Figure 1. Immunofluorescent staining of dividing human cells in culture. Human osteosarcoma cells (U2OS) were stained with a fluorescent DNA-binding dye (blue), as well as with antibodies against components of microtubules (green) and centrosomes (orange). Cells were photographed at different phases of cell division, as indicated.

Figure 2. (a) The cell division cycle. The durations of individual phases depend on the organism and cell type and are therefore not drawn to scale. Note that M phase progression represents a continuum, except for the metaphase to anaphase transition, which is marked by sister chromatid separation and hence defines the moment of birth of the two daughter cells. (b) A metaphase spindle. This simplified representation illustrates the distribution of three distinct types of spindle microtubules (astral MTs, polar MTs and kinetochore MTs) relative to centrosomes, chromosomes and cell cortex (plasma membrane).

Figure 3. A simplified model of the major controls over entry into, and exit from, mitosis in mammalian cells, highlighting the role of a phosphatase, CDC25, in regulating the onset of mitosis, and that of a ubiquitin ligase, anaphase-promoting complex (APC), in controlling the exit from mitosis. Both CDC25 and APC are targets of checkpoint mechanisms that stop cell cycle progression until conditions are appropriate for cells to divide. CDK, cyclin-dependent kinase. (For further explanation see main text.)