A snapshot of the Escherichia coli genome. K-12 MG1655 is the most commonly used laboratory strain of E. coli. The origin and terminus of replication are shown as green and red lines, respectively. Replication proceeds in two directions from the origin, dividing the genome into two regions, or replicores, which are replicated separately. The two outer black circles provide genome reference points in terms of number of base pairs (outer) and minutes (inner). The use of 100 minutes as a genomic yardstick in this chromosome is derived from the approximately 100 minutes it takes to transfer the E. coli chromosome from donor cell to recipient during bacterial conjugation. Yellow and orange markings on the inner circle denote protein-coding genes. Green and red arrows indicate the locations of genes for tRNA and rRNA, respectively.