Bacterial plasmids are small, circular, duplex DNA molecules whose natural function is to confer antibiotic resistance to the host cell. Plasmids have several propeies that make them extremely useful as cloning vectors. They exist as single or multiple copies within the bacterium and replicate independently from the bacterial DNA. The precise location of restriction enzyme cleavage sites for inseing the foreign DNA is available. Plasmids are smaller than the host chromosome and are therefore easily separated from the latter, and the desired plasmid-inseed DNA is readily removed by cutting the plasmid with the enzyme specific for the restriction site into which the original piece of DNA was inseed. Phages usually have linear DNA molecules into which foreign DNA can be inseed at several restriction enzyme sites. The chimeric DNA is collected after the phage proceeds through its lytic cycle and produces mature, infective phage paicles. A major advantage of phage vectors is that while plasmids accept DNA pieces about 6-10 kb long, phages can accept DNA fragments 10-20 kb long, a limitation imposed by the amount of DNA that can be packed into the phage head. Larger fragments of DNA can be cloned in cosmids, which combine the best features of plasmids and phages. Cosmids are plasmids that contain the DNA sequences, so-called cos sites, required for packaging lambda DNA into the phage paicle. These vectors grow in the plasmid form in bacteria, but since much of the unnecessary lambda DNA has been removed, more chimeric DNA can be packaged into the paicle head. It is not unusual for cosmids to carry inses of chimeric DNA that are 35-50 kb long. Even larger pieces of DNA can be incorporated into bacterial aificial chromosome (BAC), yeast aificial chromosome (YAC), or E. coli bacteriophage P1-based (PAC) vectors.