Conjugative plasmids in bacterial genetics Which statement best describes the defining genetic feature of conjugative plasmids that enables cell-to-cell DNA transfer?

Introduction / Context:
Conjugative plasmids are mobile genetic elements that can move directly between bacteria via conjugation. Understanding what makes a plasmid “conjugative” is essential for topics such as horizontal gene transfer, the spread of antibiotic resistance, and molecular cloning strategies.

Given Data / Assumptions:

• Bacterial conjugation requires physical contact, typically via a pilus.
• Different plasmids may or may not encode the machinery needed for transfer.
• Some plasmids also carry accessory genes (e.g., antibiotic resistance), but these are not the defining trait.

Concept / Approach:
Plasmids that are truly conjugative encode a suite of transfer functions, classically referred to as tra genes. These genes direct pilus formation, mating pair stabilization, nicking/transfer of a single DNA strand, and re-circularization in the recipient. Resistance genes are common passengers but are not required for conjugation competence.

Step-by-Step Solution:

Verification / Alternative check:
Classic F plasmid biology demonstrates that tra gene presence correlates with conjugation capability. Nonconjugative plasmids lack these loci and depend on helpers for mobilization.

Why Other Options Are Wrong:

• Antibiotic resistance (option a) is common but not defining.
• “Do not exhibit resistance” (option b) is unrelated to conjugation status.
• “Do not carry transfer genes” (option d) describes nonconjugative plasmids.
• Chromosomal integration (option e) pertains to episomes, not a universal trait.

Common Pitfalls:
Equating “conjugative” with “resistance plasmid.” Many resistance plasmids are conjugative, but resistance can also be on nonconjugative plasmids or chromosomes.

Final Answer:
Carry transfer genes known collectively as tra genes