Cell Cycle, Mitosis and Meiosis

Short version. The cell cycle alternates four phases — G1, S, G2, M — driven by cyclin-CDK oscillations, gated at four checkpoints (G1/S restriction point, intra-S, G2/M, spindle assembly checkpoint). Mitosis partitions duplicated chromosomes equally into two diploid daughters. Meiosis halves the chromosome number through two divisions, generating genetic diversity in two ways: independent assortment of homologues and crossover recombination between homologues. The recombination machinery is initiated by SPO11-induced double-strand breaks, targeted by the PRDM9 zinc-finger protein, and visualised cytologically as MLH1 foci on the synaptonemal complex. Errors at meiosis I — failure of crossover, premature loss of cohesion, or merotelic spindle attachment — are the source of most aneuploidy in human pregnancy.

The cell cycle: four phases, four control points

A dividing somatic cell progresses through four phases. G1 is the period of growth between mitosis and the next round of DNA synthesis, during which cells decide whether to commit to another division. S phase is the period during which the genome is replicated, producing two sister chromatids per chromosome. G2 is the second growth phase, during which the cell prepares for division. M phase is mitosis itself, divided into prophase, prometaphase, metaphase, anaphase, and telophase, followed by cytokinesis. Cells not actively dividing are said to be in G0 — reversibly quiescent (most adult somatic cells most of the time) or permanently post-mitotic (neurons, mature cardiomyocytes).

The engine of the cell cycle is the cyclin-dependent kinase (CDK) family. CDK activity is controlled in three ways: by association with a cyclin partner (without which CDK is inactive); by phosphorylation of activating and inhibitory residues; and by binding of CDK inhibitors. The cyclins themselves are synthesised and destroyed in regular oscillations: cyclin D in mid-G1, cyclin E at the G1/S boundary, cyclin A through S and G2, and cyclin B at the G2/M boundary. The cyclin/CDK pairs and the substrates they phosphorylate drive each transition forward and erase the substrates of the previous phase.

The genetic logic of this system was worked out in budding yeast (Hartwell 1971 and successors) and fission yeast (Nurse and colleagues, including Nurse 1990); the cyclin component was discovered biochemically in sea urchin and clam embryos by Tim Hunt (Hunt 1989; the original observation, that a protein synthesised after fertilisation was destroyed each cleavage cycle, was reported in 1983). The three shared the 2001 Nobel Prize in Physiology or Medicine. The textbook reference for cell-cycle control is Morgan, The Cell Cycle: Principles of Control (2007).

The four checkpoints

Checkpoints are surveillance systems that pause the cycle until specific conditions are satisfied. There are four classical checkpoints in mammalian cells.

1. G1/S restriction point (RB-E2F). The decision to commit to a division cycle is taken in G1 at the restriction point. Mitogen-driven cyclin D-CDK4/6 activity, followed by cyclin E-CDK2, phosphorylates the retinoblastoma protein (RB), releasing the E2F transcription factors that drive expression of S-phase genes. The CDK inhibitors p21 (CDKN1A), p27 (CDKN1B), and the INK4 family (p16^INK4a) brake this progression in response to stress, antimitogenic signals, and DNA damage. Sherr and Roberts (1999) review the G1 control system in detail. Loss of RB or p16^INK4a is among the most frequent events in human cancer and corresponds to loss of the restriction-point gate.

2. Intra-S checkpoint. Stalled or damaged replication forks activate the ATR kinase, which phosphorylates CHK1 to slow further origin firing, stabilise stalled forks, and prevent entry into mitosis until replication is completed.

3. G2/M DNA-damage checkpoint. Double-strand breaks activate the ATM kinase, which phosphorylates CHK2; both ATR/CHK1 and ATM/CHK2 inhibit the CDC25 phosphatase that activates CDK1, holding cells in G2 until breaks are repaired. The transcription factor p53 (TP53) is a central node: it is stabilised by ATM/ATR-mediated phosphorylation and induces p21 (a CDK inhibitor), apoptotic effectors, and senescence regulators. p53 is the most frequently mutated gene in human cancer, and germline TP53 mutations cause Li-Fraumeni syndrome.

