Do not encode functions essential to bacterial growth. Vary in size and copy number (<3000 bp to >100,000. Cells can harbour more than one type. Now p followed by capital letters: e. g. pbr322 (bolivar and rodriguez #322 plasmid derivative of cole1) Replicate independently of chromosome: replicon = autonomously replicating dna molecule. Two general mechanisms of replication: theta. Used by most plasmids: rolling circle (rc plasmids) 1st stage circular dsdna to form circular dsdna and. 2nd stage complementary strand synthesis of circular circular ssdna ssdna (cid:1) (cid:1) (cid:1) Unidirectional replication fork returns to starting point. Used only once per round of replication. Step 2: removal of rna primer by dna polymerase i and ligase: sso replicated last, mechanism involves host-encoded proteins only, net result: 2 dsdna plasmids from one. Host range: determined by ori region of plasmid, narrow can only replicate in a particular bacterium and closely related species (e. g. cole1 plasmids, broad replicate in a wide range of hosts.