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Cell Division Why do cells divide? The Importance of Cell Division 1. Healing and Tissue Repair Replacement of dead cells with new ones E.g. peeling dead skin after a sunburn E.g. injuries (cut, pulled muscle, broken bone) Few cells live for the adult lifespan of a multicellular organism Skin cells are replaced quickly while brain cells have a very limited reproductive capacity Aging - cells are dying more quickly than they are replaced aside Scars form as the result of injury to the dermis. The dermis is a layer of skin that is both deep and sensitive. When damage to this layer of skin occurs, the body produces new collagen fibers for the purpose of mending the injury. When the wound is finished healing, a scar is formed. The Importance of Cell Division cont’d 2. Growth Cell division increases the number of cells Allows for an increase in the size of the organism from baby to adult So why can’t cells just grow bigger? As a cell grows, the volume of cytoplasm increases faster than the surface area of the cell If a cell becomes too large, there would not be enough exchange of materials (oxygen, food) through the cell membrane to sustain it Also, the nucleus would be too far from all the other organelles Makes sense right? The Importance of Cell Division cont’d Reproduction of Some Organisms Asexual Reproduction Form of genetic reproduction that only requires one parent Offspring have the exact DNA as the parent E.g. Yeast, Bacteria, and Protists (amoeba) 3. How Cells Divide Cells go through a process called Mitosis Mitosis is part of the cell cycle There are three distinct stages of cell growth and division Interphase Mitosis Cytokinesis The Cell Cycle 1. 2. 3. Interphase Consists of 3 stages: A) Growth 1 (G1) B) Synthesis (S) C) Growth 2 (G2) Mitosis- 4 stages (PMAT) Cytokinesis- the actual splitting of the cytoplasm The Cell Cycle Stage 1 - Interphase Longest duration of cell stages Cell carries out life activities (growth, cellular respiration) Genetic material (DNA) is in very long, thin strands called chromatin. They are invisible. Just before mitosis, the DNA will replicate forming two identical strands. Interphase Cells remain in interphase if: There are not enough nutrients to provide cell growth DNA has not been replicated DNA is damaged Signals form the surrounding cells tell it not to divide DNA may be repaired if it is early in the cell cycle When there is too much damage – cell will be destroyed Stage 2 -Mitosis There are four phases in mitosis Prophase Metaphase Anaphase Telophase 1st: a few more cell parts There are some important features/organelles involved in mitosis: Centrioles-made of microtubules which direct spindle fiber production Spindle fibers- link centrioles to chromatids Chromatids – duplicated chromosomes Centromeres- link sister chromatids Telomeres- ends of chromosomes Prophase The long strands of DNA condense into compact forms that are visible – chromosomes Chromosomes have two identical halves called sister chromatids Nuclear membrane dissolves Metaphase Chromosomes line up in the middle of the cell Spindle fiber joins the centromere to the centriole at the edge of the cell Anaphase Sister chromatids are pulled apart and migrate to opposite sides of the cell. Become daughter chromosomes Telophase Chromosomes reach ends of cell Nuclear membrane reforms around chromosomes Chromosomes begin to uncoil (decondense) Cleavage furrow forms Stage 3 - Cytokinesis Cytoplasm and organelles are divided between the two nuclei Cell membrane pinches off producing two identical cells (daughter cells) Different in plant cells- instead, a cell plate is formed Plant Cell Division Animations http://0.tqn.com/d/biology/1/0/d/_/mitosis.gif http://www.cellsalive.com/mitosis.htm http://www.sumanasinc.com/webcontent/anim ations/content/mitosis.html