Binary fission

Binary fission is a form of asexual reproduction in which one cell divides into two cells of about equal size, each with an identical copy of the genome. It is the usual way bacteria and archaea multiply, and mitochondria and chloroplasts, which descend from bacteria, divide in a similar way. It is simpler than mitosis because a prokaryote typically has a single circular chromosome and no nucleus to take apart.

In a bacterium, DNA replication starts at one site on the chromosome, the origin of replication, and proceeds in both directions around the circle. As the cell lengthens, the two new chromosomes move apart toward opposite ends. A protein called FtsZ, a relative of the tubulin that forms microtubules in eukaryotes, then gathers into a ring at the middle of the cell. This Z ring recruits other proteins that build a partition of new cell wall, the septum, which pinches the cell into two.

The time one round takes is called the generation time, or doubling time. Under ideal laboratory conditions Escherichia coli can divide about every 20 minutes, so a population grows exponentially, while the bacterium that causes tuberculosis needs many hours per division.