When connected to the cell membrane, they are called episomes.  While not essential to bacterial life they may confer a selective advantage.  One type produces antibiotic proteins that can kill other 
When connected to the cell membrane, they are called episomes.  While not essential to bacterial life they may confer a selective advantage.  One type produces antibiotic proteins that can kill other bacteria and another confers resistance to antibiotics.  These two types are Col and R factors respectively.  They are useful in genetic engineering because they replicate autonomously and produce lots of desired DNA molecules.  For 10 points, name these genetic elements of bacteria that replicate independent of the chromosomes.
When connected to the cell membrane, they are called episomes.  
plasmids