3.5 Cell Cycle, Mitosis, Meiosis
Key Takeaways
- The cell cycle consists of interphase (G1, S, G2) and the mitotic (M) phase; DNA is replicated during S phase, so a 46-chromosome human cell briefly carries 92 chromatids before division.
- Three major checkpoints — G1/S (restriction point), G2/M, and the spindle assembly checkpoint — verify cell size, DNA integrity, and chromosome attachment before division proceeds.
- Mitosis (prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis) produces two genetically identical diploid daughter cells from a single somatic parent.
- Meiosis I separates homologous chromosomes (reductional division), while meiosis II separates sister chromatids; the result is four genetically distinct haploid gametes from one diploid germ-line cell.
- Meiosis generates genetic variation by crossing over in prophase I and independent assortment of homologs in metaphase I, the two mechanisms most often tested on the Praxis.
Why Cells Divide at All
Multicellular organisms divide cells for growth, replacement, and reproduction. Single-celled organisms divide to reproduce. The cell cycle coordinates DNA replication, organelle duplication, and cytoplasmic division so that each daughter cell receives a complete, accurate copy of the genome. When the cycle goes wrong, you get cancer.
The Cell Cycle
The cell cycle has two large phases: interphase (when the cell grows and copies its DNA) and the mitotic (M) phase (when the cell divides).
| Phase | What happens |
|---|---|
| G1 (Gap 1) | Cell grows, makes proteins and organelles; commits to dividing at the G1 checkpoint |
| S (Synthesis) | DNA replication; each chromosome becomes two identical sister chromatids |
| G2 (Gap 2) | More growth; final preparations for mitosis; G2/M checkpoint verifies DNA integrity |
| M | Mitosis + cytokinesis (the actual division) |
| G0 | Non-dividing resting state (mature neurons, most adult cardiomyocytes) |
A human somatic cell enters S phase with 46 chromosomes and exits with 46 chromosomes that each consist of 2 sister chromatids — so 92 DNA molecules are temporarily present.
Checkpoints
Three main checkpoints decide whether the cell cycle advances. Each is a chance to pause, repair, or trigger apoptosis if problems cannot be fixed.
- G1/S (restriction point) — Is the cell big enough? Are nutrients available? Is the DNA undamaged? Most cancer-relevant; loss of the tumor suppressor p53 disables this checkpoint.
- G2/M — Has DNA replicated fully and correctly? Are any DNA breaks repaired?
- Spindle assembly checkpoint (metaphase) — Are all chromosomes attached to spindle microtubules at their kinetochores? Anaphase will not start until every chromosome is correctly attached.
Mitosis
Mitosis divides the nucleus into two genetically identical nuclei. Cytokinesis then splits the cytoplasm. The result is two diploid (2n) daughter cells identical to the parent.
| Phase | Defining event |
|---|---|
| Prophase | Chromatin condenses into visible chromosomes; nucleolus disappears; spindle begins to form |
| Prometaphase | Nuclear envelope breaks down; spindle microtubules attach to kinetochores at centromeres |
| Metaphase | Chromosomes align on the metaphase plate (equator) |
| Anaphase | Sister chromatids separate; each is pulled to an opposite pole |
| Telophase | Chromosomes decondense; two new nuclear envelopes form; spindle disassembles |
| Cytokinesis | Cytoplasm divides — animal cells pinch in via a contractile actin ring; plant cells lay down a cell plate that becomes a new wall |
A helpful Praxis mnemonic for the sequence is "PMAT" (Prophase, Metaphase, Anaphase, Telophase). Add "PMmAT" if you also want to include prometaphase.
Meiosis
Meiosis occurs only in germ-line cells (in animals, the ovaries and testes). It takes one diploid (2n) cell through two divisions to produce four haploid (n) daughter cells, each genetically unique.
Meiosis I — Reductional Division
- Prophase I — Homologous chromosomes pair up (synapsis), forming structures called tetrads or bivalents. Crossing over occurs at chiasmata, swapping segments between homologs. This step is the single largest source of genetic variation in sexually reproducing organisms.
- Metaphase I — Homologous pairs (not individual chromosomes) line up on the metaphase plate. The orientation of each pair is random, producing independent assortment (Mendel's second law). For humans with 23 pairs, this alone generates 2^23 ≈ 8.4 million possible combinations.
- Anaphase I — Homologs separate to opposite poles. Sister chromatids stay together. Chromosome number is halved here — this is why meiosis I is "reductional."
- Telophase I and cytokinesis — Two haploid cells form, each with chromosomes still made of two sister chromatids.
Meiosis II — Equational Division (Mitosis-Like)
Meiosis II proceeds without another round of DNA replication. Sister chromatids separate, exactly as in mitosis. The result is four haploid cells.
Mitosis vs. Meiosis at a Glance
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Daughter cells | 2 | 4 |
| Ploidy of daughters | Diploid (2n) | Haploid (n) |
| Genetic identity | Identical to parent | Genetically unique |
| Crossing over | No | Yes (prophase I) |
| Homolog pairing | No | Yes (synapsis) |
| Where it happens | Somatic cells (growth, repair) | Germ cells (gamete formation) |
| Function | Growth, repair, asexual reproduction | Sexual reproduction, genetic variation |
Sources of Variation in Meiosis
Three mechanisms — all on the Praxis — explain why siblings differ:
- Crossing over in prophase I.
- Independent assortment in metaphase I.
- Random fertilization — any sperm meets any egg.
The first two happen in meiosis; the third happens because of meiosis.
A diploid plant cell has 12 chromosomes. After meiosis I is complete (but before meiosis II), how many chromosomes and chromatids are in each daughter cell?
Independent assortment of homologous chromosomes occurs during which stage of meiosis and produces what biological consequence?