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Cell Cycle Regulation [Ch 9] different cells divide at different rates AP Biology 2007-2008 Control 2 checkpoints 1. G1/S DNA replication 2. G2/M Checkpoint separation of sister chromatids If DNA synthesis is correct, mitosis starts sister chromatids centromere single-stranded AP Biology chromosomes double-stranded chromosomes G1/S checkpoint most critical checkpoint Go signal = it divides No signal = exits cycle & switches to G0 AP Biology G0 phase non-dividing state most human cells in G0 phase M Mitosis G2 Gap 2 S Synthesis AP Biology nerve & muscle cells in G0 & never divide G1 Gap 1 G0 Resting Cell Communication how cells know when to divide Cells communicate with signals Signals are proteins that can be . . . activators [go] inhibitors [stop] AP Biology “Go” signals internal signals = “promoting” factors external signals = “growth” factors How do signals work? phosphorylation kinase enzymes AP Biology inactivated Cdk Signal Types cyclin regulatory protein bind to kinase enzyme Cdk cyclin-dependent kinase Enzyme cascade activated Cdk Cdk-cyclin complex triggers passage through different stages of cell cycle AP Biology regulation of cell cycle is so key to life that the genes for these regulatory proteins have been highly conserved through evolution the genes are basically the same in yeast, insects, plants & animals AP Biology Growth factor signals cells tell other cells when to divide growth factor nuclear pore nuclear membrane P P cell division cell surface receptor protein kinase cascade Cdk P P E2F chromosome P APcytoplasm Biology nucleus Example Platelet Derived Growth Factor (PDGF) binding of PDGF to receptors stimulates cell division in connective tissue heal wounds AP Biology Cancer proto-oncogenes growth factor genes become oncogenes (cancer-causing) when mutated if switched “ON” stimulates cell growth tumor-suppressor genes genes inhibit cell division if switched “OFF” can cause cancer cell division is not inhibited like it should be AP Biology Cancer & Cell Growth Cancer = failure to CONTROL cell division How? lose checkpoints gene p53 plays a key role in G1/S restriction point p53 protein halts cell division if it detects damaged DNA stimulates repair enzymes to fix DNA forces cell into G0 resting stage keeps cell in G1 arrest causes apoptosis of damaged cell ALL cancers have to shut down p53 activity AP Biology p53 — master regulator gene NORMAL p53 p53 allows cells with repaired DNA to divide. p53 protein DNA repair enzyme p53 protein Step 1 Step 2 Step 3 DNA damage is caused by heat, radiation, or chemicals. Cell division stops, and p53 triggers enzymes to repair damaged region. p53 triggers the destruction of cells damaged beyond repair. ABNORMAL p53 abnormal p53 protein Step 1 DNA damage is caused by heat, radiation, or AP chemicals. Biology cancer cell Step 2 The p53 protein fails to stop cell division and repair DNA. Cell divides without repair to damaged DNA. Step 3 Damaged cells continue to divide. If other damage accumulates, the cell can turn cancerous.