FIGURE 6-13 Basic components of a plasmid cloning vector that can replicate within an E. coli cell. Plasmid vectors contain a selectable gene, such as amp^r, which encodes the enzyme β-lactamase and confers resistance to ampicillin. Exogenous DNA can be inserted into the bracketed region without disturbing the ability of the plasmid to replicate or express the amp^r gene. Plasmid vectors also contain a replication origin (ORI) sequence where DNA replication can be initiated by host-cell enzymes. Inclusion of a synthetic polylinker containing the recognition sequences for several different restriction enzymes increases the versatility of a plasmid vector. The vector is designed so that each site in the polylinker is unique on the plasmid.