Molecular biology is studying the structure and functions of the molecules of organic substances that make up 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 the location 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Historical reference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 ribosomic acid can be found in the studies of R. Weinberg and S. Penman in the 60s of the XX century, which described short polynucleotide molecules related to ribonucleic acids, but differing in spatial structure and sedimentation coefficient from information and transport RNAs. Most often, their molecules were found in the nucleolus, as well as in cellular organelles - ribosomes responsible for the synthesis of cellular protein. They were called ribosomal (ribosomal ribonucleic acids).
RNA Characterization
Ribonucleic acid, like DNA, is a polymer whose monomers are 4 types of nucleotides: adenine, guanine, uracil and cytidine, connected by phosphodiester bonds into long single-chain molecules, twisted in a spiral or having more complex conformations. There are double-stranded ribosomal ribonucleic acids found in RNA viruses and duplicating the functions of DNA: preservation and transmission of hereditary traits.
Three types of acids are found most often 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 accounts for about 80% of the total amount of ribonucleic acids in a cell and 60% of the mass of a ribosome, an organoid that synthesizes a cellular protein. All of the above species are synthesized (transcribed) in certain sections of DNA called RNA genes. The synthesis involves molecules of a special enzyme - RNA polymerase. The place in the cell where rRNA is synthesized is the nucleolus located in the karyoplasm of the nucleus.
Nucleolus, its role in the synthesis
In the life of a cell, called the cell cycle, the period between its divisions - interphase - is distinguished. At this time, dense bodies of a granular structure, called nucleoli, are clearly visible in the cell nucleus and are an obligatory component of both plant and animal cells.
In molecular biology, it was found 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 the genes responsible for the structure and synthesis of ribosomal acids were discovered. They were called the nucleolar organizer.
The nucleolar organizer
Until the 60s of the XX century, it was believed in biology that the nucleolar organizer, located on the site of the secondary constriction in the 13th, 14th, 15th, 21st and 22nd pairs of chromosomes, looks like a single section. Scientists involved in the study of chromosomal lesions called aberrations have found that at the time of chromosome rupture, nucleoli form on each of its parts at the site of the secondary constriction.
Thus, we can state the following: the nucleolar organizer does not consist of one, but of several loci (genes) responsible for the formation of the nucleolus. It is in it that the ribosomal ribonucleic acids of rRNA are synthesized, forming subunits of 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 the components of the future organelle, the so-called subunits, are formed. Through the pores in the nuclear membrane, they pass into the cytoplasm and form combined structures in it, which also includes i-RNA and t-RNA molecules, called polysomes.
Ribosomes themselves can separate under the influence of calcium ions and exist separately in the form of subunits. The reverse process occurs in the compartments of the cell cytoplasm, where the processes of translation - assembly of the molecules of cellular proteins proceed. The more active the cell, the more intensive metabolic processes take place in it, the more ribosomes it contains. For example, red bone marrow cells, 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 foregoing, the structure, types and functioning of rRNA genes depend on the nucleolar organizers. They contain loci containing genes encoding ribosomal RNA. O. Miller, conducting research on ovogenesis in newt cells, established a mechanism for the functioning of these genes. They synthesized copies of r-RNA (the so-called primary transcriptants) containing about 13x103 nucleotides and having a sedimentation coefficient of 45 S. Then this chain underwent a maturation process ending 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 studied the synthesis of ribosomal RNA and proved that the nucleolar DNA serves as a template (matrix) for the formation of r-RNA transcriptant. He also established that the number of immature ribosomal acids (pre-r-RNA) that are formed depends on the number of RNA polymerase enzyme molecules. Then they mature (processing), and r-RNA molecules begin to bind to peptides immediately, resulting in the formation of a ribonucleoprotein - the building material of the ribosome.
Features of ribosomal acids of eukaryotic cells
Having common structural principles 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 analysis. It was found that the magnitude of the eukaryotic ribosome, and hence the r-RNAs included in it, is larger and the sedimentation coefficient is 80 S. The organelle, losing magnesium ions, can be divided into two subunits with 60 S and 40 S. Small particle contains one molecule of acid, and the largest - three, that is, 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, located false in karyoplasm nucleus.
Prokaryotic ribosomal RNA
Unlike r-RNAs that enter nuclear cells, bacterial ribosomal ribonucleic acids are transcribed on a compacted area of the cytoplasm containing DNA called the nucleoid. It contains rRNA genes. Transcription, the general characteristic of which can be represented as a process of copying information from r-RNA of DNA genes into the nucleotide sequence of ribosomal ribonucleic acid, taking into account the complementarity rule of the genetic code: adenine nucleoide corresponds to uracil and guanine to cytosine.
R-RNA bacteria have a lower molecular weight and smaller sizes than nuclear cells. Their sedimentation coefficient is 70 S, and the two subunits have indices of 50 S and 30 S. The smaller particle contains one r-RNA molecule, and the larger particle contains two.
The role of ribonucleic acid in the translation process
The main function of r-RNA is to ensure the process of biosynthesis of cellular protein - translation. It is carried out only in the presence of ribosomes containing r-RNA. Uniting in groups, they bind to a molecule of informational DNA, forming a polysome. Molecules of transport ribosomal ribonucleic acid, which carry amino acids that, once in the polysome, bind to each other by peptide bonds, form a polymer - protein, from the cytoplasm of the cell . It is the most important organic compound of the cell, performing many important functions: building, transport, energy, enzymatic, protective and signal.
This article examined the characteristics, structure, and description of ribosomal nucleic acids, which are organic biopolymers of plant, animal, and human cells.