Rna synthesis in rna template
Viral RNA induces the formation of RNA-dependent RNA polymerase in host cells, which is involved in the replication of viral RNA. Examples are viruses of influenza, rabies, mumps, and measles. RNA replicase constructs complementary RNA strand on the template virus RNA. It serves as a template for RNA synthesis, the same type of initial viral RNA. Both stages are catalyzed by the same enzyme, although each involves different protein factors.
Since the RNA replicase is related only to the virus, obviously, effective antiviral drugs can be developed on this basis.
3.2. Protein biosynthesis
Protein synthesis is closely connected to the concept of genetic code.
Genetic code is the way to codify amino acid sequence with the help of sequence of nucleotides in all living organisms.
The properties of the genetic code
1. Triple character. The unit of code is a combination of three nucleotides (triplet or codon). All in all there are 64 codons, in other words, each amino acid is codified by three nucleotides in mRNA.
2. Continuity. There are no commas or punctuation signs between the triplets, that is, information is read continuously.
3. Non-overlapping. One and the same nucleotide could not be simultaneously in the structure of two or more triplets.
4. Specificity. A particular codon codes for only one amino acid (AUU is isoleucine, UUU is phenylalanine).
5. Degenerate. One amino acid could correlate to several codons (isoleucine is AUU, AUC, AUA).
6. Universality. Genetic code work is the same in the organism of different structures from viruses to humans.
Preparatory stage of protein synthesis
20 amino acids, 20 aminoacyl-tRNA-synthetases, more than 20 tRNA, ATP, magnesium ions are necessary.
Amino acid reacts with ATP with the formation of aminoacyl-adenylate, then amino acid is transported from aminoacyladenylate to acceptor stem of tRNA (3’-end):
R–CH(NH2)–COOH + АТP R–CH(NH2)–COO–АМP + PPi
aminoacyladenylate
R–CH(NH2)–COO–АМP + tRNA
R–CH(NH2)–COO–tRNA+ АМP
An enzyme here is aminoacyl-tRNA-synthetase. The presence of magnesium ions is necessary. Aminoacyl-tRNA-synthetases are characterised by absolute specificity. Each amino acid has its own enzyme.
Transfer RNA (tRNA) provides connection between codons of mRNA and amino acids of the future polypeptide chain. There is not less than one tRNA for each amino acid out of 20 amino acids. Each tRNA has about 80 nucleotides. They have the structure of clover leaf in 2-dimensional shape and L-structure in the 3-dimensional shape (Fig. 3).
Fig. 3. The structure of tRNA
The acceptor stem is the site for amino acids binding. It contains CCA sequence.
The TC (pseudouridilic) loop provides aminoacyl-tRNA binding with ribosome.
The D (dihydrouridilic) loop is a site for the recognition of aminoacyl-tRNA synthetase.
Anticodon loop contains an anticodon which is complimentary to the codon in the mRNA It has 3 nucleotide residues.
A destination of the variable loop is unknown.