Question
Explain the process of protein synthesis from processed mRNA.
✓
Answer
Translation:
- 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.
- Initiation:
- Assembly of ribosome on mRNA.
- Activation of amino acids and its delivery to tRNA.
- Elongation:
- Repeated cycle of amino acid delivery.
- Peptide bond formation and movement along the mRNA called translocation.
- Termination:
- The release of a polypeptide chain.
- 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.
- 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.
- 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.
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