Stages of Translation Initiation Elongation and Termination

A ribosome begins to read mRNA from a strictly definite point of its sequence, i.e. from the beginning of its coding region. It has already been noted that this point generally does not coincide with the 5'-terminal mRNA nucleotide and as a rule is located at a certain, sometimes significant, distance from the 5'-end of the polynucleotide chain. The ribosome should in some way identify the readout origin, bind to it, and then begin translation. The series of events that provide for the beginning of translation is called initiation. Initiation requires a special initiation codon, initiator tRNA, and proteins, which are referred to as initiation factors.

After initiation the ribosome consecutively reads mRNA codons in the direction of its 3'-end. The mRNA readout implies concomitant synthesis of the polypeptide chain coded by the mRNA. Synthesis takes place on the ribosome by the sequential addition of amino acid residues to the nascent polypeptide chain; it is in this way that the peptide elongation is accomplished. Each new amino acid residue is added to the carboxyl terminus (C-terminus) of the peptide; in other words, the C-terminus of the peptide is the end that grows. The addition of one amino acid residue corresponds to the readout of one nucleotide triplet. This whole process involving the actual translation of mRNA coding region is termed elongation.

When a ribosome reaches the mRNA termination codon, synthesis of the polypeptide stops. In the presence of the termination codon the ribosome does not bind any aminoacyl-tRNA; instead, specialized proteins called termination factors come into play. These factors induce the release of the synthesized polypeptide from the ribosome. This stage is designated as termination. After termination the ribosome may either jump off the mRNA or continue to slip along it without, however, translating. When a ribosome comes across a new initiation codon either on a new mRNA chain or on the same chain downstream from the termination codon, a new initiation takes place. Thus each ribosome passes through the whole translation cycle including initiation, elongation, and termination; such an epicycle results in the readout of the whole mRNA coding sequence and synthesis of a complete polypeptide. Thereafter, a given ribosome may repeat the cycle with the same mRNA chain, another mRNA chain, or another coding sequence in the same chain (Fig. 4.4).

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  • lisa
    What are the stages of translationinitiation, elongation and termination?
    8 years ago

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