Plasmid

Plasmids are (typically) circular double-stranded DNA molecules that are separate from the chromosomal DNA (Fig. 1). They usually occur in bacteria, sometimes in eukaryotic organisms (e.g., the 2-micrometre-ring in Saccharomyces cerevisiae). Their size varies from 1 to over 400 kilobase pairs (kbp). There are from one copy, for large plasmids, to hundreds of copies of the same plasmid present in a single cell.

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Plasmids often contain genes or gene-cassettes that confer a selective advantage to the bacterium harboring them, e.g., the ability to make the bacterium antibiotic resistant. Every plasmid contains at least one DNA sequence that serves as an origin of replication or ori (a starting point for DNA replication), which enables the plasmid DNA to be duplicated independently from the chromosomal DNA (Fig. 2).

DNA: :For other uses, see DNA (disambiguation)....

Saccharomyces cerevisiae: Saccharomyces cerevisiae is a species of budding yeast. It is perhaps the most relevant yeast for mankind, both for its use since ancient times in baking and brewing, and for being one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia col...

Kilobase pair: A kilobase, also referred to as kilobase pair, Kb, and Kbp, is literally one thousand base pairs; this is a measure of the length of a piece of nucleic acid (DNA or RNA). Because nucleic acids in the wild are normally double-stranded, each nucleotide is normally matched up with its complement to f...