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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.
conjugative - mediate DNA transfer through conjugation and therefore spread rapidly among the bacterial cells of a population; e.g., F plasmid, many R and some col plasmids. nonconjugative - do not mediate DNA through conjugation, e.g., many R and col plasmids. The pBR322 plasmid is one of the first plasmids widely used as a cloning vector.
The most common method is alkaline lysis, which involves the use of a high concentration of a basic solution, such as sodium hydroxide, to lyse the bacterial cells. [15] [16] [17] When bacteria are lysed under alkaline conditions (pH 12.0–12.5) both chromosomal DNA and protein are denatured; the plasmid DNA however, remains stable.
Various Azospirillum species possess seven replicons; A. lipoferum, for instance, has one bacterial chromosome, five chromids, and one plasmid. [4] Plasmids and bacteriophages are usually replicated as single replicons, but large plasmids in Gram-negative bacteria have been shown to carry several replicons. [5]
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]
AT-rich region: important for DNA strand opening and is located adjacent to the inverted repeat sequences. [1] [3] [5] [8] [11] [12] The oriT is a noncoding region of the bacterial DNA. [13] Due to its important role in initiating bacterial conjugation, the oriT is both an enzymatic substrate and recognition site for the relaxase proteins.
Gene therapy using plasmids is a potential treatment option for individuals who have a dysfunctional gene causing a disorder. Recombinant DNA of a functional copy of the defective gene can be incorporated into a plasmid host cell. This provides the host cell with a functional gene to treat the disorder. [5]