Using Drug Resistance as a Selection Technique

The solid nutrient medium in the petri dish contains the antibacterial drug ampiciliin This drug kills . coli however, the cells that harbor the plasmid readily grow in the presence of the drug because part of the plasmid contains a gene that codes for a drug resistance factor. A bacterium that has accepted a plasmid is said to have acquired ampidllin resistance. 

Note that plasmids exist as circular dsDNA when they occur in bacteria, that is, when they are replicated and transcribed in viw. When plasmids arc extracted from bacteria they also occur as drcular dsDNA, but they can easily be converted to linear dsDNA in a test tube for the purpose of receiving inserts. Special enzymes called restriction enzymes recognize specific and rarely occurring sequences of base pairs and catalyze the hydrolysis of the dsDNA at a specific point within or near the sequence. In this way, a plasmid consisting of several thousand base pairs can be cut at one site only and converted to a single piece of linear dsDNA, 

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