The genomes of prokaryotes are compact compared with those of eukaryotes, as they lack introns, and the genes tend to be expressed in groups known as operons. The circular chromosome of the bacterium Escherichia coli consists of a DNA molecule approximately 4.6 million nucleotides long.
In addition to the main chromosome, bacteria are also characterised by the presence of extra-chromosomal genetic elements called plasmids. These relatively small circular DNA molecules usually contain genes that are not essential to growth or reproduction.