binary fission
/ BY-nuh-ree FISH-un /
Bacteria do not bother with the elaborate machinery of nuclei, spindles, and a multi-stage cycle that complex cells use to divide. They have a simpler problem and a simpler solution: copy the single loop of DNA, and split straight down the middle. Binary fission is the fast, straightforward way bacteria and other simple (prokaryotic) cells reproduce by splitting one cell into two.
In binary fission, the cell's single circular chromosome is copied, starting from one point and running both ways around the loop. As copying proceeds, the two resulting DNA loops are actively moved to opposite ends of the cell. The cell then grows longer and a ring of protein (made of a tubulin relative called FtsZ) contracts at the middle, pulling the membrane inward and, in bacteria, building a new wall across the gap until the cell splits into two complete daughter cells. There is no nucleus to dismantle and no mitotic spindle, which is why the whole process is so quick.
Binary fission matters because it lets bacteria multiply astonishingly fast — under good conditions some divide every 20 minutes, so a single cell can become billions within a day, which is why infections flare and food spoils so quickly. But there is an important caveat: because each daughter is a genetic copy of the parent, binary fission alone produces little variety. Bacteria gain genetic novelty through mutation and by swapping genes sideways (horizontal gene transfer), not through the parent-mixing that sexual reproduction provides.
A single E. coli bacterium dividing every 20 minutes would, in theory, become over a billion cells in about 10 hours — which is why a tiny contamination can spoil food overnight or cause an infection to take hold fast.
Dividing every 20 minutes, one bacterium becomes a billion in hours.
Binary fission is not the same as mitosis: prokaryotes have no nucleus and no mitotic spindle, so calling bacterial division 'mitosis' is a common mistake — they separate their DNA by a different, simpler mechanism.