BIO 121 Lecture 19-20 The Eukaryotic Cell Cycle & Cell Death California State University

The eukaryotic cell cycle consists of interphase (G1,S, and G2 phases) and M phase (cytokinesis and mitosis). The cell cycle controls cell number, cell type (cell cycle exit is required for differentiation and cell cycle regulators have been found to also influence differentiation), and cell quality (if a cell is defective, the cell will either arrest or undergo apoptosis, depending on the extent of the damage or errors). Entry into the cell cycle from G1 can be influenced by mitogens, which initiate signal transduction pathways (like Ras-MAPK) to initiate expression of immediate early genes. One immediate early gene is myc, a transcription factor that causes the expression of G1 cyclin. When G1 cyclin is expressed it can bind to its Cdk, thereby allowing G1-Cdk phosphorylation of critical proteins to move the cell into S phase. One such substrate is Rb, which binds and inhibits E2F. Once Rb is phosphorylated, this activity is inhibited, and it releases E2F. E2F is a transcription factor that causes the expression of many genes important for S phase, including S cyclin. Cyclin-Cdk complexes are critical cell cycle regulators. Cyclins are proteins that bind Cdks (cyclins dependent kinases). Cdks are always present, but only have active kinase activity if they are bound to their appropriate cyclin. Cyclin expression “cycles” throughout the cell cycle; specific cyclins are expressed during specific phases.