T-DNA borders of the Ti plasmid On the Ti plasmid, the T-region (T-DNA) is flanked by direct repeat border sequences of what length?

Introduction / Context:
T-DNA transfer begins with precise endonucleolytic nicks at conserved border repeats. Knowing the length of these repeats is central to vector design, primer placement, and understanding how Vir proteins recognize and process the transfer DNA in Agrobacterium-mediated transformation.

Given Data / Assumptions:

• Border repeats occur at left and right ends of the T-DNA.
• VirD1/VirD2 nick at or near the right border to initiate transfer.
• The repeat length is conserved among canonical Ti plasmids.

Concept / Approach:
The classic border element is a 25 base-pair direct repeat sequence present at both T-DNA ends, with the right border serving as the primary nicking site for generating the single-stranded T-strand. These sequences are retained in most binary vectors to ensure proper processing and transfer.

Step-by-Step Solution:

Verification / Alternative check:
Sequence alignments of pTi plasmids and widely used binary vectors (e.g., pBIN19 derivatives) show the 25 bp border sequences flanking the T-DNA cassette.

Why Other Options Are Wrong:

• 12, 20, 30, or 100 bp: do not match the canonical 25 bp border repeat length.

Common Pitfalls:
Confusing the 25 bp border repeats with longer promoter or terminator sequences in the T-DNA payload; the borders are short and specific.

Final Answer:
25 bp