6.5 The Cell Cycle, Mitosis and Cancer

After S phase each chromosome consists of two identical sister chromatids joined at a centromere. DNA content has doubled but chromosome number has not, because chromosome number is conventionally counted by centromeres. Histone-associated DNA condenses and supercoils for movement, temporarily restricting transcription while reducing tangling and breakage.

In prophase chromosomes condense, spindle microtubules form and the nuclear envelope breaks down. In metaphase chromosomes align at the equator with sister kinetochores attached to opposite poles. In anaphase centromeres separate and sister chromatids—now daughter chromosomes—move apart. In telophase chromosomes decondense, spindles disassemble and nuclear envelopes reform.

Cytokinesis is distinct from mitosis. An animal cell uses an actin–myosin contractile ring to form a cleavage furrow. A plant cell cannot pinch through its wall; Golgi-derived vesicles fuse at the centre to form a cell plate and deliver wall materials. Cytoplasm is usually shared approximately equally, but oogenesis produces one large ovum and small polar bodies, while budding yeast produces unequal cells.