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Transcript
Cell Cycle and Mitosis Cell Division = Nuclear Division + Cytokinesis Mitosis – produces 2 identical daughter cells with same genetic material as parent cell Meiosis – “reduction division”, daughter cells contain half the genetic material of parent cell Chromosome Structure • • • • Chromatin condenses Forms chromosomes 2 sister chromatids Connected by centromere • Each chromatid is one coiled molecule of DNA Chromosome Structure The Cell Cycle Interphase G1+S+G2 M phase Mitosis + Cytokinesis Interphase • G1: growth • S: growth and duplication of DNA – Creates 2 sister chromatids • G2: growth and preparation for division Mitosis: Part One Prophase • Nucleoli disappear • Chromatin condenses • Nuclear envelope breaks down • Mitotic spindle assembles – MTOC’s move to opposite sides of cell – Microtubules form (tubulin) between – MT attach to centromere at kinetochore Mitosis: Part Two Metaphase • Chromosomes distribute along metaphase plate • Ends with MT pulling chromosomes apart (2 chromatids then called chromosomes, each with its own centromere and kinetochore) Anaphase • MT shorten, pulling chromosomes towards opposite sides of cell (tubulin uncoupled at chromosome end) • Other MT (not attached to chromosomes) interact to push MTOC’s apart Telophase • Nuclear envelope develops around each pole (forms 2 nuclei) • Chromosomes disperse into chromatin • Nucleoli reappear Cytokinesis – Animal cells • microfilaments form ring inside membrane between 2 nuclei • MF shorten to pinch off cell into 2 • Groove = cleavage furrow Cytokinesis – Plant cells • Vesicles from Golgi migrate to plane inbetween 2 nuclei • Vesicles fuse to form cell plate • Cell plate becomes membrane for daughter cells • Cell walls develop between membranes Cell Cycle Regulation Why divide? • Surface-to-volume Ratio – Volume increases faster than surface area – At some point, the cell would be unable to exchange enough materials to maintain cell function • Genome-to-volume Ratio – Limited capacity to control cell processes – At some point, cell would exceed ability of genome to regulate activities • Cell must then stop growing or divide Checkpoints • At specific points in cell cycle • Cell evaluates internal & external conditions • Determines whether to continue through cell cycle Cyclin-dependent Kinases (Cdk’s) • Phosphorylate proteins that regulate the cell cycle • Activated by cyclin (regulatory protein) Cyclin • S-cyclin builds up during G1 to trigger DNA replication • M-cyclin builds up during G2 to trigger mitosis • Cdk is recycled, but cyclin is degraded MPF = Mitosis-promoting factor (M-cyclin + Cdk) Growth factors • Receptors on outside of cell membrane for external molecules (growth factors) that stimulate cell division • Ex: for wound healing, growth factors are secreted by damaged cells and platelets to stimulate growth of scar tissue Growth factors, Cdk, & G1 Checkpoint Anchorage Dependence • Most cells only divide when attached to a substrate – Flat external surface – Neighboring cell – Culture dish Density-dependent Inhibition • Cells stop dividing when surrounding cell density reaches a certain maximum Cancer • Uncontrolled cell growth and division • Transformed cell = cell that has become cancerous • Proliferate (divide) without regard to: – – – – Cell cycle checkpoints Density-dependent inhibition Anchorage dependence Other regulatory mechanisms