The T-DNA segment of Agrobacterium tumefaciens Ti plasmid is approximately 23,000 base pairs (bp) in size, making option (3) correct.​

Question Breakdown

Agrobacterium tumefaciens transfers T-DNA from its Ti plasmid into damaged plant cells, where it integrates randomly into the nuclear genome, causing crown gall tumors. The question focuses specifically on T-DNA length, a key fact in plant molecular biology and genetic engineering.​

Option Analysis

• (1) 80,000 bp: Incorrect; far exceeds actual T-DNA size, possibly confused with entire Ti plasmid (~200,000 bp).​

• (2) 40,000 bp: Incorrect; double the correct value, no biological basis in standard Ti plasmids.​

• (3) 23,000 bp: Correct; standard T-DNA size carries virulence genes (e.g., cytokinin, auxin biosynthesis) and opine synthesis genes.​

• (4) 50,000 bp: Incorrect; sometimes cited for modified binary vectors, but native tumor-inducing T-DNA is ~23 kb.​

The T-DNA size Agrobacterium tumefaciens Ti plasmid measures 23,000 base pairs, critical for GATE Life Sciences students studying plant genetic engineering and bacterial gene transfer. This segment integrates into plant chromosomes at wound sites, transforming cells into tumors while enabling modern biotech tools like GM crops.​

T-DNA Mechanism

T-DNA borders (25 bp direct repeats) define the transferable region processed by VirD2 endonuclease into single-stranded T-strand, escorted by proteins into plant nucleus.​

• Carries iaa, tms (hormone genes) + nos, ocs (opine genes).

• Integration: Random via non-homologous recombination.

Size Significance

Native T-DNA: ~23 kb fits type IV secretion system limits. Disarmed vectors replace tumor genes with GOI for transformation.​

Exam Strategy

GATE traps: Confuse T-DNA (23 kb) vs. full Ti. Remember: Contact → Vir induction → T-DNA transfer → Integration. Key for plant biotech questions.