Q.41 In the chain elongation process, the factor involved in translocation of ribosome from one codon to the next along the mRNA is:
In protein synthesis, the chain elongation process requires precise movement of the ribosome along mRNA, known as translocation, to add amino acids one by one. This MCQ targets that step, with EF-G as the correct factor driving it.
Question Breakdown
The query asks: “In the chain elongation process, the factor involved in translocation of ribosome from one codon to the next along the mRNA is:” Options are (1) GDP, (2) NADP, (3) EF-G, (4) EF-Tu.
During elongation, after peptide bond formation, peptidyl-tRNA sits in the P site and the next aminoacyl-tRNA in the A site, with mRNA paused. Translocation shifts everything by one codon, moving the ribosome forward.
Option Analysis
GDP
GDP is a nucleotide, not a protein factor. It binds to elongation factors like EF-G or EF-Tu after GTP hydrolysis but does not directly perform translocation. It signals inactivity, so incorrect.
NADP
NADP acts as a coenzyme in redox reactions, like photosynthesis or pentose phosphate pathway in plants and microbes. It has no role in bacterial translation elongation or ribosome movement. Irrelevant here.
EF-G (Correct)
EF-G (Elongation Factor G) is the GTPase that catalyzes translocation. It binds the pre-translocation ribosome (with hybrid A/P and P/E tRNAs), hydrolyzes GTP, and drives rotation of the 30S subunit head and body, shifting mRNA and tRNAs to P and E sites.
GTP hydrolysis by EF-G unlocks the ribosome, accelerates spontaneous movement by 10,000-fold, and stabilizes the post-translocation state. Without it, translocation stalls.
EF-Tu
EF-Tu delivers aminoacyl-tRNA to the A site during elongation, proofreads codon-anticodon match, and hydrolyzes GTP for release. It acts before peptide bonding, not in translocation.
Correct Answer
(3) EF-G. This matches bacterial translation mechanisms in exams like GATE Life Sciences.
Chain Elongation Overview
Chain elongation cycles through: aa-tRNA/EF-Tu/GTP entry (EF-Tu step), peptidyl transfer by peptidyl transferase, then EF-G/GTP-driven translocation. Repeat until stop codon.
| Step | Factor | Role |
|---|---|---|
| Codon recognition & A-site entry | EF-Tu | Delivers aa-tRNA, GTP hydrolysis |
| Peptide bond | Ribosome (peptidyl transferase) | Links amino acids |
| Translocation | EF-G | Moves tRNA/mRNA by one codon |
EF-G’s structure mimics the tRNA-mRNA complex, aiding its pawl-like or motor function in unlocking and biasing forward movement. For plant biology contexts, note similar eEF2 in eukaryotes.
