Plasmids (term coined by J.Lederberg) are autonomous self-replicating molecules of DNA that are maintalned as discrete, extrachromosomal genetic elements in bacteria. It is a replicon (replicon is self-replicating moles nucleic acid). Plasmids are mostly DNA molecules. RNA plasmids (both single stranded and double strandad form) are very rare and reported in few plants, fungi and even animals. Bacterial plasmids are usually much smaller than bacterial chromosome, varying from less than 5 kbp to more than several hundred kbp, though plasmids as large 2Mbp also occur in some bacteria. They usually encode traits that are not essential for bacterial viability, and replicate independently of the chromosome. Most plasmids are circular, negatively supercoiled, dsDNA molecules, linear plasmids have also been reported in genera Borrelia and Streptomyces.

Types of plasmid

Conjugative and mobilizable plasmid:

Plasmids can be classified into two categories according to their mobility- conjugative (self-transmissible) and mobilizable. A conjugative plasmid is a self-transmissible plasmid in that it possesses all the necessary genes to transmit itself to another bacterium by conjugation. It contains both Mpf genes (involved in mating pair formation) and Dtr genes (involved in DNA transfer and replication). Mpf genes encode proteins responsible for pilus-mediated cell-to-cell contact and formation of Type IV secretion system  which act as a channel through which proteins or protein-DNA complexes can be translocated. Dtr genes encode proteins that bind to the DNA at the origin of transfer, oriT, forming a complex called relaxosome. The key catalytic component of the relaxosome is the relaxase. The relaxase produces a nick within oriT and becomes transiently attached to the 5' end of the DNA single strand.

The F-plasmid is an example of self-transmissible plasmid. Bacteria that possess a copy of F-plasmid are called F-Dositive or F-plus (denoted F*). Cells wich has the of lack F plasmids are called F-negative or F-minus (F). The F-plasmid is also an episome. A plasmid that can exist autonomously in a cell or can integrate itself into the bacterial chromosome is termed as an episome. A bacterial cell that contains an F-plasmid integrated with the bacterial chromosome is referred to as an Hfr (High frequency recombination) cell. Integration involves homologous recombination between two covalently closed circular DNA molecules forming one circular molecule containing both of the original DNA structures. It is thought that the insertion sequences (IS sequence) present in the F-plasmid and those in the host chromosome serve as regions of homology for the insertional event.

R-plasmids make the host cell resistant to one or more antibiotics. They typically have genes that code for enzymes capable of destroying or modifying antibiotics. Some R-plasmids have only a single antibiotic resistance gene, whereas others have many. Most R-plasmids are mobilizable plasmids.

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