556 Chapter 17 • Genetic Regulation of Cell Number: Normal and Cancer Cells that is thought to be secreted in a variety of tissues un-der growth-inhibitory conditions. The TGF-± ligand binds to the TGF-± receptor and activates the receptor’s serine/threonine kinase activity. The ensuing signal-transduction cascade eventually blocks the phosphoryla-tion and inactivation of the Rb protein. Recall, from ear-lier in the chapter, the role that Rb plays in regulating the cell cycle by preventing activation of the E2F tran-scription factor. Blocking Rb inactivation thus keeps E2F expression turned off. Consequently, the ingredients necessary for DNA replication are not present and the cell cycle will progress no farther. APOPTOSIS: POSITIVE EXTRACELLULAR CONTROLS Often, the command for self-destruction comes from a neighboring cell. For example, within the immune sys-tem, only a small percentage of B cells and T cells ma-ture to make functional antibody or receptor protein, re-spectively. The nonfunctional immature B cells and T cells must be eliminated by induced self-destruction; otherwise, the numbers of these unnecessary cells would
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.