Biochemistry

in the polypeptide chain is translated. 61 triplets from 64 possible triplets encode certain amino acids, that is, these triplets are "coding". Some triplets do not code for amino acids, but their role is termination (from the latin. termination - termination) synthesis of the nascent polypeptide chain. The code is innate, which means that almost all amino acids are encoded by more than one triplet of nucleotides. So, the amino acids leucine, arginine and serine are encoded by six codons, methionine and tryptophan have only one codon, the remaining 15 amino acids - from 2 to 4. The translation process is carried out using T-RNAs loaded with amino acids. Aminoacyl-T-RNA binds with its complementary triplet (anticodon) to the M-RNA codon in the ribosome. Another aminoacyl-T-RNA is attached to the adjacent codon of M-RNA. The first T-RNA at the same time attaches its amino acid residue with the carboxyl terminus to the amino group of the second amino acid to form a dipeptide, and it is released and separated from the ribosome. Further, as the ribosome moves along the M-RNA chain, the dipeptide joins the carboxyl terminus with the amino group of the third amino acid to form and release the second T-RNA, and so on, until the entire ribosome encoding this protein to the M-RNA corresponding to the ribosome DNA gene. Then, protein synthesis is terminated, and the resulting polypeptide is release from the ribosome. The first ribosome is followed by a second, third, etc., which sequentially read information on the same M-RNA strand in the polysome. Thus, the growth of the polypeptide chain occurs from the N- terminus to the C-terminus (if you suspend protein synthesis, for example, using the antibiotic puromycin, you can get unfinished polypeptide chains with the C-termination incomplete at different stages). Aminoacyl-T-RNA binds first to the small ribosomal subunit, and then to the large one, and on which the polypeptide chain grows. Components of the protein biosynthesis system are synthesized mainly in a cell nucleus. All RNA synthesis involved in this process takes place on the DNA matrix during transcription: R-RNA, M-RNA, T-RNA. So, T-RNA and M-RNA are synthesized in the form of very large molecules and undergo a “ maturation ” stage in the cell nucleus, during which a significant part of the M-RNA molecule is breakdown without leaving the cytoplasm, and a functioning molecule, which part originally synthesized, enters the cytoplasm at the site of protein synthesis. Before entering the polysome, M-RNA from the moment of synthesis binds to special protein particles - “ informants ” and is transferred to ribosomes in the form of a ribonucleoprotein complex. Ribosomes, also “ mature ” in the cytoplasm, some of the proteins attach to the precursors of ribosomes emerging from the nuclei, already in the cytoplasm. 185