The genome of the bacterium Escherichia coli contains approximately 4.6 million base pairs. This familiar figure usually refers to the laboratory strain K-12 derivative MG1655, whose complete circular chromosome was published in 1997.
Unlike the multiple linear chromosomes found in humans, the main E. coli genome is generally one circular DNA molecule located in the nucleoid region rather than inside a membrane-bound nucleus. The chromosome carries the instructions for essential processes such as metabolism, cell division, DNA repair, and protein production.
The 4.6-million-base-pair estimate is not an immutable number for every E. coli cell. Different strains can have somewhat different chromosome sizes and may carry plasmids, which are smaller, independently replicating DNA molecules. Pathogenic strains can also possess extra genes, including some involved in toxin production or antibiotic resistance.
The reference K-12 MG1655 genome contains about 4,288 annotated protein-coding genes, along with seven ribosomal-RNA operons and 86 transfer-RNA genes. The wrong options represent useful scale comparisons, but 9.2 million is roughly double the standard bacterial-genome estimate and 2.3 million is roughly half of it.