Amitosis

Amitosis (a- + mitosis) is cell proliferation that does not occur by mitosis, the mechanism usually identified as essential for cell division in eukaryotes. Its name, like the word non-fiction, is mostly about what it is not. It does not involve maximal condensation of chromatin into chromosomes, observable by light microscopy as they line up in pairs along the metaphase plate. It does not involve these paired structures being pulled in opposite directions by a mitotic spindle to form daughter cells. Rather, it effects nuclear proliferation without the involvement of chromosomes, unsettling for cell biologists who have come to rely on the mitotic figure as reassurance that chromatin is being equally distributed into daughter cells. The phenomenon of amitosis, even though it is an accepted as occurring in ciliates, continues to meet with skepticism about its role in mammalian cell proliferation, perhaps because it lacks the reassuring iconography of mitosis. Of course the relatively recent discovery of copy number variations (CNV's) in mammalian cells within an organ, significantly challenges the age-old assumption that every cell in an organism must inherit an exact copy of the parental genome to be functional. Rather than CNV's resulting from mitosis gone awry, some of this variation may arise from amitosis, and may be both desirable and necessary. Furthermore, it is well to remember that ciliates possess a mechanism for adjusting copy numbers of individual genes during amitosis of the macronucleus.(Prescott, 1994)