The insertion of F creates an Hfr cell. Hypothetical markers 1 and 2 are shown on F to depict the direction of insertion. The origin (O) is the mobilization point where insertion into the E. coli chromosome occurs; the pairing region is homologous with a region on the E. coli chromosome; a through d are representative genes in the E. coli chromosome. Pairing regions (hatched) are identical in plasmid and chromosome. They are derived from mobile elements called insertion sequences (see Chapter 15). In this example, the Hfr cell created by the insertion of F would transfer its genes in the order a, d, c, b.