Q.78 Which of the following event(s) occur(s) during translation in prokaryotes?
(A) tRNA binding to the start codon of mRNA on the 30s subunit of ribosome
(B) Anticodon of tRNA binding to the start codon of mRNA on the 50s subunit of
ribosome
(C) The ribosome continues to move along the mRNA to add new amino acids to the
polypeptide
(D) The polypeptide is released when the ribosome reaches the stop codon

Translation in prokaryotes decodes mRNA into proteins using 70S ribosomes split into 30S and 50S subunits. This process unfolds in three phases: initiation, elongation, and termination, each with specific events. Correct identification of these events is crucial for exams like CSIR NET Life Sciences.​

Question Breakdown

The query asks which events occur during prokaryotic translation among four options. All options relate to core phases, but precision matters for subunits and sites.​

Option Analysis

Option (A): tRNA binding to the start codon of mRNA on the 30S subunit of ribosome
Initiation starts with the 30S subunit binding mRNA at the Shine-Dalgarno sequence, then scanning to the AUG start codon. The initiator fMet-tRNA^fMet^ binds to this codon in the P site of the 30S subunit, aided by initiation factors IF1, IF2, and IF3. This forms the 30S initiation complex before the 50S subunit joins.​
Correct – This precisely describes prokaryotic initiation.​

Option (B): Anticodon of tRNA binding to the start codon of mRNA on the 50S subunit of ribosome
The 50S subunit joins after 30S initiation complex formation, completing the 70S ribosome. Start codon recognition and initiator tRNA binding occur exclusively on the 30S subunit; the 50S lacks direct mRNA interaction sites for codon-anticodon pairing. This option misplaces the event to the large subunit.​
Incorrect – Binding happens on 30S, not 50S.​

Option (C): The ribosome continues to move along the mRNA to add new amino acids to the polypeptide
During elongation, after the first peptide bond, EF-Tu delivers aminoacyl-tRNA to the A site. Peptidyl transferase forms the bond, then EF-G drives translocation: the ribosome shifts 3 nucleotides (5′ to 3′), moving peptidyl-tRNA from A to P site and exposing the next codon. This cycle repeats, adding amino acids to the C-terminus.​
Correct – Describes translocation in elongation.​

Option (D): The polypeptide is released when the ribosome reaches the stop codon
Termination occurs when a stop codon (UAA, UAG, UGA) enters the A site. Release factors RF1/RF2 (codon-specific) and RF3 bind, triggering peptidyl transferase to hydrolyze the ester bond between the polypeptide and P-site tRNA. The completed chain releases, subunits dissociate.​​
Correct – Standard termination mechanism.​

Correct Answer

Options (A), (C), and (D) occur during translation in prokaryotes. Option (B) does not, due to incorrect subunit specification.​