Plant gene transfer — What is the approximate size range of the virulence (Ti) plasmid carried by Agrobacterium tumefaciens that mediates T-DNA transfer to plant cells?

Introduction / Context:
The tumor-inducing (Ti) plasmid of Agrobacterium tumefaciens is central to natural genetic transformation of plants and to plant biotechnology. Knowing the typical size range of this virulence plasmid helps students appreciate cloning strategies, vector design, and why binary/co-integrative systems are used rather than moving the entire plasmid into Escherichia coli.

Given Data / Assumptions:

• The Ti plasmid carries vir genes for DNA processing/transfer and a T-DNA region integrated into plant genomes.
• Large plasmids are difficult to manipulate directly in standard lab hosts.
• Published Ti plasmids commonly fall in the low-hundreds of kilobase size range.

Concept / Approach:
Classical nopaline- and octopine-type Ti plasmids are typically around two hundred kilobases. Because of this size, binary vector systems split functions: a small E. coli–Agrobacterium shuttle plasmid carries T-DNA, while a resident helper plasmid in Agrobacterium supplies vir functions in trans.

Step-by-Step Solution:

Verification / Alternative check:
Genetic maps in plant biotech texts consistently depict Ti plasmids ~200 kb, supporting the 140–235 kb bracket as representative across common isolates.

Why Other Options Are Wrong:

• 40–80 kb and 80–120 kb: too small for full vir/T-DNA complement of classical Ti plasmids.
• >235 kb: some variants can be very large, but this is not the typical range emphasized in foundational courses.
• 8–20 kb: corresponds to small cloning vectors, not Ti plasmids.

Common Pitfalls:
Assuming all plasmids are similar in size; megaplasmids like Ti plasmids necessitate specialized vector systems.

Final Answer:
140-235 kb