108. In a plant transformation vector, the selectable marker gene is ideally located next to the “left border” of T-DNA region, because:
A. Left border of T-DNA enters last into the plant cell.
B. Right border of T-DNA enters last into the plant cell.
C. It ensures complete transfer of T-DNA into the plant cell.
Choose the most appropriate answer from the options given below:
A. A and B Only
B. B and C Only
C. A and C Only
D. C Only

Detailed Explanation:

In plant genetic engineering, the T-DNA region of a plasmid from Agrobacterium tumefaciens is transferred into the plant genome. This T-DNA is flanked by two distinct sequences — the Right Border (RB) and the Left Border (LB).

Direction of T-DNA Transfer:

• The transfer of T-DNA begins at the right border and proceeds towards the left border.

• This means the right border enters the plant cell first, while the left border enters last.

Why Place the Selectable Marker Gene Near the Left Border?

1. Ensuring Complete Transfer:

   • Since transfer proceeds from right to left, genes located near the left border are the last to be integrated.

   • If the selectable marker gene is placed near the left border, only cells that have received the entire T-DNA (including the marker) will survive under selection.

   • This ensures selection of fully transformed plant cells.

2. Minimizing Partial Transfers:

   • If the selectable marker were placed near the right border, partial transfers could still confer resistance without complete gene integration.

Correct Answer:

C. A and C Only

• A is correct: Left border enters last.

• C is correct: Placement near the left border ensures full T-DNA transfer.

• B is incorrect: The right border enters first, not last.

Conclusion:

Strategic placement of the selectable marker gene near the left border of T-DNA is critical in plant transformation. It acts as a safeguard, ensuring only fully transformed cells survive, which increases the efficiency and reliability of genetic engineering experiments.