PLASMIDS
Many bacteria have accessory DNA molecules in addition to their larger chromosome. These molecules, called plasmids, are extensively used in genetic engineering. In order to be useful in labs, these plasmids need to have an origin of replication (ori), which enables them to replicate within a bacterial cell. They also need to have multiple restriction enzyme sites to enable cutting and pasting of DNA into a plasmid.

Most plasmids have one or two identifiable markers that give a distinct phenotype to the bacterial cell. Examples of such markers include antibiotic resistance (ampR) or expression of an enzyme that catalyzes a reaction that produces a color change (lacZ).