Binary fission
Binary fission, asexual reproduction by a separation of the body into two new bodies, often referred to as daughter cells. In the process of binary fission, an organism duplicates its genetic material, typically deoxyribonucleic acid (DNA), and then divides into two parts (cytokinesis). Each daughter cell receives one copy of the genetic material.
Binary fission is the primary method of reproduction of prokaryotic organisms. In bacteria, following the initial step of replication of genetic material, the cell grows and elongates. Next, a septum, or dividing partition, forms toward the midpoint of the cell; this is facilitated by a protein known as FtsZ, which curls into a ring to guide the growth of the cell membrane inward, thereby creating the septum. The cytoplasm divides on either side of the septum to produce two daughter cells, which then separate.
In protists, binary fission is often differentiated into types, such as transverse or longitudinal, depending on the axis of cell separation. Regular transverse fission in some organisms, such as tapeworms and scyphostome polyps, is called strobilation. Commonly, this results in a chain, called a strobilus, of the fission products—the proglottids of tapeworms and the ephyrae of scyphozoan jellyfish; each proglottid or ephyra matures in turn and separates from the end of the strobilus. A few metazoan (multicellular) species regularly undergo a body division into several units simultaneously, a process called fragmentation.
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Planarian fission and fragmentation generally represent direct reproduction in which each portion regenerates missing parts to become a complete new animal. Strobilation products, however, are only indirectly reproductive: proglottids are not regenerative but carry and release great numbers of eggs and die; ephyrae do not produce new polyps but mature into sexually reproducing medusae, the larvae of which become polyps.
Binary fission can take place rapidly, particularly in bacteria such as Escherichia coli, which are able to divide roughly every 20 minutes under optimal conditions. When division is repeated again and again, exponential population growth is possible, wherein twice as many cells are produced with each generation. Offspring generally are very similar genetically to the parent, but every now and then, during DNA replication, mutations occur; such spontaneous mutations are an important driver of genetic variation within populations.