Most bacteria replicate through a process known as binary fission. A bacterium begins by growing to about twice its normal size and replicating its genetic material, then splits into two identical cells.
Before a bacterial cell divides into two, it must create two identical copies of its DNA and send one copy to each side of the cell. This occurs at the same time that the cell is increasing in volume by also replicating its proteins and other cell structures. Once the DNA has been divided, a special cell wall, known as the equatorial plate, begins to grow partway across the original bacterial cell. The equatorial plate eventually grows all of the way across the cell, dividing it into two.
Binary fission is a very rapid process. When conditions are right, which for most bacterial species means that the temperature is warm and plenty of moisture is available, bacteria can divide once every 20 minutes. This allows bacterial populations to increase exponentially, since each division leads to a doubling in the number of bacteria present.
A few species of bacteria replicate by methods other than binary fission. Some cyanobacteria replicate by a process called budding, in which a small offspring bacterium grows as a bud on a larger, mature bacterium before eventually splitting off on its own.