Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Fabry disease in humans is an X-linked recessive disease. The probability (in percentage) for a phenotypically normal father and a carrier mother to have a son with Fabry disease is ________%.
Options:
(A) landing → attachment → tail contraction → penetration and unplugging → DNA ejection
(B) attachment → landing → penetration and unplugging → tail contraction → DNA ejection
(C) landing → tail contraction → attachment → DNA ejection → penetration and unplugging
(D) attachment → tail contraction → landing → penetration and unplugging → DNA ejection
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2023 Q.20)**
**1. Governing Concept**:
- **Topic**: Virology & T-Even Bacteriophage Infection Mechanism
- Infection of an *E. coli* host cell by contractile myoviruses (such as bacteriophage T4) follows an invariant biophysical sequence:
1. **Landing**: Long tail fibers reversibly contact outer membrane receptors (LPS/OmpC).
2. **Attachment**: Short tail fibers pin irreversibly to the cell surface.
3. **Tail contraction**: Contractile tail sheath shortens, driving the rigid inner tail tube through the outer membrane.
4. **Penetration and unplugging**: Baseplate tail lysozyme degrades peptidoglycan, and the tail plug is displaced.
5. **DNA ejection**: High internal capsid pressure ejects genomic dsDNA through the tube into host cytoplasm.
**2. Option Analysis**:
- **Option (A) [landing → attachment → tail contraction → penetration and unplugging → DNA ejection] is CORRECT**: Represents the exact chronological biophysical sequence.
- **Option (B) is INCORRECT**: Attachment cannot precede landing; tail contraction must occur before penetration.
- **Option (C) is INCORRECT**: Tail contraction cannot occur prior to stable attachment.
- **Option (D) is INCORRECT**: Contraction cannot happen prior to landing.
**3. Final Answer**:
- **Correct Choice**: **A**
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