Bacteria are unicellular organisms that lack a nuclear membrane. Most bacterial genomes that have been studied consist of a circular chromosome that contains a single double-stranded DNA molecule several million base pairs in length (Figure 7.4). For example, the genome of E. coli has approximately 4.6 million base pairs of DNA. However, some bacteria contain multiple chromosomes. For example, Vibrio cholerae, which causes cholera, has two circular chromosomes, and Rhizobium meliloti has three chromosomes. There are even a few bacteria that have linear chromosomes. Many bacterial chromosomes are organized efficiently. For example, more than 90% of the DNA in E. coli encodes proteins. In contrast, only about 1% of human DNA encodes proteins.