Discussion
A plasmid is best described as
Answer: D
Correct answer: a small circular DNA molecule that replicates independently of the chromosome
A plasmid is an extrachromosomal DNA molecule, usually circular and double-stranded, carrying its own origin of replication so that it replicates independently of the host chromosome. It is called a replicon for that reason. Plasmids are generally dispensable for growth but carry genes conferring advantage in particular conditions: R plasmids confer antibiotic resistance, F plasmids confer the ability to conjugate, Col plasmids encode bacteriocins, and virulence plasmids encode toxins. Their spread by conjugation is the principal route by which antibiotic resistance moves between bacteria, including between species, which is why resistance can appear far faster than mutation alone would allow. In the laboratory they are the standard cloning vectors: a plasmid with a selectable marker, an origin of replication and a multiple cloning site accepts foreign DNA and carries it into a host for replication and expression. A plasmid that can also integrate into the chromosome is an episome.
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