Fundamental concepts and definition:
are autonomous extrachromosomal replicons found commonly in prokaryotes, but also in eukaryotes and their organelles. They are generally covalently closed and supefioiled circles of double-stranded DNA with sizes ranging from 1to 300 kbp. They contain fmm one to over 100 genes.
Some plasmids integrate into the host genome and are described as episomes
Plasmids are parasitic structures (seysh DNA (q.v.)) which move between populations by conjugation, transformation, transduction or cell fusion. Minimally, plasmids encode products required for their replication and maintenance in the host.
Most bacterial plasmids also carry genes conferring a desirable but dispensable phenotype upon their host (e.g. antibiotic resistance). Such genes are often found on transposable elements which can jump between different plasmids in the same cell.
Conversely, eukaryotic nuclear plasmids are generally cryptic plasmids (they confer no phenotype) and some appear to have originated from chromosomal DNA. Terms used in plasmid biology are defined in Table 20.2
The fundamental distinction between plasmids and viruses is that the former are maintained as stable extrachromosomal replicons which do not encode the coat proteins that enable viruses to form infectious particles (they can therefore only exist outside the cell as naked DNA). Some viruses can exist as plasmids, e.g. the P1 prophage is maintained as a plasmid during temperate infection, but the converse is never true.
Plasmids are used widely as cloning vectors, but this chapter primarily concerns the behavior of natural plasmids.