Plasmid Created 2026-10-13 Updated 2026-10-18
A plasmid is an extrachromosomal DNA replicon: genetic material maintained by DNA replication separately from the host's main chromosomes. Separate copying does not mean independence of the host's enzymes or resources. Many plasmids are circular, but naturally occurring linear plasmids also exist.[ref]
Conservation of plasmid maintenance functions between linear and circular plasmids in Borrelia burgdorferi by Philip E. Stewart and others. Stewart, Chaconas and Rosa (2003), abstract opening sentence: natural linear and circular plasmids
Engineered plasmids can carry genes for genetic transformation and functional complementation. Typical laboratory plasmids combine an origin of replication, a selectable marker and a plasmid insert. These are engineering roles, not requirements that every natural plasmid carry an experimentally chosen selectable marker or plasmid insert.[ref]
Molecular Biology Reference by Addgene. Plasmid Elements, Figure 2 and table: origin, antibiotic-resistance gene, insert and promoter; description applies to engineered vectors
The useful separation is between keeping a DNA fragment, finding its carriers and testing its function. Molecular cloning uses propagation to recover more copies for study. Plasmid restriction mapping tests a proposed arrangement, while DNA sequencing examines its letters. A successful genetic transformation or a biological colony showing antibiotic resistance answers neither question completely. The yeast cross-species complementation experiment goes a step further by separating plasmid selection from a test of functional complementation.