4. Spindle assembly checkpoint (SAC). At metaphase, anaphase is held until every kinetochore has formed a stable, bipolar attachment to the spindle. Unattached kinetochores generate a diffusible "wait anaphase" signal — the mitotic checkpoint complex (BUBR1, BUB3, MAD2, CDC20) — that inhibits the anaphase-promoting complex/cyclosome (APC/C) and so prevents securin destruction. When all kinetochores are attached, the signal extinguishes, the APC/C activates, securin is destroyed, separase is unleashed, cohesin is cleaved, and sisters separate. Musacchio and Salmon (2007) review the SAC mechanism.

Mitosis: equal division

Mitosis is conventionally divided into five stages.

Prophase. Chromosomes condense, the nucleolus disassembles, and the centrosomes migrate to opposite poles, beginning to nucleate microtubules.
Prometaphase. The nuclear envelope breaks down, and microtubules invade the nuclear region. Kinetochores assembled on the centromeres of each sister chromatid capture microtubules; correct (amphitelic) attachment requires that the two sister kinetochores attach to opposite poles. Erroneous attachments — syntelic (both sisters to one pole), monotelic (one sister attached, one not), merotelic (one kinetochore attached to both poles) — are corrected by the Aurora B kinase, which destabilises tensionless attachments.
Metaphase. Chromosomes align at the metaphase plate. The spindle assembly checkpoint holds the cell here until all kinetochores are attached and under tension.
Anaphase. Cohesin is cleaved by separase. Sister chromatids move to opposite poles. Anaphase A is the shortening of kinetochore microtubules; anaphase B is the elongation of the spindle itself.
Telophase. Chromosomes decondense, the nuclear envelope reforms, and the spindle disassembles. Cytokinesis — the physical division of the cytoplasm by the actomyosin contractile ring — partitions the daughter cells.

The result is two diploid daughter cells, each with the same complement of chromosomes as the parent.

Meiosis: reductional and equational division

Meiosis is the cell division that produces haploid gametes (sperm and oocytes) from diploid germline cells. It comprises one round of DNA replication followed by two divisions, meiosis I and meiosis II. Meiosis I (the reductional division) separates homologous chromosomes; meiosis II (the equational division) separates sister chromatids. The net effect is to halve the chromosome number, from 2n=46 to n=23.

Meiosis I is where the special biology happens. After S phase, each chromosome consists of two sister chromatids, paired at the centromere. In leptotene, chromosomes begin to condense. In zygotene, homologous chromosomes pair and align along their length, forming the synaptonemal complex — a tripartite proteinaceous structure with two lateral elements (one per homologue) and a central element. By pachytene, synapsis is complete and recombination is in progress. Diplotene sees the synaptonemal complex disassemble while homologues remain held together at chiasmata — the cytological manifestation of crossovers. In diakinesis the chromosomes condense further. Metaphase I aligns homologues (not sisters) at the spindle equator; anaphase I separates homologues to opposite poles; meiosis II then separates sisters in a mitosis-like division.

The genetic consequence of meiosis I is twofold. Independent assortment: maternal and paternal homologues align at the equator independently of one another, so the 2²³ possible combinations of maternal and paternal contributions are sampled. Recombination: each pair of homologues experiences at least one crossover, and on average two to three crossovers, mixing alleles from the two parental haplotypes. The genetic distance between two loci on the same chromosome — measured in centiMorgans — is essentially the recombination probability between them, the foundation of linkage analysis used in Mendelian inheritance calculation.

SPO11, double-strand breaks, and PRDM9 hotspots

Recombination is initiated, not by accident, but by a programmed double-strand break introduced into one DNA strand by the topoisomerase-VI-like enzyme SPO11. Keeney, Giroux and Kleckner (1997) identified SPO11 as the catalytic core, covalently linked to the broken end via a 5' phosphotyrosyl bond. The break is resected to expose a 3' single-stranded tail, which invades the homologous chromosome to form a displacement loop. Resolution of the resulting recombination intermediates produces either a crossover (an exchange of flanking sequence between homologues) or a non-crossover (a local patch of gene conversion).

