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The Cell Cycle Why do cells divide? Reproduction Replace damaged cells/Repair Allows organism to grow Density Dependent Inhibition - Space fills, division stops - Smaller cells are more efficient at: - Obtaining O2 H2O Nutrients (glucose amino acids, etc.) - Ridding themselves of CO2 wastes Mathematical Proof: cm2 cm3 Surface Area Volume Compare: 1cm Surface Area = 1x6 = 6 Volume 1x1x1 1 More surface area per unit of volume 3cm Surface Area = 1x6(9) = 54 = 2 Volume 3x3x3 27 1 Less surface area per unit of volume Cell Cycle So cells divide to stay small and efficient. Let’s look at their life cycle: G Phases = intense growth intense activity S Phases = DNA synthesis Chromosome division and separation How long does this all take? Depends on the type of cell! Skin cells – 2 or less hrs Digestive – 6 hrs Nerves – don’t Mitosis: Cell division Interphase: G1, S, G2 Chromosome Changes During Cell Cycle Chromosome Sister Chromatids joined by a Centromere Chromosomes headed for 2 separate daughter cells Chromatin Cell Division Cell Division occurs in two ways: to produce two different kinds of cells for two different functions Mitosis produces genetically identical daughter cells that will take over the job of the parent cell Meiosis produces genetically different cells that will be used in sexual reproduction,,,egg + sperm Mitosis Occurs in 4 (5) stages: Prophase Metaphase Anaphase Telophase (Cytokinesis) Describe changes involving DNA Describes changes involving cytoplasm and the rest of the cell Interphase Cell is full sized Nucleus is intact DNA has already been replicated Centrioles (animal cells only) are prepared to assist in division Animal Cell Plant Cell Centrioles Prophase Animal Cell Chromosomes form Nuclear envelope breaks up Nucleolus breaks up Spindle apparatus forms Centrioles (animal cells only) move to opposite ends of the cell Plant Cell Early Late Metaphase Animal Cell Chromosomes line up in the middle of the cell Centrioles (animal cells only) are as far apart as possible Spindle attaches to centromeres Plant Cell Anaphase Animal Cell Sister chromatids separate Spindle shortens Cytokinesis begins Animal Cells = “pinching” Plant Cells = cell plate forms Plant Cell Telophase Animal Cell Chromosomes have moved to opposite ends of the cell Chromosomes begin to unwind into chromatin Nuclear envelope begins to reform Spindle apparatus dissociates Cytokinesis continues Animal Cells = pinching more Plant Cells = cell plate grows until wall is complete Plant Cell Early Late Cytokinesis Cell will separate throughout anaphase and telophase Daughter cells result; half the size as parent cell at the end of G2 Animal Cell Plant Cell
