Cell Division

The Cell Cycle

The cell cycle consists of interphase and the mitotic (M) phase. Interphase is divided into three sub-phases: G1 (cell growth, organelle duplication, and preparation for DNA synthesis), S phase (DNA replication, producing sister chromatids joined at the centromere), and G2 (further growth, protein synthesis, and preparation for mitosis).

Cells that exit the cycle and stop dividing enter a quiescent state called G0. Many differentiated cells (such as neurons and mature muscle fibers) remain in G0 permanently, while others can re-enter the cycle in response to growth factor signals.

Mitosis (PMAT)

Mitosis is the division of the nucleus into two genetically identical daughter nuclei. It proceeds through four continuous stages commonly remembered by the acronym PMAT:

Prophase - chromatin condenses into visible chromosomes, the mitotic spindle begins to form from centrosomes, and the nuclear envelope starts to break down. Metaphase - chromosomes align at the metaphase plate (cell equator), with kinetochore microtubules attached to both sister chromatids of each chromosome.

Anaphase - cohesin proteins holding sister chromatids together are cleaved by separase, and motor proteins pull the chromatids toward opposite poles. Telophase - chromosomes decondense, nuclear envelopes re-form around each set, and the spindle disassembles. Cytokinesis (division of the cytoplasm) usually overlaps with telophase, forming a cleavage furrow in animal cells or a cell plate in plant cells.

Cell Cycle Checkpoints and Regulation

The cell cycle is governed by internal checkpoints that prevent errors from propagating. Cyclins are regulatory proteins whose concentrations fluctuate throughout the cycle. They bind and activate cyclin-dependent kinases (CDKs), which phosphorylate target proteins to drive cell cycle progression.

The G1 checkpoint (restriction point) assesses cell size, nutrient availability, and DNA integrity. The G2 checkpoint verifies that DNA replication is complete and error-free. The spindle assembly checkpoint (M checkpoint) ensures all kinetochores are properly attached to spindle microtubules before anaphase proceeds.

Tumor suppressors such as p53 and Rb act as brakes on the cell cycle. When p53 detects DNA damage, it can halt the cycle at G1, activate DNA repair, or trigger apoptosis if the damage is irreparable. Loss of tumor suppressor function or gain-of-function mutations in proto-oncogenes (becoming oncogenes) can lead to uncontrolled proliferation and cancer.

Meiosis I and II

Meiosis is the specialized division that produces haploid gametes (sperm and eggs) from diploid parent cells. It consists of two rounds of division: meiosis I (reductional division, separating homologous chromosomes) and meiosis II (equational division, separating sister chromatids, similar to mitosis).

During prophase I, homologous chromosomes pair up (synapsis) and exchange segments through crossing over (recombination), creating new allele combinations. At metaphase I, homologous pairs line up randomly at the metaphase plate - independent assortment - meaning each gamete receives a unique mix of maternal and paternal chromosomes. Together, crossing over, independent assortment, and random fertilization generate the genetic variation essential for natural selection.

Mitosis vs. Meiosis

Both processes involve chromosome segregation, but they differ in purpose, number of divisions, and genetic outcome.

Sources of Genetic Variation

Sexual reproduction generates offspring with novel combinations of alleles through three mechanisms. Crossing over in prophase I shuffles alleles between homologous chromosomes. Independent assortment at metaphase I means each pair of homologs sorts independently, producing 2^n possible gamete combinations (where n is the haploid number - for humans, 2^23 = over 8 million combinations per parent).

Random fertilization further multiplies variation, as any sperm can fuse with any egg. Together, these mechanisms ensure that (with the exception of identical twins) no two individuals produced by sexual reproduction are genetically identical, providing the raw material for evolution by natural selection.