that carry information about the structure of proteins necessary to perform one function in prokaryotes. This group of genes is called operon. At the beginning of each operon is a kind of landing site for RNA polymerase called promoter.this is a specific sequence of DNA nucleotides that the enzyme "recognizes" due to chemical affinity. Only by attaching to the promoter, RNA polymerase is able to start RNA synthesis. Having reached the end of the operon, the enzyme encounters a signal (in the form of a certain sequence of nucleotides) indicating the end of reading. The finished mRNA moves away from DNA and goes to the site of protein synthesis.

There are four stages in the transcription process: 1) RNA binding- polymerase with a promoter; 2) initiation- the beginning of the synthesis. It consists in the formation of the first phosphodiester bond between ATP or GTP and the second nucleotide of the synthesized RNA molecule; 3) elongation– RNA chain growth; those. sequential addition of nucleotides to each other in the order in which their complementary nucleotides are in the transcribed DNA strand. The elongation rate is 50 nucleotides per second; four) termination- completion of RNA synthesis.

After passing through the pores of the nuclear membrane, mRNA is sent to the ribosomes, where genetic information is deciphered - it is translated from the "language" of nucleotides to the "language" of amino acids. The synthesis of polypeptide chains according to the mRNA template, which occurs in ribosomes, is called broadcast(lat. translation - translation).

Amino acids, from which proteins are synthesized, are delivered to ribosomes with the help of special RNAs called transport RNAs (tRNAs). There are as many different tRNAs in a cell as there are codons that code for amino acids. At the top of the "sheet" of each tRNA there is a sequence of three nucleotides that are complementary to the nucleotides of the codon in the mRNA. They call her anticodon. A special enzyme, a kodase, recognizes tRNA and attaches an amino acid to the leaf petiole, only the one encoded by the triplet complementary to the anticodon. The energy of one ATP molecule is spent on the formation of a covalent bond between tRNA and its “own” amino acid.

In order for an amino acid to be included in the polypeptide chain, it must break away from the tRNA. This becomes possible when the tRNA enters the ribosome and the anticodon recognizes its codon in the mRNA. The ribosome has two sites for binding two tRNA molecules. One of these areas, called acceptor, tRNA enters with an amino acid and attaches to its codon (I). Does this amino acid attach to itself (accept) the growing chain of protein (II)? A peptide bond is formed between them. tRNA, which is now attached together with the mRNA codon in donor section of the ribosome. A new tRNA comes to the vacated acceptor site, bound to the amino acid, which is encrypted by the next codon (III). From the donor site, the detached polypeptide chain is again transferred here and extended by one more link. Amino acids in the growing chain are connected in the sequence in which the codons encoding them are located in the mRNA.

When one of the three triplets is found on the ribosome ( UAA, UAG, UGA), which are "punctuation marks" between genes, no tRNA can take a place in the acceptor site. The fact is that there are no anticodons that are complementary to the nucleotide sequences of "punctuation marks". The detached chain has nothing to attach to in the acceptor site, and it leaves the ribosome. Protein synthesis is complete.

In prokaryotes, protein synthesis begins with the codon AUG, located in the first place in the copy from each gene, occupies such a position in the ribosome that the anticodon of a special tRNA interacts with it, connected with formylmentionine. This modified form of the amino acid methionine immediately enters the donor site and plays the role of a capital letter in the phrase - the synthesis of any polypeptide chain begins with it in the bacterial cell. When the triplet AUG is not in the first place, but inside a copy from the gene, it encodes the amino acid methionine. After completion of the synthesis of the polypeptide chain, formylmethionine is cleaved from it and is absent in the finished protein.

To increase the production of proteins, mRNA often passes simultaneously not one, but several ribosomes. What structure united by one mRNA molecule is called polysome. On each ribosome, identical proteins are synthesized in this bead-like assembly line.

Amino acids are continuously supplied to ribosomes by tRNA. Having donated the amino acid, the tRNA leaves the ribosome and is connected with the help of a codase. The high coherence of all the "services of the plant" for the production of proteins allows, within a few seconds, to synthesize polypeptide chains consisting of hundreds of amino acids.

Properties of the genetic code. Through the process of transcription in a cell, information is transferred from DNA to protein.

DNA → mRNA → protein

The genetic information contained in DNA and mRNA is contained in the sequence of nucleotides in molecules.

How does the translation of information from the "language" of nucleotides into the "language" of amino acids take place? This translation is carried out using the genetic code. code or cipher, is a system of symbols for translating one form of information into another. Genetic code is a system for recording information about the sequence of amino acids in proteins using the sequence of nucleotides in mRNA.

What are the properties of the genetic code?

triplet code. RNA contains four nucleotides: A, G, C, W. If we tried to designate one amino acid with one nucleotide, then 16 out of 20 amino acids would remain unencrypted. A two-letter code would encrypt 16 amino acids. Nature has created a three-letter, or triplet, code. It means that each of the 20 amino acids is coded for by a sequence of three nucleotides called a triplet or codon.

The code is degenerate. It means that each amino acid is encoded by more than one codon. Exceptions: meteonine and tryptophan, each of which is encoded by one triplet.

The code is unambiguous. Each codon codes for only one amino acid.

There are "punctuation marks" between genes. In printed text, there is a period at the end of each phrase. Several related phrases make up a paragraph. In the language of genetic information, such a paragraph is an operon and its complementary mRNA. Each gene in the prokaryotic operon or an individual eukaryotic gene encodes one polypeptide chain - a phrase. Since in a number of cases several different polypeptide chains are sequentially created along the mRNA template, they must be separated from each other. To do this, there are three special triplets in the genetic year - UAA, UAG, UGA, each of which indicates the cessation of the synthesis of one polypeptide chain. Thus, these triplets perform the function of punctuation marks. They are at the end of every gene.

