Chapter 9 - Chapter 9 How Do Prokaryotic and Eukaryotic...

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Chapter 9 How Do Prokaryotic and Eukaryotic Cells Divide? Unicellular organisms use cell division to reproduce. Multicellular organisms also use it for growth and repair Prokaryotic Cell Reproduction Four general cell division events: Signal to initiate division Replication of DNA Segregation of DNA àฏ to the new cells Cytokinesis àฏ cell membrane/ cell wall separates two new cells Prokaryotes divide by binary fission External factors (nutrient concentration, environmental conditions) act as reproductive signals that initiate cell division For many bacteria, abundant food supplies speed up the division cycle Most have just one circular chromosome DNA replicates at center of cell The two DNA molecules may attach to prokaryotic cytoskeleton àฏ assists segregation (either as tracks or actively pulling DNA apart) Cytokinesis separates cell into two by pinching in of plasma membrane Protein fibers from ring around center of cell New cell wall material formed as cell pinches in Eukaryotic Cell Reproduction Eukaryotic cells divide by mitosis or meiosis usually many chromosomes a nucleus must replicate and divide Three steps of eukaryotic cell division: Replication of the DNA
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Mitosis = nuclear division and distribution of DNA into two new nuclei with identical genome Cytokinesis = division of the cytoplasm into two cells Meiosis is specialized cell division for sexual reproduction producing gametes Gametes typically have one-half the original amount of DNA from parent Gametes (egg and sperm) from separate parents join at fertilization , forming zygote How Is Eukaryotic Cell Division Controlled? The eukaryotic cell cycle has two phases: mitosis interphase Interphase is a long period between cell divisions Some cells stay in interphase indefinitely. Others divide regularly or occasionally Interphase has three subphases: G1 (Gap 1) cell growth before the beginning of DNA synthesis. S (synthesis) DNA replication G2 (Gap 2) cell growth between S and mitosis Subphase transitions depend on several Cdks ( cyclin-dependent kinases ) Activated Cdks phosphorylate other proteins (ex: retino-blastoma protein) Cdks are activated by binding to a cyclin Different cyclins help Cdks phosphorylate different target proteins RB (retinoblastoma protein) is key to progressing past “restriction point” When RB becomes inactivated, cell can progress from G1 to S phase Retinoblastoma protein restricts progression into S-phase when active Phosphorylation deactivates RB protein, allowing progression into S-phase past restriction point Proteins such as p21, p53, and RB that normally block the cell cycle are known as tumor suppressors Cyclin-Cdk complexes act as checkpoints .
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Example: If DNA is damaged during G1, p21 protein is made p21 binds to G1 Cdks, preventing their activation Cell cycle stops while DNA is repaired
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