In this article we will discuss about the translation of amino acid to tRNA in cytoplasm.
(i) Activation of Amino acids:
Amino acids must be necessarily activated before their involvement in peptide synthesis (Fig. 4-10).
The process is accomplished by the binding of an amino acid to ATP enzymatically and its consequent transference to tRNA.
(ii) Transfer of amino acid to tRNA:
There is formation of aminoacyl-tRNA as follows:
There is formation of monoadenylate of amino acid (Fig. 4-10).
It is believed that both the reactions are catalyzed by amino-acyl tRNA synthetases. Thus, for 20 amino acids, at least 20 different enzymes and 20 tRNA species would be needed.
In summary, the cytoplasm has as many tRNA as the number of amino acids and corresponding synthetases.
Each tRNA consists of nearly 80 nucleotides and at places they are paired attaining a clover leaf structure (Fig. 4-11).
All the tRNA molecules have CCA as the terminal sequence. Each tRNA is characterized by at least three features: site for the recognition of specific amino-acyl-tRNA synthetase, site for the attachment of an amino acid and site for the recognition of template or anticodon.
The recognition potentiality helps the tRNA to select the definite amino acid from amongst the mixture of their different assortment.
(iii) Initiation of polypeptide:
To begin with, mRNA is associated with the ribosomes and is called initiation process. It is a highly complex phenomenon and also involves binding of mRNA starter without formylation.
The complex is formed with the initiation factors and small subunit of ribosome. Immediately the bigger subunit joins the complex and the whole of ribosome becomes functional.
In E. coli following steps are involved in initiation:
Of these reactions the first two are stimulated by the initiation factor F1.
Compared to bacteria, in higher organisms, methionine is not formylated and the 40S subunit associates with met – tRNA fmet without the help of mRNA.
(iv) Elongation of polypeptide:
Once 70S-mRNA-f-met-tRNA fmet complex is formed, there is elongation of polypeptide chain by the regular addition of amino acids.
It is generally assumed that during this step, only single codon of mRNA at a ribosome is exposed (Fig. 4-10A, B, C, D, 4-12). That is to mean
The terms first, second and third nucleotide refers to the individual nucleotide of a triplet codon. Given the position of the bases in a codon, it is possible to find the corresponding amino acid. For example, the codon AUG on mRNA specifies methionine, whereas CAU specifies histidine. UAA, UAG and UGA are chain terminator CT codons.
CI is a chain initiator codon that one codon is read at a time. The decoding or deciphering of codon is accomplished through anticodon of amino acyl tRNA which is complementary to the codon. Evidently, anticodon acts as a template recognition site. In other words, specific tRNA brings specific amino acid in position. Immediately after mRNA is moved in one direction.
Thus facilitating the deciphering of the new codon, when another amino acyl tRNA brings another specific amino acid. Likewise the mRNA is moved one codon in one direction. In this way, the whole sequence of events is repeated and specific number and types of amino acids are linked together. The amino acids are associated with each other enzymatically to constitute peptide linkage.
As the process goes on, the tRNA are let off one by one in the cytoplasm to be recharged with their specific amino acid. The mRNA carries the genetic message. The tRNA, rRNA, etc. are the components which help in the successful accomplishment of the translation. Elongation of polypeptides consists of three substeps.
i. Binding of amino acid-rRNA to site A of ribosome and its shifting to P site making available site A for the next amino acyl tRNA. For the reaction of amino acyl tRNA and A site GTP and TF (transfer factors or translocase) are needed (Fig. 4-10, C, D).
ii. Peptide bond formation occurs between—COOH of peptidyl tRNA of P site and—NH2 of amino acyl tRNA at A site.
iii. Translocation of peptidyl tRNA from A to P site.
(iv) Polypeptide termination:
The peptide chain is terminated or released from the last tRNA as well as ribosome by some termination factor. The termination is decided and/or facilitated through the codons like UAA, UAG, and UGA. Thereafter, the two subunits of a ribosome are separated and are available for the new cycle of synthesis. Release factors R1, R2, and R3 help in the recognition of the termination codons.