There are no "punctuation marks" within the gene.

The code is universal. The genetic code is the same for all creatures living on Earth. In bacteria and fungi, wheat and cotton, fish and worms, frogs and humans, the same triplets encode the same amino acids.

Principles of DNA replication. The continuity of genetic material in the generations of cells and organisms is ensured by the process replication - duplication of DNA molecules. This complex process is carried out by a complex of several enzymes and proteins that do not have catalytic activity, which are necessary to give polynucleotide chains the desired conformation. As a result of replication, two identical double helixes of DNA are formed. These so-called daughter molecules are no different from each other and from the original parent DNA molecule. Replication occurs in the cell before division, so each daughter cell receives exactly the same DNA molecules that the mother cell had. The replication process is based on a number of principles:

Only in this case, DNA polymerases are able to move along the parent strands and use them as templates for the error-free synthesis of daughter strands. But the complete unwinding of helices, consisting of many millions of base pairs, is associated with such a significant number of rotations and such energy costs that are impossible under cell conditions. Therefore, replication in eukaryotes begins simultaneously in some places of the DNA molecule. The region between two points where the synthesis of daughter chains begins is called replicon. He is unit of replication.

Each DNA molecule in a eukaryotic cell contains many replicons. In each replicon, one can see a replication fork - that part of the DNA molecule that has already unraveled under the action of special enzymes. Each strand in the fork serves as a template for the synthesis of a complementary daughter strand. During replication, the fork moves along the parent molecule, while new sections of DNA are untwisted. Since DNA polymerases can move only in one direction along the matrix strands, and the strands are oriented antiparallel, two different enzymatic complexes simultaneously synthesize in each fork. Moreover, in each fork, one daughter (leading) chain grows continuously, and the other (lagging) chain is synthesized by separate fragments several nucleotides long. Such enzymes, named after the Japanese scientist who discovered them fragments of Okazaki are linked by DNA ligase to form a continuous chain. The mechanism of formation of daughter chains of DNA fragments is called discontinuous.

Need for primer DNA polymerase is not able to start the synthesis of the leading strand, nor the synthesis of the Okazaki fragments of the lagging strand. It can only build up an already existing polynucleotide strand by sequentially attaching deoxyribonucleotides to its 3'-OH end. Where does the initial 5' end of the growing DNA strand come from? It is synthesized on the DNA template by a special RNA polymerase called primase(English Primer - seed). The size of the ribonucleotide primer is small (less than 20 nucleotides) in comparison with the size of the DNA chain formed by DNA poimerase. Fulfilled his Functions The RNA primer is removed by a special enzyme, and the gap formed during this is closed by DNA polymerase, which uses the 3'-OH end of the neighboring Okazaki fragment as a primer.

The problem of underreplication of the ends of linear DNA molecules. Removal of extreme RNA primers, complementary to the 3'-ends of both strands of the linear parent DNA molecule, leads to the fact that the daughter strands are shorter than 10-20 nucleotides. This is the problem of underreplication of the ends of linear molecules.

The problem of underreplication of the 3' ends of linear DNA molecules is solved by eukaryotic cells using a special enzyme - telomerase.

Telomerase is a DNA polymerase that completes the 3'-terminal DNA molecules of chromosomes with short repeating sequences. They, located one after another, form a regular terminal structure up to 10 thousand nucleotides long. In addition to the protein part, telomerase contains RNA, which acts as a template for extending DNA with repeats.

Scheme of elongation of the ends of DNA molecules. First, complementary binding of the protruding DNA end to the template site of telomerase RNA occurs, then telomerase builds up DNA, using its 3'-OH end as a seed, and RNA, which is part of the enzyme, as a template. This stage is called elongation. After that, translocation occurs, i.e. movement of DNA, extended by one repeat, relative to the enzyme. This is followed by elongation and another translocation.

As a result, specialized end structures of chromosomes are formed. They consist of repeatedly repeated short DNA sequences and specific proteins.

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

Reference. Classification

Scientifically, metabolism is metabolism.

Metabolism is usually divided into two stages:
during catabolism, complex organic molecules break down into simpler ones, with the production of energy; (energy 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. It is here that 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 can be seen, there is an outward resemblance to the known structure of the DNA molecule (deoxyribonucleic acid).
However, RNA can be both double-stranded and single-stranded.
Nucleotides (five- and hexagons in the figure)
In addition, an RNA strand consists of four nucleotides (or nitrogenous bases, which is the same thing): adenine, uracil, guanine, and cytosine.
The DNA strand consists of a different 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 figure:


The hexagons in the figures are benzene rings, in which, instead of carbon, other elements are embedded, in this case, it is nitrogen.

Benzene. For reference.

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

Compound of nitrogenous bases

Complementary nucleotides are linked (hybridized) with each other 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; conversely, short sections will be unstable. This determines the function of a particular RNA.
The figure shows a fragment of a complementary RNA region. Nitrogenous bases shaded in blue

RNA structure

The linkage of many groups of nucleotides form RNA hairpins (primary structure):


Many pins in the tape are interlocked in a double helix. In expanded form, such a structure resembles a tree (Secondary Structure):


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


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

Conclusion

To calculate the conformations that RNA will accept, according to their primary sequence, there are