Most eubacteria have DNA that is present in a single large circular chromosome. In addition, there may be numerous much smaller circles, called plasmids. Plasmids usually carry one or a few genes. These often are for specialized functions, such as metabolism of a particular nutrient or antibiotic.
Despite the absence of a nucleus, the chromosome is usually confined to a small region of the cell, called the nucleoid, and is attached to the inner membrane. The bacterial genome is smaller than that of a eukaryote. For example, E. coli has only 4.6 million base pairs of DNA, versus three billion in humans. As in eukaryotes, the DNA is tightly coiled to fit it into the cell. Unlike eukaryotes, however, the DNA is not attached to histone proteins.
Much of what we know about DNA replication has come from study of bacteria, particularly E. coli, and the details of this process are discussed elsewhere in this encyclopedia. Unlike eukaryotic replication, prokaryotic replication begins at a single point, and proceeds around the circle in both directions. The result is two circular chromosomes, which are separated during cell division. Plasmids replicate by a similar process.