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A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; however, plasmids are sometimes present in archaea and eukaryotic organisms.
Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmids that often encode advantageous traits but are not essential to their bacterial host. Plasmids can be easily gained or lost by a bacterium and can be transferred between bacteria as a form of horizontal gene transfer.
A bacterial artificial chromosome (BAC) is a DNA construct, based on a functional fertility plasmid (or F-plasmid), used for transforming and cloning in bacteria, usually E. coli. [ 1 ] [ 2 ] [ 3 ] F-plasmids play a crucial role because they contain partition genes that promote the even distribution of plasmids after bacterial cell division.
The DNA of the bacterial cell is silenced before entry into the cell by a repressor protein which is encoded for by the prophage. Upon the transfer of the bacterial cell's DNA into the host cell, the repressor protein is no longer encoded for, and the bacterial cell's original DNA is then turned on in the host cell.
DNA gyrase is not the sole enzyme responsible for decatenation. In an experiment by Zechiedrich, Khodursky and Cozzarelli in 1997, it was found that topoisomerase IV is the only important decatenase of DNA replication intermediates in bacteria. [20] When DNA gyrase alone was inhibited, most of the catenanes were unlinked.
Extrachromosomal DNA exists in prokaryotes outside the nucleoid region as circular or linear plasmids. Bacterial plasmids are typically short sequences, consisting of 1 to a few hundred kilobase (kb) segments, and contain an origin of replication which allows the plasmid to replicate independently of the bacterial chromosome. [10]
More than five decades ago, Jacob, Brenner, and Cuzin proposed the replicon hypothesis to explain the regulation of chromosomal DNA synthesis in E. coli. [18] The model postulates that a diffusible, trans-acting factor, a so-called initiator, interacts with a cis-acting DNA element, the replicator, to promote replication onset at a nearby origin.
3-The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell. 4-Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors. A pilus (Latin for 'hair'; pl.: pili) is a hair-like cell-surface appendage found on many bacteria and archaea. [1]