Transcript Mitosis
Cell Division: Mitosis Chapter 8 Prokaryotic Cell Division • Binary fission: dividing in half • Steps – Chromosomes duplicate and move to ends of cell – Cell elongates – Plasma membrane pinches off into 2 ‘daughter cells’ Chromosomes • 1 chromosome is 1 DNA molecule w/ 100’s to 1000’s of genes • Composed of chromatin – DNA and protein • Number varies within species – – – – – 46 in humans 78 in dogs 38 in cats 254 in shrimp 1200 Adders-tongue (fern) Eukaryotic Cell Division • Chromosomes replicated to form 2 sister chromatids – Joined by centromere • Division splits chromosome apart – Each becomes a chromosome – Count centromeres to determine number The Eukaryotic Cell Cycle (Overview) • Interphase – About 90% of time – Chromosomes not visible yet – 3 Subphases • G1 phase = cell growth and maturation • S phase = synthesis, chromosomes replicated • G2 phase = growth and prep for division • G0 phase = not dividing or quienescent • Mitotic (M Phase) – Actual cell division – Mitosis – nuclear division • • • • Prophase Metaphase Anaphase Telophase – Cytokinesis – cytoplasmic division • Repeat Mitotic Phase Prophase Events • Sister chromatids condense • Nuclear envelope begins to disappear • Centrosomes (centrioles in animal cells) move to opposite ends of cell • Mitotic spindles form Metaphase Events • Centrosomes at opposite ends of cells • Sister chromatids line up with centromere on metaphase plate • Microtubules attached to each chromatid at the centromere Anaphase Events • Sister chromatids separate • Single chromosomes move toward opposite ends of the cell – Microtubule ‘tug of war’ • Cell elongates Telophase Events • Daughter nuclei form • Nuclear envelope reforms • Chromosomes begin to uncoil • Mitosis is complete Cytokinesis • Division of cytoplasm – Begins at the end of telophase (late anaphase too) • Animal cells – Cleavage furrow forms • Microfilaments interact to pinch plasma membrane in 2 • Plant cells – Cell plate forms • Vesicles collect in the middle until fuse with plasma membrane Apoptosis • Programmed cell death • Necessary for normal development – Maintains balance of cell growth – Organism shape/form development – Sunburn peeling • Tightly regulated to maintain balance Cell Division Control • Cyclical control system regulates with ‘checkpoints’ – 3 major ones: G1, G2, and M – Ensures previous processes complete before allowing continuation of division • Possible number of divisions varies – Ends of chromosomes dictate – Skin cells and those of GI tract divide regularly – Liver cells rarely divide • Need growth factors, proteins secreted by the body to stimulate growth – Cells respond to specific ones • Density-dependent inhibition – Cells stop dividing if become crowded – Remove cells and dividing restarts • Anchorage dependence – Must have contact w/ a solid surface Cancer • Cancer is a disease of the cell cycle – Only 1 cell needs to be transformed into a cancer cell – Immune system normally detects then destroys – Undetected can proliferate = tumor • Benign stay in original location – Can be removed by surgery so don’t disrupt organs • Malignant can spread into other areas – Metastasis: cancer cells spread via the circulatory system from original site • Don’t follow normal signaling – – – – No density-dependent inhibition (overlap) No check points Make own growth factors to increase proliferation Unlimited number of divisions • Often named for site of origin – 4 categories Cancerous Cell Growth Categories of Cancer • Carcinomas – External or internal coverings of the body – E.g. skin or intestinal linings • Leukemias and lymphomas – Blood forming tissues – E.g. bone marrow or lymph bodies • Sarcomas – Tissues that support the body – Eg. Bones or muscle Cancer Treatments • Radiation for localized cells – Damages DNA of cancer cells more than normal ones – Lost ability to repair • Chemotherapy for metastatic tumors – Drugs interfere with cell division (normal and abnormal) and cause side effects • Nausea from intestinal cells • Hair loss from hair follicle cells • Infections from immune system cells