Cell and Nuclear Division
The cell cycle, mitosis, meiosis, cytokinesis, chromosome behavior, and how errors in segregation affect chromosome number.
Cell division supports growth, tissue repair, asexual reproduction, and gamete production. Accurate chromosome duplication and segregation are essential because daughter cells must receive the correct genetic information.
The cell cycle
During G1 the cell grows and carries out normal metabolism. DNA is replicated during S phase. During G2 the cell continues to grow and prepares the structures needed for nuclear division. Cyclins and cyclin-dependent kinases help regulate progression through checkpoints.
DNA replication produces two sister chromatids for each chromosome. Sister chromatids are identical DNA copies joined in the centromere region until they separate during division.
Mitosis
- Prophase: chromosomes condense, spindle organization begins, and the nuclear envelope breaks down.
- Metaphase: chromosomes align at the metaphase plate with spindle microtubules attached to kinetochores.
- Anaphase: sister chromatids separate and move toward opposite poles.
- Telophase: chromosomes reach the poles, decondense, and new nuclear envelopes form.
Mitosis maintains chromosome number and, barring mutation, produces daughter nuclei with the same genetic information as the parent nucleus.
Meiosis creates genetic variation
Meiosis contains two nuclear divisions after a single round of DNA replication. In meiosis I homologous chromosomes pair as bivalents, crossing over can occur between non-sister chromatids, and homologous chromosomes segregate. Meiosis II separates sister chromatids.
Stages of meiosis and chromosome segregation
Reference asset tracked internally.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of nuclear divisions | 1 | 2 |
| Typical products | 2 nuclei | 4 nuclei |
| Chromosome number | Maintained | Halved |
| Genetic similarity | Usually genetically very similar | Genetically varied because of crossing over and independent assortment |
| Major role | Growth, repair, asexual reproduction | Gamete/spore production |
Independent assortment and segregation
At metaphase I, each homologous pair can orient independently of other pairs. At anaphase I, homologous chromosomes segregate into different daughter cells. These processes help produce different combinations of maternal and paternal chromosomes in gametes.
Non-disjunction
Non-disjunction is the failure of homologous chromosomes in meiosis I or sister chromatids in meiosis II to separate correctly. It can produce gametes with an extra or missing chromosome. After fertilization, this may result in trisomy or monosomy.
Cytokinesis
Animal cells typically form a cleavage furrow using a contractile ring of actin and myosin. Plant cells build a cell plate from vesicles that fuse at the center of the cell and develop into new membrane and wall material.
Mitotic index
Mitotic index is the fraction of observed cells that are undergoing mitosis. A high value indicates a high proliferation rate. It can be associated with rapidly growing tissues or tumors, but it is not by itself a diagnosis of cancer.