HOw do scientist use resistence to antibiotics to identify bacteria which contain recombinant plasmids?
The plasmid DNA contains an antibiotic resistence gene. If a bacterium has taken up a plasmid, then it will be resistent to the antibiotic. Sometimes the plasmid has two resistence genes to separate antibiotics. The addition site for the added DNA is in the middle of one of these genes. This way, you can tell which bacteria have taken up plasmids and also those which have taken up plasmids which contain the foreign DNA you are cloning. First grow your bacteria on plates with one antibiotic on (the whole gene). Blot the colonies to a plate with the other antibiotic, giving an identical pattern of colonies. The ones you want are those which grow on the first plate, but not on the second. See illustration of this: http://www.science.siu.
