Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
In animal cell culture, a CO2 enriched atmosphere in the incubator chamber is used to maintain the
culture pH between 6.9 and 7.4. Which one of the following statements is CORRECT?
Options:
(A) Higher the bicarbonate concentration in the medium, higher should be the requirement of
gaseous CO2
(B) Lower the bicarbonate concentration in the medium, higher should be the requirement of
gaseous CO2
(C) Higher the bicarbonate concentration in the medium, lower should be the requirement of
gaseous CO2
(D) CO2 requirement is independent of bicarbonate concentration in the medium
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.40)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Plant Biotechnology & Genetic Engineering (*Agrobacterium* Ti Plasmid Biology)
**2. Detailed Step-by-Step Analysis**:
- In *Agrobacterium tumefaciens*-mediated plant transformation, the **Ti (tumor-inducing) plasmid** contains distinct functional regions:
1. **T-DNA**: The segment transferred into the plant genome, delimited by **25-bp direct border repeat sequences** (Left Border, LB, and Right Border, RB).
2. **Internal T-DNA Genes**: Oncogenes (*iaaM*, *iaaH*, *ipt*) for auxin and cytokinin synthesis, and opine biosynthesis genes (*nos*, *ocs*). These are expressed only inside the plant cell. They are **NOT** required for the transfer process itself and can be deleted entirely (yielding disarmed vectors).
3. **Virulence (*vir*) region**: A regulon of ~24 genes (*virA, virG, virB, virC, virD, virE*) that encode the molecular transfer machinery:
- VirD1/VirD2 endonuclease cleaves at the **border repeats** to generate single-stranded T-strands.
- VirB/VirD4 assemble a Type IV secretion system channel spanning the bacterial membranes.
- VirE2 coats the single-stranded T-strand.
- Therefore, the only genetic components **essential for T-DNA excision and transfer** are the **Border repeat sequences** (in cis) and the **vir genes** (in trans).
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Oncogenes are dispensable and disarmed in biotechnology vectors.
- **Option (B) [CORRECT]**: Border repeat sequences (defining T-DNA boundaries) and *vir* genes (providing the endonuclease and transfer machinery) are strictly essential.
- **Option (C) [INCORRECT]**: Opine genes are non-essential passengers.
- **Option (D) [INCORRECT]**: Neither opine genes nor oncogenes are required for transfer.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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