Every life form's genetic material is packaged in one or more chromosomes. Its genome is the nucleic acid in one complete set of chromosomes, excluding nonessential ones. This chapter is about the composition, size, shape, and number of chromosomes in bacteria, archaea, mitochondria, chloroplasts, and viruses – all but the eukarya. The nucleic acid of chromosomes is double stranded DNA, except for a few single-stranded DNA plasmids and except for some viruses.
Bacteria and Archaea
Bacteria and archaea have two kinds of chromosomes, essential chromosomes, which are required for the survival and normal functioning of the cell, and plasmids, which are not absolutely necessary for survival and reproduction. Most bacteria and archaea whose genomes have been analyzed have only one essential chromosome, and in nondividing (nonreproducing) cells there is one copy of that chromosome. The number of different types of plasmids per cell varies from zero to several, and the number of copies of each plasmid ranges from 1 to ~102, depending on the plasmid. Usually, essential chromosome is simply referred to as chromosome.
In most bacteria the genome is located in an amorphous region, the nucleoid (Figure 4.1), which takes up from a quarter to half the cell's volume. It is not known whether plasmids are also restricted to the nucleoid. The parts of the chromosome where DNA replication begins and ends are attached to the cell's membrane. Unlike the nucleus of a eukaryal cell, a membrane does not bound the nucleoid region.