Cell Cycle and Cell Division

A typical mammalian cell cycle lasts about 24 hours.

Karyokinesis is the division of the nucleus, in which duplicated chromosomes are separated into two daughter nuclei. Cytokinesis is the division of the cytoplasm, by which the cell splits into two daughter cells. In mitosis, karyokinesis usually occurs first, followed by cytokinesis.

During interphase, the cell prepares for division. It has three stages:

- G₁ phase: the cell is metabolically active and grows, but does not replicate DNA.
- S phase: DNA replication occurs, so the DNA amount doubles, though chromosome number remains the same.
- G₂ phase: the cell continues to grow and proteins are synthesised in preparation for mitosis.

So, interphase is the period of growth, DNA synthesis, and preparation for cell division.

The G₀ (quiescent) phase is the inactive stage entered by cells that stop dividing. These cells are still metabolically active, but they do not proliferate unless they are called back into division when needed.

Mitosis is called equational division because the chromosome number remains the same in the daughter cells as in the parent cell. The duplicated chromosomes are separated equally, so each daughter cell gets an identical genetic complement with the same number of chromosomes.

The stage in which chromosomes are moved to the spindle equator and aligned on the metaphase plate is metaphase.

The stage in which the centromere splits and the sister chromatids separate is anaphase.

Pairing of homologous chromosomes takes place during zygotene of prophase I in meiosis, when synapsis occurs.

Crossing over between homologous chromosomes takes place during pachytene of prophase I in meiosis.

- Synapsis: the pairing of homologous chromosomes during zygotene of prophase I.
- Bivalent: the pair formed by two synapsed homologous chromosomes; it is also called a tetrad.
- Chiasmata: the X-shaped points visible in diplotene where homologous chromosomes remain attached after crossing over.

These are key events of prophase I of meiosis.