Figure 1. The stages of mitosis during the normal cell cycle. Cells enter mitosis from a resting state into interphase, the stage in which DNA is replicated. Chromosomes (shown in green) become visible in early prophase and continue to condense through late prophase. During prophase, the spindle is formed by the two centrioles, and in late prophase the nuclear membrane breaks down. Replicated chromosomes align on the equatorial plane during metaphase, sister chromatids disjoin during anaphase, and cell division is completed in telophase, leaving two identical daughter cells that return to interphase.

Figure 2. The regulation of mitosis (M) and the interphase phases of Gap 1 (G₁), Synthesis (S), Gap 2 (G₂) depends on numerous checkpoints. Mitotic cyclin-dependent kinases (Cdks) and their associated cyclins (Cyc) regulate progression from one phase of mitosis to another. Cdk2/CycE control progression from G₁ to S; CycA,B/Cdk1 from S to G₂. APC/C regulates chromosome segregation through degradation of proteins such as CycB that permits cytokinesis to progress. Mitotic checkpoint genes such as BUB and MAD produce proteins that inhibit the activity of APC/C.

Figure 3. Sister chromatids are held together by cohesins to ensure the proper orientation of the chromosomes on the spindle and their separation at anaphase. Release of the cohesin complexes in the arms is accomplished by phosphorylation, while those at the centromeres require the action of separase, which is activated after its inhibitor, securin, is degraded.

Figure 4. The stages of meiosis during gametogenesis. Cells enter prophase I after DNA replication, although the chromosomes appear as single strands. Homologues begin to pair in zygotene, with chiasmata (points of recombination or crossing-over) becoming evident as chromosomes continue to condense during pachytene. Further condensation and separation cause individual chromatids to become distinct during diplotene. A second decondensation occurs in the female during dictyotene that lasts for decades. Recondensation and movement of the chiasmata towards the chromosome ends take place during diakinesis. Paired homologues, each with two chromatids, line up on the equatorial plate during metaphase, and homologues disjoin towards the two poles during anaphase I. Telophase I (not shown) is generally very short and is punctuated by completion of cell division. Chromosome behaviour during the second phase of meiosis is strikingly similar to that in mitosis except that there are only 23 chromosomes present, each composed of two chromatids that disjoin at anaphase II to form haploid gametes.

Figure 5. Meiotic nondisjunction results in trisomy (47 chromosomes) or monosomy (45 chromosomes) after fertilization of the aneuploid gamete with a normal gamete. (a) Normal segregation (disjunction) of one pair of acrocentric chromosomes. One chromosome has satellites on the short arm to distinguish the two members of the pair. These represent a heterozygous marker near the centromeres. (b) Nondisjunction at meiosis I (MI). Note that the marker satellites are heterozygous in the gametes. (c) Nondisjunction at meiosis II (MII). Note that the marker satellites are homozygous in the gametes.