Where rRNA is synthesized. Ribosomal ribonucleic acids rRNA: characteristic, structure and description

Molecular biology is concerned with the study of the structure and functions of molecules of organic substances that make up the living cells of plants, animals and humans. A special place among them is assigned to a group of compounds called nucleic (nuclear) acids.

There are two types: deoxyribonucleic acid (DNA) and ribonucleic acid . The latter has several modifications: i-RNA, t-RNA and r-RNA, differing in their functions and location of localization in the cell. This article is devoted to the study of the following questions: where rRNA is synthesized in prokaryotic and eukaryotic cells, what is its structure and significance.

Historical reference

The first scientific references to ribosomal acid can be found in the studies of R. Weinberg and S. Penman in the 1960s, who described short polynucleotide molecules related to ribonucleic acids, but differed in spatial structure and sedimentation coefficient from information and transport RNA. Most often their molecules were found in the nucleolus, as well as in cell organelles - ribosomes, responsible for the synthesis of cellular protein. They were called ribosomal (ribosomal ribonucleic acids).

RNA characteristic

Ribonucleic acid, like DNA, is a polymer whose monomers are nucleotides of four kinds: adenine, guanine, uracil and cytidine, connected by phosphodiester bonds to long single-stranded molecules twisted in the form of a spiral or having more complex conformations. There are also double-stranded ribosomal ribonucleic acids that occur in RNA-containing viruses and duplicate DNA functions: preservation and transmission of hereditary traits.

Three types of acids are found in the cell most often: matrix, or information, RNA, transport ribosomal ribonucleic acid, to which amino acids are attached, as well as ribosomal acid, located in the nucleolus and cell cytoplasm.

Ribosomal RNA is about 80% of the total amount of ribonucleic acids in the cell and 60% of the mass of the ribosome, the organoid that synthesizes the cellular protein. All of the above species are synthesized (transcribed) in certain parts of the DNA, called RNA genes. In the process of synthesis involved molecules of a special enzyme - RNA polymerase. The place in the cell where rRNA is synthesized is the nucleolus located in the nuclei karyoplasm.

Nucleolus, its role in the synthesis

In the life of a cell called the cell cycle, the period between its divisions, the interphase , is distinguished. At this time, the dense corpuscles of the granular structure, called nucleoli, are clearly seen in the cell nucleus and are an obligatory component of both the plant and animal cells.

In molecular biology, it has been established that the nucleoli are those organelles where rRNA is synthesized. Further studies of cytologists led to the discovery of sections of cellular DNA in which genes responsible for the structure and synthesis of ribosomal acids were discovered. They were called the nucleolus organizer.

Nucleolus organizer

Until the 1960s, in biology, there was an opinion that the nucleolus organizer, which is located on the site of secondary constriction in the 13th, 14th, 15th, 21st and 22nd pairs of chromosomes, has the appearance of a single site. Scientists involved in the study of chromosomal aberrations, called abberations, found that at the time of chromosome rupture, the nucleolus on each of its parts is formed at the site of the secondary constriction.

Thus, we can state the following: the nucleolar organizer consists not of one, but of several loci (genes) responsible for the formation of the nucleolus. It is in it that the ribosomal ribonucleic acids rRNA are synthesized, forming subunits of the protein-synthesizing organelles of the cell-ribosomes.

What are ribosomes?

As mentioned earlier, all three main types of RNA exist in the cell, where they are synthesized in certain areas - DNA genes. Ribosomal RNAs formed as a result of transcription form complexes with proteins-ribonucleoproteins, from which constituent parts of the future organelle form the so-called subunits. Through the pores in the nuclear membrane, they pass into the cytoplasm and form in it integrated structures, including also molecules of i-RNA and t-RNA, called polysomes.

The ribosomes themselves can be separated by the action of calcium ions and can exist separately as subunits. The reverse process takes place in the compartment of the cellular cytoplasm, where translation processes take place - the assembly of molecules of cellular proteins. The more active the cell, the more intense the metabolism processes in it, the more ribosomes it contains. For example, red marrow cells, hepatocytes of vertebrates and humans are characterized by a large number of these organelles in the cytoplasm.

How are r-RNA genes encoded?

Proceeding from the above, the structure, types and functioning of rRNA genes depend on nucleolar organizers. They contain loci containing genes encoding ribosomal RNA. O. Miller, carrying out studies of ovogenesis in the cells of newts, established the mechanism of the functioning of these genes. Copies of r-RNA (the so-called primary transcriptants) containing about 13x103 nucleotides and having a sedimentation coefficient of 45 S were synthesized from them. This chain then underwent a maturation process that resulted in the formation of three r-RNA molecules with sedimentation coefficients of 5.8 S, 28 S and 18 S.

The mechanism of formation of r-RNA

Let us return to the experiments of Miller, who investigated the synthesis of ribosomal RNAs and proved that nucleolus DNA serves as a template for the formation of a p-RNA transcript. He also established that the number of immature ribosomal acids (pre-r-RNA) that are formed depends on the number of molecules of the enzyme RNA polymerase. Then their maturation (processing) takes place, and the molecules of r-RNA immediately begin to bind to the peptides, resulting in the formation of ribonucleoprotein, the building material of the ribosome.

Features of ribosomal acids of eukaryotic cells

Having common principles of structure and general functional mechanisms, ribosomes of prokaryotic and nuclear organisms still have cytomolecular differences. To find them out, scientists applied a method of research, called X - ray structural analysis. It was found that the size of the eukaryotic ribosome, and hence of the p-RNA entering it, is larger and the sedimentation coefficient is equal to 80 S. The organelle, losing magnesium ions, can be divided into two subunits with the exponents 60 S and 40 S. The small particle contains One molecule of acid, and the larger one - three, that is, the nuclear cells contain ribosomes consisting of 4 polynucleotide acid helices of the following characteristics: 28 S RNA - 5 thousand nucleotides, 18 S - 2 thousand 5 S - 120 nucleotides, 5, 8 S - 160. The site where rRNA is synthesized in eukaryotic cells is the nucleolus, false in karyoplasm nucleus.

Ribosomal Prokaryotic RNA

Unlike p-RNA entering nuclear cells, the ribosomal ribonucleic acids of bacteria are transcribed on a compacted part of the cytoplasm containing DNA and called the nucleoid. It contains rRNA genes. Transcription, the general characteristic of which can be represented as a process of rewriting information from the r-RNA of DNA genes to the nucleotide sequence of ribosomal ribonucleic acid, taking into account the rule of complementarity of the genetic code: the adenine nucleotide corresponds to uracil and the guanine to cytosine.

P-RNA bacteria have a smaller molecular weight and smaller dimensions than that of nuclear cells. Their sedimentation coefficient is 70 S, and the two subunits have a score of 50 S and 30 S. The smaller particle contains one molecule of p-RNA, and the larger one contains two.

The role of ribonucleic acid during translation

The main function of p-RNA is to ensure the process of biosynthesis of cellular protein-translation. It occurs only in the presence of ribosomes containing p-RNA. Uniting in groups, they communicate with an information DNA molecule, forming a polysome. To it, from the cytoplasm of the cell, molecules of the transport ribosomal ribonucleic acid, carrying amino acids, which, having got into the polysome, are linked together by peptide bonds, forming a polymer-protein. It is the most important organic compound of the cell, fulfilling many important functions: construction, transport, energy, enzymatic, protective and signal.

In this article, the characteristics, structure and description of ribosomal nucleic acids, which are organic biopolymers of plant, animal and human cells, were considered.