Question

Explain protein synthesis with respect to initiation, elongation and termination.

Answer 1

Protein synthesis (translation) decodes the codons of mRNA to assemble amino acids into a polypeptide on a ribosome.

1. Initiation

1. The small ribosomal subunit binds to mRNA and locates the start codon, usually [AUG].
2. An initiator tRNA carrying methionine, with a complementary anticodon, binds to the start codon. The large subunit then joins; the initiator tRNA occupies the P site, forming the initiation complex.

2. Elongation

1. A charged tRNA carrying the amino acid specified by the next codon enters the A site.
2. The ribosome catalyses peptide-bond formation, transferring the growing chain to the amino acid at the A site.
3. The ribosome moves one codon along the mRNA (translocation). The tRNA carrying the chain shifts to the P site, the empty tRNA leaves, and the next charged tRNA enters. This cycle lengthens the polypeptide.

3. Termination

1. When a stop codon—[UAA}, [UAG} or [UGA}—enters the A site, no tRNA binds to it. A release factor binds instead.
2. The completed polypeptide is released, and the ribosome separates from the mRNA into its subunits.

Thus, translation converts the nucleotide sequence of mRNA into a specific amino-acid sequence.