Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
Which set of the following events occurs during the elongation step of translation?
P. Attachment of mRNA with the smaller subunit of ribosome
Q. Loading of correct aminoacyl-tRNA into the A site
R. Formation of a peptide bond between the amino acyl-tRNA in the A site and the peptide
chain that is attached to the peptidyl-tRNA in the P site
S. Dissociation of the ribosomal subunits
T. Translocation of peptidyl-tRNA from the A site to the P site of the ribosome
Options:
(A) Surface immunoglobulins ̶ B cell antigen receptor
(B) Affinity maturation ̶ isotype switching
(C) Fc region of antibodies ̶ binding to complement proteins
(D) Spleen, the secondary lymphoid organ ̶ no connection with the lymphatic system
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.37)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Molecular Biology (Mechanism of Translation Elongation Cycle)
**2. Detailed Step-by-Step Derivation**:
- Translation is divided into initiation, elongation, and termination:
- **Initiation**: The small ribosomal subunit binds mRNA, recruits initiator tRNA to the P-site, and the large subunit joins (Event P).
- **Elongation Cycle**: A recurring three-step cycle:
1. **Step 1 (Decoding)**: The correct cognate aminoacyl-tRNA complexed with EF-Tu$\cdot$GTP (or eEF1A) is loaded into the ribosomal **A site** (**Event Q**).
2. **Step 2 (Peptidyl Transfer)**: The 23S/28S rRNA peptidyl transferase ribozyme catalyzes peptide bond formation between the amino acid on aminoacyl-tRNA in the A-site and the ester-linked growing peptide on peptidyl-tRNA in the **P site** (**Event R**).
3. **Step 3 (Translocation)**: EF-G$\cdot$GTP (or eEF2) hydrolyzes GTP, causing the ribosome to advance one codon ($3\text{ nucleotides}$) along mRNA, which translocates the uncharged tRNA to the E-site and the newly formed peptidyl-tRNA from the **A site to the P site** (**Event T**).
- **Termination & Ribosome Recycling**: Stop codons trigger release factors, leading to polypeptide release and dissociation of ribosomal subunits (Events S).
- Therefore, the specific events occurring during the **elongation step** are **Q, R, and T**.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Event P occurs during initiation.
- **Option (B) [INCORRECT]**: Event P is an initiation step.
- **Option (C) [CORRECT]**: Events Q (A-site loading), R (peptide bond synthesis), and T (ribosomal translocation) define the elongation cycle.
- **Option (D) [INCORRECT]**: Event S occurs during termination and recycling.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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