The prokaryotes (Greek for 'before nucleus' - including Eubacteria and
Archaea) lack a discrete nucleus, and the chromosomes of prokaryotic
cells are not enclosed by a separate membrane.
Most bacteria contain a single, circular chromosome. (There are
exceptions: some bacteria - for example, the genus Streptomyces –
possess linear chromosomes, and Vibrio cholerae, the causative agent
of cholera, has two circular chromosomes.) The chromosome - together
with ribosomes and proteins associated with gene expression - is
located in a region of the cell cytoplasm known as the nucleoid.