Overview
Cell division allows organisms to grow, repair damaged tissue, and reproduce. There are two fundamentally different kinds of nuclear division. Mitosis copies a cell to make two identical cells and underlies growth and tissue repair. Meiosis occurs only in the formation of eggs and sperm and produces genetically varied cells with half the normal chromosome number. Understanding both processes explains how a single fertilized egg becomes a whole body, and how traits are shuffled from one generation to the next.
The Cell Cycle
Most of a cell’s life is spent in interphase, the growth-and-preparation period, punctuated by the brief division stage.
| Phase | What happens |
|---|---|
| G1 | Cell grows, makes proteins and organelles |
| S | DNA is replicated; each chromosome becomes two sister chromatids |
| G2 | Cell continues to grow and checks the copied DNA |
| M (mitosis) | Nucleus divides |
| Cytokinesis | Cytoplasm splits into two cells |
Checkpoints at the G1/S and G2/M boundaries verify that conditions are right and that DNA is undamaged before the cell commits to dividing. Cells that are not actively dividing rest in a state called G0.
Mitosis
Mitosis is a continuous process divided into named phases for study. A helpful memory aid is PMAT.
- Prophase — Chromatin condenses into visible chromosomes (each already two sister chromatids). The nuclear envelope begins to break down and the mitotic spindle forms.
- Metaphase — Chromosomes line up single-file along the cell’s midline (the metaphase plate). Spindle fibers attach to each centromere.
- Anaphase — Sister chromatids separate and are pulled to opposite poles. Each pole now has a complete set.
- Telophase — Nuclear envelopes re-form around the two sets of chromosomes, which begin to decondense.
Cytokinesis then pinches the cytoplasm in two, producing two genetically identical diploid daughter cells (46 chromosomes each in humans).
Meiosis
Meiosis begins like mitosis, with one round of DNA replication, but is followed by two rounds of division and yields four haploid cells.
Meiosis I — the reduction division
Here the number of chromosomes is halved. Homologous chromosomes (the maternal and paternal copies of each chromosome) pair up. While paired, they may swap matching segments in a process called crossing over, mixing alleles between the two chromosomes. The homologous pairs then line up and separate, so each new cell receives only one chromosome of each pair. The independent, random alignment of different pairs (independent assortment) further scrambles the combinations.
Meiosis II — like mitosis
The two cells from meiosis I divide again, this time separating sister chromatids much as in mitosis. The final product is four genetically unique haploid gametes.
Mitosis vs. Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cells | 2 | 4 |
| Chromosome number | Diploid (unchanged) | Haploid (halved) |
| Genetic result | Identical to parent | Genetically unique |
| Crossing over | No | Yes (meiosis I) |
| Purpose | Growth, repair | Gamete formation |
Nondisjunction and Chromosome Disorders
If chromosomes or chromatids fail to separate correctly, a phenomenon called nondisjunction, a gamete may end up with an extra or missing chromosome. Fertilization then produces an embryo with an abnormal chromosome count. The best-known example is trisomy 21 (Down syndrome), in which cells carry three copies of chromosome 21. Nondisjunction of the sex chromosomes causes conditions such as Turner syndrome (a single X) and Klinefelter syndrome (XXY).
Clinical relevance
Because cancer is fundamentally a disease of uncontrolled cell division, many chemotherapy drugs target the machinery of mitosis: taxanes stabilize spindle microtubules while vinca alkaloids block their assembly, both halting cells in metaphase. Prenatal screening and karyotyping detect nondisjunction disorders such as trisomy 21, and the increased frequency of these conditions with maternal age reflects errors accumulating in eggs arrested midway through meiosis for years. Understanding the cell cycle also explains why rapidly dividing tissues, such as bone marrow and the gut lining, are the first to suffer side effects during cancer treatment.