Ti plasmid genetics — Where are the virulence (vir) genes that mediate T-DNA processing and transfer located relative to the T-DNA region?
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AOn the T-DNA itself
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BOutside the T-DNA region on the Ti plasmid (separate vir region)
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COnly on the plant genome after integration
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DOn a chloroplast plasmid
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EOn the bacterial chromosome only, not the plasmid
Answer
Correct Answer: Outside the T-DNA region on the Ti plasmid (separate vir region)
Explanation
Introduction / Context:Agrobacterium tumefaciens uses vir genes to excise, process, and transfer T-DNA into plant cells. These vir genes are essential for infection but are not themselves transferred to the plant; they reside in a distinct region of the Ti plasmid.
Given Data / Assumptions:
- T-DNA borders delimit the DNA that is transferred.
- virA/virG and other vir operons encode proteins for sensing, processing, and transfer.
- Only the T-DNA between border repeats integrates into the plant genome.
Concept / Approach:Separate the transfer cargo (T-DNA) from the bacterial machinery (vir region). The vir genes operate in trans to mobilize the T-DNA, remaining in the bacterium.
Step-by-Step Solution:
Identify vir region location → outside T-DNA.Recognize that only T-DNA integrates into plant DNA.Choose the option reflecting this separation.Verification / Alternative check:Binary vector systems split T-DNA and vir functions onto separate plasmids, proving vir can act in trans.
Why Other Options Are Wrong:
a) If vir were on T-DNA, they would transfer, but they do not.c,d,e) Contradict known Ti plasmid architecture and function.Common Pitfalls:Confusing border sequences (on T-DNA) with vir genes (outside T-DNA).
Final Answer:Outside the T-DNA region on the Ti plasmid (separate vir region).