mitosis

The usual process of nuclear division in the somatic cells of eukaryotes. Mitosis is classically divided into four stages. The chromosomes are actually replicated prior to mitosis during the S phase of the cell cycle. During the first stage, prophase, the chromosomes condense and become visible as double strands (each strand being termed a chromatid) and the nuclear envelope breaks down. At the same time the mitotic spindle forms by the polymerization of microtubules and the chromosomes are attached to spindle fibres at their kinetochores. In metaphase the chromosomes align in a central plane perpendicular to the long axis of the spindle. This is termed the metaphase plate. During anaphase the paired chromatids are apparently pulled to opposite poles of the spindle by means of the spindle fibre microtubules attached to the kinetochore, though the actual mechanism for this movement is still controversial. This separation of chromatids is completed during telophase, when they can be regarded as chromosomes proper. The chromosomes now lengthen and become diffuse and new nuclear envelopes form round the two sets of chromosomes. This is usually followed by cell division or cytokinesis in which the cytoplasm is also divided to give two daughter cells. Mitosis ensures that each daughter cell has a diploid set of chromosomes that is identical to that of the parent cell.