What is the principal difference between a self-transmissible (conjugative) plasmid and a mobilizable plasmid?

Introduction / Context:
Plasmid transfer depends on specialized genes. Conjugative (self-transmissible) plasmids encode the full set of transfer (tra) functions, including pilus formation and DNA processing. Mobilizable plasmids generally contain an origin of transfer (oriT) and relaxase but lack the mating pair formation genes, relying on a helper plasmid for transfer.

Given Data / Assumptions:

• We must compare genetic content required for transfer.
• Both plasmid types can move under some conditions, but only one supplies all required machinery.

Concept / Approach:
Self-transmissible plasmids encode both DNA processing and mating pair formation components. Mobilizable plasmids only encode a subset (often relaxase and oriT) and piggyback on a co-resident conjugative plasmid.

Step-by-Step Solution:

Verification / Alternative check:
Many IncF and IncP plasmids are self-transmissible. Small cloning plasmids with oriT are mobilizable only in the presence of a helper plasmid.

Why Other Options Are Wrong:

• Transfers both DNA strands: Conjugation typically transfers a single DNA strand; the second strand is synthesized in the recipient.
• Transfers antibiotic resistance genes: Either plasmid type may carry resistance; this is not the defining difference.
• Usually has a transposon inserted: Not a defining feature.

Common Pitfalls:

• Confusing oriT presence (mobilizable) with full conjugation capability.
• Assuming resistance genes imply self-transmissibility.

Final Answer:
Carries genes encoding the mating apparatus