T-DNA transfer initiates at:
Ti plasmid T-DNA is delimited by two imperfect 25 base pair direct repeats designated right and left borders. Transfer is inherently polar because right border contains optimal nick site plus adjacent overdrive sequence that recruits VirC1 and VirD2 complex. Nicking at right border creates free 5' end that remains capped by VirD2 and defines leading end for Type IV secretion. Displacement synthesis then proceeds toward left border, which functions merely as termination point. Mutants lacking right border are completely avirulent, while left border deletion still allows transfer with vector backbone read-through, confirming initiation.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.