Figure 1. (a) Cells in G1 may decide to exit the cell cycle to enter a quiescent G0 phase or pass the restriction point, progressing through the cell cycle. Cells duplicate their DNA before entering mitosis. (b) Cells in mitosis undergo a series of distinct morphological changes. In prophase, chromatin condenses forming visible chromosomes, centrosomes begin to form the mitotic spindle. During metaphase, chromosomes attach to each pole of the mitotic spindle by connecting their kinetochores to microtubules. In anaphase, the spindle poles begin to move apart separating the chromosomes. Chromosome separation is completed in telophase and cells are separated into two daughters by cytokinesis.

Figure 2. - and -tubulin are the basic subunits of microtubules. -tubulin is bound by stable guanosine triphosphate (GTP), -tubulin by unstable GTP, which can be hydrolysed to GDP. - and -tubulin monomers form a stable heterodimer. Heterodimers make longitudinal contacts forming a protofilament. Thirteen protofilaments make lateral contacts generating a microtubule cylinder. If the plus (+) end of the microtubule is GTP bound (on the -tubulin subunits), additional tubulin dimers can be added, once a dimer is incorporated the GTP on -tubulin is hydrolysed to GDP. If GTP bound tubulin dimer concentration is low, GTP in the microtubule is hydrolysed. GDP microtubule caps are unstable leading to depolymerization of the microtubule, microtubules peel apart to release tubulin subunits.