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Cell Cycle Control System Chapter 12.3 Its Time to Play Simon Says!! Molecular Control system • Timing and rate of cell division in different parts of the plant and animal are crucial to normal growth, development, and maintenance. • Frequency varies among different types of cells – Ex: Frequent divisions of skin cells Liver cells reserve as need arises Nerve and muscle cells don’t even divide Cytoplasmic signals • 1970s experiments revealed that molecular signals from the cytoplasm were driving the cell cycle. Cell Cycle Control System A cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle. Other Examples • Take a minute. Can you think of more examples of control systems? Cycle Control System • Checkpoints are critical control points where stop and go ahead signals regulate the cycle. • Some cells have built in stop signals that halt the cycle until another signal overrides and starts cycle. • Checkpoints can register signals from both inside and outside the cell Checkpoints • Three major checkpoints: G1 – the restriction point G2 –MPF facilitates to cell dividing stage M – mitotic division Regulatory Molecules • Protein kinases enzymes that activate or inactivate proteins by phosphorylating them • Cyclins are proteins that cyclically fluctuate their concentration in the cell *determine active or inactive state of kinases. • CdK – cyclin-dependent kinases in varying numbers control all stages of cell cycle Internal Checkpoints • G1 phase – activator CdK will cause cycle to complete itself; whereas lack of cyclins will halt in G0 phase • G2 phase – MPF “ M-phase promoting factor” will trigger passage to mitosis or cell dividing phase. • M phase – kinetochores send signal that delays anaphase until all chromosomes are attached to spindle External Checkpoints Growth factors – protein released by cells that stimulate other cells to divide *mitogen – mitosis promoter PDGF – made by platelets; required for division of fibroblasts to clot (G1 phase) Density dependent inhibition – occurs when crowded cells stop dividing *results from physical contact, and lack of adequate growth factors and nutrients Anchorage dependence – cells must be attached to a substratum such as extracelluar matrix; cytoskeleton attachment Cancer Cells • Cancer cells divide excessively and invade other tissues. • Cancer cell do not respond to most of the body’s control mechanisms. • Cancer cell still grow even when growth factors have been depleted. • Cancer cells stop dividing at random points of the cycle. Cancer Cells Transformation – process that converts a normal cell to a cancer cell Tumors: benign – remains at original site malignant – invades other organs and impairs function Cancer Cells Metastasis- spread of cancer cells to locations distant from original site. *cells can secrete signal molecules that cause blood vessels to grow toward tumor *cells can also separate and travel in blood and lymph to other parts of the body Treatments Radiation – localized tumors or proliferation *targets and damages DNA of cancer cell Chemotherapy – malignant or metastasis *most target and prevent cells from proceeding past metaphase *side effects – exerts same targets on normal cells Ex: intestinal cells - nausea hair follicle cells – hair loss immune cells - increased infections