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Plasmids

	Plasmids - In 1960 Jacob, Schaeffer and Wol1man used the term episome to describe genetic elements which can exist either independently of the bacterial chromosome or integrated into it. The term plasmid was originally used to describe only extra-chromosomal genetic units. In current usage the terms do not have the precise meanings given above, and are sometimes used synonymously. Plasmids are circular, double-stranded molecules of DNA that exist independently of chromosomal DNA in bacterial cells.
They replicate during cell division and are inherited by both daughter cells. Plasmids carry genes which determine a variety of biological functions. They may also carry genes which specify the ability to transfer plasmids from donor to recipient cell by conjugation. Some plasmids may contain as few as three to four genes. The essential functions of the cell are coded by chromosomal genes. The bacterial cell does not require any naturally occurring plasmids for viability or growth. Plasmid functions are additional to the functions of chromosomal genes, and confer selection advantage to, the bacterial cell under varied environmental conditions.
A plasmid may be conjugative or non-Conjugative. A conjugative p1asmids carries genes that promote the transfer of the plasmid from the host cell to a recipient cell by conjugation. It may also promote the transfer of chromosomal and or non-conjugative plasmid DNA through a process called mobilization. A non- conjugative plasmid is unable to promote its own transfer by conjugation. Recipient cells which acquire plasmids during conjugation are termed transconjugants. The molecular weight of conjugative plasmids is an excess of 20 x 106, while non-conjugative plasmids are much smaller. They are apparently not large enough to carry genes coding for plasmid transfer during conjugation
Plasmids are circular dsDNA molecules. In the resting state the DNA is coiled in a right-handed superhelical coil. There is one superhelical turn for every 400-600 base pairs. The twisted conformatation is known as covalently closed circular (ccc) DNA. Cleavage or nicking of one of the two DNA strands results in the release of the twists of the ccc form to give rise to an open circular form

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