Mitotic division (“M phase”) is the
culmination of the eukaryotic cell cycle for somatic cells. Mitotic cell
division is divided into six phases. The first is prophase, which is
characterized by chromosome condensation (the reorganization of the sister
chromatids into compact rod-like structures). Following condensation, assembly
of the mitotic spindle apparatus occurs outside the nucleus between the two
centrosomes which have duplicated and moved apart to the poles of the cell.
The second stage of mitosis is
prometaphase, which is marked by the disintegration of the nuclear envelope. This
is followed by metaphase, where sister chromatids are attached to opposite
spindle poles by microtubules bound to protein complexes called kinetochores. In
animal cells, 10-40 microtubule-binding sites are associated with any one
kinetochore. In yeast, each kinetochore contains only one attachment site. At
this point, the chromosomes are seen to be aligned at the cell’s equator (the
metaphase plate). The sister chromatids are themselves held together by the
protein cohesin.
At anaphase, the sister chromatids
separate to form two daughter chromosomes that are pulled towards opposite
poles of the spindle. Microtubules bound to kinetochores, as well as the
centrosome, are reeled in towards the cell’s periphery by specialized dynein
motor proteins that ‘walk’ towards the minus end of the microtubule but are
held stationary by cargo-binding domains that are anchored to the cell cortex.
The next phase in the cycle is
telophase, the stage at which the daughter chromosomes de-condense at the
spindle poles and a new nuclear envelope is assembled. A contractile ring is
then formed, marking the final stage of the process -- cytokinesis. The
contractile ring is comprised of actin and myosin filaments. The cell thus
differentiates to form two new daughter cells, each with a nucleus containing a
complete and identical set of chromosomes.
