Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

• Translation is the process of synthesis of protein from mRNA with the help of ribosome.
• A translational unit in mRNA from 5'→3' comprises of a start codon, region coding for a polypeptide, a stop codon and untranslated regions (UTRs) at both 5'-end and 3'-end for efficient process.

There are three stages of protein synthesis:

1. Initiation:

• Assembly of ribosome on mRNA.
• Activation of amino acids and its delivery to tRNA.

2. Elongation:

• Repeated cycle of amino acid delivery.
• Peptide bond formation and movement along the mRNA called translocation.

3. Termination:

• The release of a polypeptide chain.

1. Initiation:

• In prokaryotes, initiation requires the large and small ribosome subunits, the mRNA, initiation tRNA and three initiation factors (IFs).

• Activation of amino acid: Amino acids become activated by binding with aminoacyl tRNA synthetase enzyme in the presence of ATP.

• Transfer of amino acid to tRNA: The AA - AMP - Enzyme complex formed reacts with specific tRNA to form aminoacyl-tRNA complex. AA - AMP - Enzyme complex + tRNA → AA - tRNA + AMP + Enzyme.

• The cap region of mRNA binds to the smaller subunit of ribosome.

• The ribosome has two sites, A-site and P-site.

• The smaller subunit first binds to the initiator mRNA and then binds to the larger subunit so that initiation codon (AUG) lies on the P-site.

• The initiation tRNA, i.e., methionyl tRNA then binds to the P-site.

2. Elongation of polypeptide chain:

• Another charged aminoacyl tRNA complex binds to the A-site of the ribosome at the second codon.
• A peptide bond is formed between carboxyl group (-COOH) of amino acid at P-site and amino group (-NH) of amino acid at A-site by the enzyme peptidyl transferase.
• The ribosome slides over mRNA from codon to codon in the 5′ → 3′ direction.
• According to the sequence of codons, amino acids are attached to one another by peptide bonds and a polypeptide chain is formed.

3. Termination of polypeptide:

• When the A-site of ribosome reaches a termination codon which does not code for any amino acid, no charged tRNA binds to the A-site.
• Dissociation of polypeptide from ribosome takes place, which is catalysed by a 'release factor'.
• There are three termination codons namely UGA, UAG and UAA.