Chapter 18: The Cell Cycle: Regulation & Division

The cycle is traditionally divided into interphase—encompassing the G1, S, and G2 stages—and the M phase, which includes mitosis and cytokinesis. A central discovery in molecular biology was the identification of maturation promoting factor (MPF), a complex composed of Cdk1 and cyclin B that serves as a master regulator for entering mitosis. Higher eukaryotes employ a diverse family of cyclin-dependent kinases (Cdks) and cyclins, such as the D-type cyclins which integrate external growth factor signals through the Ras/Raf/MEK/ERK pathway to overcome the G1 restriction point. This transition involves the phosphorylation of the Rb tumor suppressor protein, releasing E2F transcription factors to trigger the synthesis of genes necessary for DNA replication. To maintain genomic integrity, the cell utilizes sophisticated checkpoints; for instance, the protein p53 mediates arrest in response to DNA damage by inducing inhibitors like p21, while the spindle assembly checkpoint prevents premature chromosome separation by inhibiting the anaphase-promoting complex (APC/C) until proper kinetochore attachment is achieved. During M phase, mitotic kinases like Aurora and Polo-like kinases drive dramatic structural reorganizations, including chromatin condensation via condensins, nuclear envelope breakdown, and the assembly of the mitotic spindle. Finally, the process concludes with cytokinesis, achieved through a contractile ring in animal cells or the formation of a cell plate in plant cells, ensuring the birth of two distinct daughter cells with complete genetic complements.