[Additional chromosomes in bacteria: properties and origin].

The review considers papers published over the last 15 years that deal with the presence in cells of some bacterial genera and species of a second chromosome that is smaller than the major one (occasionally, of two additional chromosomes). These additional chromosomes differ from the major one in the set of genes and specific features of replication; however, they carry genes vitally important for the bacterium. The role of these chromosomes and their probable origin from megaplasmids are discussed.