SPO11 does not target the genome uniformly. In humans and mice, recombination is concentrated at hotspots a few kilobases wide, separated by recombination "deserts". The DNA-binding specificity of these hotspots is determined by PRDM9, a fast-evolving zinc-finger protein with histone H3 lysine 4/36 methyltransferase activity. Baudat et al. (2010) identified PRDM9 as the locus controlling the fine-scale distribution of recombination in mice and humans; the rapid evolution of PRDM9's DNA-binding zinc fingers explains why hotspot locations differ between closely related species and even between PRDM9 alleles within a population.

The cytological output of meiotic recombination is countable. MLH1, a component of one of the major crossover-resolution pathways, accumulates at maturing crossover sites and can be visualised as discrete foci on the synaptonemal complex of pachytene cells. MLH1 foci correspond approximately one-to-one with crossovers; foci counts are used as a proxy for recombination frequency in research on meiotic regulation and on the impact of age, sex, and genotype.

The origin of non-disjunction

Errors in chromosome segregation produce gametes with too many or too few chromosomes — the substrate of human aneuploidy. The frequency and origin of these errors are striking. Hassold and Hunt (2001) reviewed the molecular cytogenetic evidence that the great majority of human aneuploidies originate at female meiosis I, and that the frequency rises sharply with maternal age — from a few per cent below age 25 to more than 30% above age 40. Two non-exclusive mechanisms are proposed:

Failure of recombination. A homologue pair without a crossover (an "achiasmate" bivalent) lacks the physical chiasma that normally holds homologues together until anaphase I, and is at high risk of mis-segregation. Susceptible chromosomes (notably chromosome 21) and susceptible regions tend to be those with the lowest recombination frequency.
Premature loss of cohesion. Human oocytes are arrested in late prophase I from foetal life, sometimes for decades. Cohesin loaded onto bivalents in foetal life must remain functional until ovulation. Age-related loss of cohesin or its regulators is thought to predispose bivalents to premature separation, leading to mis-segregation at meiosis I or II.

Errors at male meiosis and at meiosis II contribute a smaller fraction of human aneuploidy, and post-zygotic errors give rise to mosaic aneuploidies. The downstream cytogenetic consequences — trisomies 21, 18, 13; sex-chromosome aneuploidies; structural rearrangements arising from mis-repair of meiotic breaks — are the subject of chromosomal abnormalities.

Why this matters for pedigree analysis

Mendel's laws are statements about meiosis. The 50:50 ratio at the heart of segregation reflects the equal probability that a heterozygous parent passes on either allele at any locus. Independent assortment between unlinked loci reflects the random orientation of bivalents at metaphase I. Linkage between nearby loci reflects the finite probability of crossover between them. When a pedigree shows linkage, segregation distortion, or recurrence of an aneuploidy, the underlying biology is meiotic. Educational tools that compute Mendelian ratios — for example, our Mendelian inheritance calculator, Punnett square calculator, or the family-history component of hereditary cancer risk modelling — are surface-level interfaces to the meiotic biology described on this page.

References

Hartwell LH. Genetic control of the cell division cycle in yeast II. Genes controlling DNA replication and its initiation. Journal of Molecular Biology 1971;59:183.
Hunt T. Maturation promoting factor, cyclin and the control of M-phase. Current Opinion in Cell Biology 1989;1:268.
Nurse P. Universal control mechanism regulating onset of M-phase. Nature 1990;344:503.
Keeney S, Giroux CN, Kleckner N. Meiosis-specific DNA double-strand breaks are catalyzed by Spo11, a member of a widely conserved protein family. Cell 1997;88:375.
Sherr CJ, Roberts JM. CDK inhibitors: positive and negative regulators of G1-phase progression. Genes & Development 1999;13:1501.
Hassold T, Hunt P. To err (meiotically) is human: the genesis of human aneuploidy. Nature Reviews Genetics 2001;2:280.
Musacchio A, Salmon ED. The spindle-assembly checkpoint in space and time. Nature Reviews Molecular Cell Biology 2007;8:379.
Baudat F, Buard J, Grey C et al. PRDM9 is a major determinant of meiotic recombination hotspots in humans and mice. Science 2010;327:836.
Morgan DO. The Cell Cycle: Principles of Control. New Science Press / Sinauer 2007.