Section 4: Bioprocess Engineering and Process Biotechnology
Mass transfer in bioprocesses (kLa)
Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)
Question Statement:
During protein synthesis, tRNAs are NOT involved in
Options:
(A) charging
(B) intiation
(C) elongation
(D) termination
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.17)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Molecular Biology (Translation Machinery & Termination Factors)
**2. Detailed Step-by-Step Analysis**:
- Let us examine the role of transfer RNA (tRNA) across the four stages of protein translation:
1. **Charging (Aminoacylation)**: Aminoacyl-tRNA synthetases covalently esterify specific amino acids onto their cognate tRNAs using ATP. Thus, tRNAs are directly involved.
2. **Initiation**: The initiator tRNA ($tRNA_i^{\text{Met}}$ in eukaryotes, $fMet-tRNA_f^{\text{Met}}$ in prokaryotes) binds to the start codon (AUG) in the ribosomal P-site. Thus, tRNAs are directly involved.
3. **Elongation**: Aminoacyl-tRNAs complexed with elongation factors (EF-Tu/eEF1A) decode mRNA codons in the ribosomal A-site, delivering amino acids for peptidyl transfer. Thus, tRNAs are directly involved.
4. **Termination**: When a stop codon (UAA, UAG, UGA) enters the ribosomal A-site, there are **no normal tRNAs** that recognize stop codons. Instead, protein **release factors** (RF1/RF2 in prokaryotes, eRF1 in eukaryotes), which mimic the shape of a tRNA molecule (molecular mimicry), bind directly to the stop codon and trigger ester bond hydrolysis by peptidyl transferase to release the nascent polypeptide chain.
- Therefore, tRNAs are **NOT** involved in **termination**.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Charging specifically modifies tRNA with amino acids.
- **Option (B) [INCORRECT]**: Initiator tRNA is an essential participant in translation initiation.
- **Option (C) [INCORRECT]**: Elongation relies on sequential aminoacyl-tRNA delivery to the A-site.
- **Option (D) [CORRECT]**: Termination is mediated entirely by protein release factors without tRNA involvement.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **D**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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