A cell will first duplicate its chromosome, elongate, and then pinch in the middle creating two genetically identical daughter cells. This relatively simple and rapid process allows for exponential growth of the organism's population. The bacterial cell undergoes division about every 20 minutes under optimal conditions. Therefore in the process of just eight hours one bacterial cell could create over 16 million descendants. During replication of the chromosome, there is no built in repair mechanism, so if mistakes occur during replication they will be passed on to daughter cells.
Some mistakes have no effect on the bacterial cell.
Other mistakes can create fatal errors and the cell will die. However, some mistakes provide new variants genes. If these genes are helpful to the bacterial cell they will be passed on to future generations. An example of this could be the creation of a variant gene that enables the organism to be resistant to certain types of antibiotics. For example, if an error occurred during the replication of the bacterial chromosome one daughter cell could gain a variant gene that now codes for an enzyme that blocks the activity for a specific antibiotic.