Key Points

The cell cycle is the sequence of events by which a cell duplicates its genome and divides. It consists of two basic phases: Interphase (the preparatory phase) and the M Phase (the actual division phase).

Interphase is divided into three phases: $G_1$ phase (cell growth), S phase (DNA synthesis/replication), and $G_2$ phase (protein synthesis and further growth). Cell growth is continuous, but DNA replication occurs only during the S phase.

During the S phase, the amount of DNA per cell doubles from 2C to 4C. However, the chromosome number remains the same; if it is $2n$ in $G_1$, it remains $2n$ after the S phase.

Some cells in adult animals, like heart cells, exit the $G_1$ phase and enter an inactive stage called the quiescent stage ($G_0$). These cells are metabolically active but do not proliferate unless needed.

The M Phase includes nuclear division (karyokinesis) and cytoplasmic division (cytokinesis). Mitosis is called equational division because the chromosome number of the parent is conserved in the daughter cells.

Mitosis is divided into four stages: Prophase (condensation of chromosomes), Metaphase (alignment at the equatorial plate), Anaphase (separation of sister chromatids), and Telophase (reformation of nuclei).

In metaphase, chromosome condensation is complete, and they align at the metaphase plate. Spindle fibers attach to the kinetochores of the sister chromatids.

Anaphase begins with the simultaneous splitting of the centromere of each chromosome. The sister chromatids then separate and move towards opposite poles.

In animal cells, cytokinesis occurs by the formation of a cleavage furrow. In plant cells, a cell plate forms in the center and grows outward to divide the cytoplasm.

Mitosis is essential for the growth of multicellular organisms, cell repair, and replacement of old cells (e.g., skin, gut lining). It results in the production of diploid daughter cells with identical genetic complements.

Meiosis reduces the chromosome number by half, producing four haploid daughter cells from a single diploid parent cell. It occurs during gametogenesis in sexually reproducing organisms.

Meiosis involves two sequential divisions, Meiosis I and Meiosis II, but only a single cycle of DNA replication. It features the pairing of homologous chromosomes and recombination between them.

Prophase I is a long and complex phase divided into five stages: Leptotene, Zygotene (synapsis), Pachytene (crossing over), Diplotene (chiasmata visible), and Diakinesis (terminalisation of chiasmata).

Synapsis is the pairing of homologous chromosomes during the zygotene stage. Crossing over, the exchange of genetic material between non-sister chromatids, occurs during the pachytene stage and leads to genetic recombination.

Meiosis I is a reductional division where homologous chromosomes separate. Meiosis II is an equational division, similar to mitosis, where sister chromatids separate.

In Anaphase I of meiosis, homologous chromosomes separate, while sister chromatids remain joined at their centromeres. In anaphase of mitosis, centromeres split and sister chromatids separate.

Meiosis maintains a constant chromosome number across generations in sexually reproducing organisms. It also increases genetic variability in a population through recombination, which is important for evolution.