When a prokaryotic cell reproduces, the circular chromosome replicates and the cell divides in a process called binary fission. Replication usually begins at a specific place on the chromosome, called the origin of replication (or simply the origin). In a process that is not well understood, the origins of the two newly replicated chromosomes move away from each other and toward opposite ends of the cell. In at least some prokaryotes, proteins bind near the origins and anchor the new chromosomes to the plasma membrane at opposite ends of the cell. Finally, a new cell wall forms between the two chromosomes, producing two cells, each with an identical copy of the chromosome. Under optimal conditions, some prokaryotic cells divide every 20 minutes. At this rate, a single cell could produce a billion descendants in a mere 10 hours.