Bacteria swap tiny loops of DNA, even across species lines
Alongside their main chromosome, many bacteria carry plasmids: small, self-copying rings of DNA holding optional extras such as antibiotic resistance or the ability to digest toxic chemicals. They pass these rings to neighbours, sometimes of entirely different species, which helps explain how useful traits spread so quickly through microbial populations.
A plasmid is a DNA molecule that lives apart from the chromosome and replicates on its own. Most are circular and double-stranded and occur in bacteria and archaea, though some eukaryotes have them and a few are linear, needing special tricks to copy their ends. If chromosomes hold the essentials for normal life, plasmids carry the kit for special circumstances. Sizes range from under one thousand base pairs to megaplasmids of several million, at which point they barely differ from small chromosomes, and a cell may hold one copy or thousands.
The American molecular biologist Joshua Lederberg coined the word in 1952 for any hereditary factor outside the chromosome. That early definition accidentally swept in bacterial viruses, so in 1968 it was narrowed to elements that live mainly outside the chromosome, replicate independently, and help move mobile genes between unrelated bacteria. Like viruses, plasmids are generally not considered alive. Unlike viruses, they have no protein coat; they travel as naked DNA, mostly through conjugation, and some encode the 'sex' pilus that makes the transfer possible. This makes them part of the mobilome, the shifting pool of genes exchanged horizontally.
Replication depends on an origin of replication, often with a gene for a starter protein and repeated sequences called iterons. Small plasmids borrow the host's copying enzymes; large ones may bring their own. A few can insert themselves into the host chromosome. The cargo varies: resistance to antibiotics or heavy metals, virulence factors that help invade a host, nitrogen fixation, or the ability to break down stubborn pollutants. Some 'cryptic' plasmids offer no obvious benefit, though recent work suggests they may contribute to uneven antibiotic resistance within populations.
Laboratories exploit all this. Engineered plasmids serve as vectors in molecular cloning, carrying recombinant DNA into host cells through a process called transformation, and custom sequences designed with software can now be ordered online from commercial suppliers.
Source: Plasmid
