Binary Fission

Binary fission is the way bacteria and archaea reproduce: a single cell copies its DNA, grows, and splits into two genetically identical daughter cells. It is asexual and can be very fast, with Escherichia coli able to double in about 20 minutes in rich conditions.

Replication begins at a single origin on the circular chromosome and proceeds in both directions, as described in DNA replication and origins of replication. As the copies move to opposite ends of the cell, the cell elongates. A ring of the protein FtsZ, related to tubulin, forms at the midpoint and recruits enzymes that build a new wall, pinching the cell into two.

Doubling each generation gives exponential growth: one cell becomes more than a million in about 7 hours at a 20-minute generation time. This speed explains how infections can spread and how bacteria evolve resistance so quickly, since any mutation that helps can be quickly selected. Growth in nature is much slower than in the laboratory because of limits on nutrients.

Eukaryotic cells divide by mitosis, with a nucleus, many chromosomes and a spindle. Binary fission uses no spindle. The organelles mitochondria and chloroplasts also divide by a fission-like process, a trace of their bacterial ancestry. Bacteria can still exchange genes by conjugation and other routes, so fission does not stop their evolution.

• Mitosis — the eukaryotic counterpart
• DNA Replication — the copying step
• Escherichia coli — the model organism for fission