Chapter 14: Cell Division

The eukaryotic cell cycle is structured into two main periods: interphase, which includes the G1 (first gap), S (DNA synthesis), and G2 (second gap) phases, and the M phase (mitosis and cytokinesis). The progression through these stages is tightly regulated by the cyclic activity of Cyclin-Dependent Kinases (Cdks), such as the crucial Maturation-Promoting Factor (MPF), which is a complex consisting of a catalytic kinase subunit and a regulatory cyclin protein whose concentration rises and falls in a predictable pattern. Cdk activity is balanced by regulatory kinases, like Wee1, and phosphatases, like Cdc25, which control the G2/M transition. The cell cycle is monitored by checkpoints (e.g., the spindle assembly checkpoint, SAC) that use surveillance mechanisms like the ATM and ATR protein kinases to detect DNA damage or replication errors, triggering arrest through inhibitors like p21 or through targeted protein destruction managed by ubiquitin ligase complexes such as SCF and the Anaphase-Promoting Complex (APC). Mitosis, the process ensuring genetically identical daughter cells, involves five stages, beginning with Prophase, marked by chromosome compaction aided by condensin and sister chromatid linkage maintained by cohesin. Following nuclear envelope breakdown, Prometaphase involves the microtubules capturing the kinetochores. In Metaphase, chromosomes align at the equatorial plate. Anaphase begins when the APC-mediated destruction of securin releases separase, which cleaves cohesin, allowing sister chromatids to separate (Anaphase A), while the spindle poles move apart (Anaphase B), driven by microtubule depolymerization and motor proteins. Cytokinesis in animal cells utilizes the contractile ring theory, powered by an actin-myosin II ring. In plant cells, cytokinesis involves the construction of the cell plate via the phragmoplast, a structure that grows outward from the center, often guided by the prior location of the preprophase band. An example of external regulation is the Piezo1 mechanosensitive ion channel, which responds to membrane tension in epithelial cells by stimulating division, a pathway linked to cancer aggressiveness. Meiosis reduces the chromosome number by half across two divisions, ensuring genetic variability. Critical to Prophase I is synapsis—the pairing of homologous chromosomes—and genetic recombination (crossing-over), which holds bivalents together through chiasmata. Meiotic errors, specifically nondisjunction, result in an abnormal chromosome number (aneuploidy), exemplified by trisomy 21 (Down syndrome), which emphasizes the sensitivity of this cellular process.