Bacteria are prokaryotes, that is they have a single chromosome and lack a nucleus. To pack the chromosome inside the cell the DNA is coiled and supercoiled; a process mediated by the DNA gyrase enzyme system (see Chapter 6). Bacterial ribosomes differ from eukaryotic ones, making them a target for antibacterial therapy. Bacteria also contain accessory DNA in the form of plasmids. For the role of plasmids in antimicrobial resistance see Chapter 7. They may also code for pathogenicity factors.