Plasmids, Extrachromosomal, Proteins, DNA Molecules, Recombinant DNA

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	Plasmids - Plasmids are autonomously replicating, extrachromosomal, covalently closed circular DNA molecules, which are present in prokaryotes. A plasmid can replicate when the chromosome is not in replication mode and when it is synthesizing proteins. As the chromosome and plasmids are not under the control of each other, plasmids are called as autonomously replicating, extrachromosomal elements.
Plasmids originally found in nature have been modified, shortened and reconstructed to enhance their ability either for general purposes or to suit (fulfill) particular experimental designs, literally for using with different types of host cells. An ideal plasmid vector for molecular cloning must have minimum amount of DNA, replicate in a relaxed manner rather than in stringent mode to ensure a good yield of DNA, contain at least two selectable markers and have only single recognition site for restriction enzyme. The last specification permits cleavage of the circle at a unique site for ligation to the insert segment. For maximum convenience in selection, the unique restriction site should be within one of the two selectable marker genes. It cannot interrupt sequences that are essential for plasmid maintenance. Vectors that approach such features have been constructed from naturally occurring DNA molecules using both, classical genetics and recombinant DNA techniques.