Chapter 19: Eukaryotic Cell Cycle

Eukaryotic Cell Cycle summary details the molecular mechanisms regulating the eukaryotic cell cycle, the fundamental process by which cells replicate their DNA and divide to produce distinct daughter cells. The text outlines the four primary phases of the cycle—G1, S, G2, and M—and identifies Cyclin-Dependent Kinases (CDKs) paired with stage-specific Cyclin subunits as the master controllers of cell cycle progression. It explores the multilayered regulation of CDK activity, which involves activating and inhibitory phosphorylation by enzymes such as CAK, Wee1, and Cdc25, as well as inhibition by Cyclin Kinase Inhibitors (CKIs) like p21, p27, and INK4 proteins. The summary explains how irreversible transitions between cycle stages are driven by proteolysis via ubiquitin-protein ligases, specifically the SCF complex and the Anaphase-Promoting Complex or Cyclosome (APC/C). Key regulatory transitions are detailed, including the G1 restriction point (START), where growth factors signaling through the Rb-E2F pathway commit cells to DNA replication, and the licensing of replication origins by the ORC and MCM helicase complexes to ensure the genome is replicated exactly once. The narrative describes the G2-M transition, driven by positive feedback loops that activate mitotic CDKs, leading to nuclear envelope breakdown, centrosome separation, and chromosome condensation via condensins. The mechanical events of mitosis are broken down, focusing on spindle assembly, kinetochore attachment, and the initiation of anaphase through the Separase-mediated cleavage of Cohesin following the APC/C-dependent degradation of Securin. The summary also analyzes critical surveillance mechanisms, or checkpoints, such as the DNA damage response involving ATM, ATR, and p53, and the Spindle Assembly Checkpoint (SAC), which prevents aneuploidy by monitoring chromosome attachment tension. Finally, the chapter contrasts mitosis with meiosis, highlighting meiosis-specific processes such as homologous recombination, synapsis, and the specialized roles of proteins like Rec8 and Shugoshin in producing haploid germ cells.