Molecular biology studies 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 given 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, which differ in their functions and location in the cell. This article is devoted to the study of the following questions: where is rRNA synthesized in prokaryotic and eukaryotic cells, what is its structure and significance.
Historical background
The first scientific mention of ribosomal acid can be found in the studies of R. Weinberg and S. Penman in the 60s of the XX century, who described short polynucleotide molecules related to ribonucleic acids, but differing in spatial structure and sedimentation coefficient from information and transport RNA. Most often, their moleculesfound 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, the monomers of which are 4 types of nucleotides: adenine, guanine, uracil and cytidine, connected by phosphodiester bonds into long single-stranded molecules, twisted in the form of a spiral or having more complex conformations. There are also double-stranded ribosomal ribonucleic acids found in RNA-containing viruses and duplicating the functions of DNA: the preservation and transmission of hereditary traits.
Three types of acids are most common in the cell, these are: matrix, or informational, 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 makes up about 80% of the total amount of ribonucleic acids in the cell and 60% of the mass of the ribosome, an organoid that synthesizes cellular protein. All of the above species are synthesized (transcribed) at certain sections of DNA, called RNA genes. In the process of synthesis, molecules of a special enzyme, RNA polymerase, are involved. The place in the cell where rRNA is synthesized is the nucleolus, located in the karyoplasmkernels.
Nucleolus, its role in synthesis
In the life of a cell, called the cell cycle, there is a period between its divisions - interphase. At this time, dense bodies of a granular structure, called nucleoli, are clearly visible in the cell nucleus, which are an indispensable component of both plant and animal cells.
In molecular biology, it has been established that the nucleoli are the organelles where rRNA is synthesized. Further research by cytologists led to the discovery of sections of cellular DNA, in which genes responsible for the structure and synthesis of ribosomal acids were found. They were called the nucleolar organizer.
Nuclear organizer
Until the 60s of the 20th century, there was an opinion in biology that the nucleolar organizer, located at the site of the secondary constriction in the 13th, 14th, 15th, 21st and 22nd pairs of chromosomes, looks like a single site. Scientists involved in the study of chromosomal damage, called aberrations, have found that at the moment of chromosome break at the site of the secondary constriction, the formation of nucleoli occurs on each of its parts.
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 ribosomal ribonucleic acids rRNA are synthesized, which form subunits of protein-synthesizing cell organelles - ribosomes.
What are ribosomes?
As mentioned earlier, all three main typesRNA exists in the cell, where they are synthesized at certain sites - DNA genes. The ribosomal RNA formed as a result of transcription form complexes with proteins - ribonucleoproteins, from which the constituent parts of the future organelle, the so-called subunits, are formed. Through the pores in the nuclear membrane, they pass into the cytoplasm and form in it the combined structures, which also include molecules of i-RNA and t-RNA, called polysomes.
The ribosomes themselves can be separated under the action of calcium ions and exist separately as subunits. The reverse process occurs in the compartments of the cell cytoplasm, where the processes of translation take place - the assembly of cellular protein molecules. The more active the cell, the more intense the metabolic processes in it, the more ribosomes it contains. For example, cells of the red bone marrow, hepatocytes of vertebrates and humans are characterized by a large number of these organelles in the cytoplasm.
How are rRNA genes encoded?
Based on the above, the structure, types and functioning of rRNA genes depend on nucleolar organizers. They contain loci containing genes encoding ribosomal RNA. O. Miller, conducting research on oogenesis in newt cells, established the mechanism of the functioning of these genes. Copies of rRNA (the so-called primary transcriptants) were synthesized from them, containing about 13x103 nucleotides and having a sedimentation coefficient of 45 S. Then this chain underwent a maturation process, ending with the formation of threerRNA molecules with sedimentation coefficients of 5, 8 S, 28 S, and 18 S.
Mechanism of rRNA formation
Let's return to the experiments of Miller, who investigated the synthesis of ribosomal RNA and proved that nucleolar DNA serves as a template (matrix) for the formation of rRNA - a transcriptant. He also established that the number of immature ribosomal acids (pre-r-RNA) that are formed depends on the number of molecules of the RNA polymerase enzyme. Then their maturation (processing) occurs, and rRNA molecules immediately begin to bind to peptides, resulting in the formation of a ribonucleoprotein, the building material of the ribosome.
Features of ribosomal acids in eukaryotic cells
Having the same principles of structure and common functional mechanisms, the ribosomes of prokaryotic and nuclear organisms still have cytomolecular differences. To find out, scientists used a research method called x-ray diffraction analysis. It was found that the size of the eukaryotic ribosome, and hence the rRNA included in it, is larger and the sedimentation coefficient is 80 S. The organelle, losing magnesium ions, can be divided into two subunits with indicators of 60 S and 40 S. A small particle contains one molecule of acid, and a large one - three, that is, nuclear cells contain ribosomes consisting of 4 polynucleotide helices of acid 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, located in the karyoplasm of the nucleus.
Ribosomal RNA of prokaryotes
Unlike r-RNA,entering the nuclear cells, the ribosomal ribonucleic acids of bacteria are transcribed in a compacted area 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 rRNA of DNA genes into a nucleotide sequence of ribosomal ribonucleic acid, taking into account the rule of complementarity of the genetic code: adenine nucleoitide corresponds to uracil, and guanine to cytosine.
R-RNA bacteria have a lower molecular weight and smaller size than that of nuclear cells. Their sedimentation coefficient is 70 S, and the two subunits have values of 50 S and 30 S. The smaller particle contains one rRNA molecule, and the larger one contains two.
The role of ribonucleic acid in the translation process
The main function of r-RNA is to ensure the process of cellular protein biosynthesis - translation. It is carried out only in the presence of ribosomes containing r-RNA. Combining into groups, they bind to the informational DNA molecule, forming a polysome. Molecules of the transport ribosomal ribonucleic acid, carrying amino acids, which, once in the polysome, bind to each other by peptide bonds, form a polymer - protein. It is the most important organic compound of the cell, which performs many important functions: building, transport, energy, enzymatic, protective and signaling.
This article examined the characteristics, structure and description of ribosomal nucleic acids, which areorganic biopolymers of plant, animal and human cells.
