Section 4: Bioprocess Engineering and Process Biotechnology
Mass transfer in bioprocesses (kLa)
Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)
Question Statement:
During a positive-negative selection process, transformed animal cells expressing
are killed in presence of ganciclovir in the medium.
Options:
(A) pyruvate kinase
(B) viral thymidine kinase
(C) viral serine/threonine kinase
(D) viral tyrosine kinase
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2020 Q.19)**
**1. Governing Concept**:
- **Topic**: Genetic Engineering & Gene Targeting (Positive-Negative Selection)
- In gene targeting in embryonic stem cells (Mansour et al.):
- **Positive selection marker**: Neomycin phosphotransferase ($neo^r$), inserted into the target exon, confers resistance to G418 (selects for vector integration).
- **Negative selection marker**: Herpes simplex virus thymidine kinase (*HSV-tk*), placed at the flank of the targeting construct.
- If random (non-homologous) integration occurs, *HSV-tk* is retained, and its expression converts the non-toxic nucleoside analog **ganciclovir** into a toxic phosphorylated compound that kills the cells.
- Thus, cells expressing **viral thymidine kinase** are killed in the presence of ganciclovir.
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Pyruvate kinase is a normal glycolytic enzyme.
- **Option (B) [viral thymidine kinase] is CORRECT**: Phosphorylates ganciclovir into a cytotoxic chain-terminator.
- **Option (C) is INCORRECT**: Non-existent selection enzyme.
- **Option (D) is INCORRECT**: Tyrosine kinases do not metabolize ganciclovir.
**3. Final Answer**:
- **Correct Choice**: **B**
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