A plasmid is a small, circular loop of double-stranded DNA found in the cytoplasm of many bacteria, separate from the main bacterial chromosome in the nucleoid. Plasmids are much smaller than the chromosome, usually a few thousand base pairs long, and a single cell may contain one copy or many copies of the same plasmid. Not every bacterium has plasmids.
Plasmids replicate independently of the bacterial chromosome, using their own origin of replication. This means a cell can build up several copies, and copies are passed to both daughter cells when the bacterium divides by binary fission.The genes on plasmids are usually not essential for the normal survival of the cell, but they can give an advantage in particular conditions. Examples include genes for antibiotic resistance (on R plasmids), genes for toxins, and genes that allow the use of unusual nutrients.Plasmids can be passed from one bacterium to another, even between different species. In conjugation, a copy of a plasmid is transferred from a donor cell to a recipient cell through a sex pilus. This is a form of horizontal gene transfer, and it is one reason antibiotic resistance can spread so quickly through bacterial populations.Because they are small, replicate on their own and are easily taken up by bacteria, plasmids are also used as vectors in genetic engineering to carry a new gene into a bacterial cell.
Plasmids replicate independently of the bacterial chromosome, using their own origin of replication. This means a cell can build up several copies, and copies are passed to both daughter cells when the bacterium divides by binary fission.The genes on plasmids are usually not essential for the normal survival of the cell, but they can give an advantage in particular conditions. Examples include genes for antibiotic resistance (on R plasmids), genes for toxins, and genes that allow the use of unusual nutrients.Plasmids can be passed from one bacterium to another, even between different species. In conjugation, a copy of a plasmid is transferred from a donor cell to a recipient cell through a sex pilus. This is a form of horizontal gene transfer, and it is one reason antibiotic resistance can spread so quickly through bacterial populations.Because they are small, replicate on their own and are easily taken up by bacteria, plasmids are also used as vectors in genetic engineering to carry a new gene into a bacterial cell.