In microbial genetics, which type of plasmid can exist either autonomously (as an independent circular DNA molecule) or integrated into the host's chromosome, depending on cellular conditions?

Introduction / Context:
Plasmids are extra-chromosomal, circular DNA molecules that replicate independently in many bacteria. A special category, called episomes, can integrate into the host chromosome and later excise to exist independently. This question checks your understanding of plasmid behavior and vocabulary used in microbial genetics and biotechnology.

Given Data / Assumptions:

• The focus is on a plasmid type that has two lifestyles: free (autonomous) and integrated.
• The host is a prokaryotic cell (e.g., Escherichia coli).
• Terms like episome and lysogen have specific meanings in genetics.

Concept / Approach:
Episomes are plasmids capable of site-specific recombination with the bacterial chromosome. Famous examples include the F factor in E. coli, which can integrate via recombination to form HFr strains, and later excise to become an autonomous F plasmid again. This duality distinguishes episomes from ordinary non-integrative plasmids.

Step-by-Step Solution:

Verification / Alternative check:
Recall the F factor behavior: when integrated, the cell is HFr; when excised, the F factor replicates as a plasmid. This is the textbook example of an episome, confirming the choice.

Why Other Options Are Wrong:

• Medisome: Not a standard term in genetics.
• Lisosome: Misspelling and not a genetics term.
• Lysogen: Refers to a bacterium carrying a prophage, not a plasmid class.

Common Pitfalls:

• Confusing episomes (plasmid–chromosome integration) with prophages (virus–chromosome integration).
• Assuming all plasmids can integrate; most cannot without specialized systems.

Final Answer:
Episome