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CELLULAR REPRODUCTION – Ch. 9 As the cell grows, its volume increases much more rapidly than the surface area. The cell might have difficulty supplying nutrients and expelling enough waste products. Diffusion over large distances is slow and inefficient. Small cells maintain more efficient transport systems. Cell Cycle – sequence of growth and division (mitosis) Why do cells divide? to grow larger to repair injuries to replace cells that die (skin cells, blood cells) GROWTH - Interphase (this is NOT a phase of Mitosis) – majority of the time – the cell is active and growing chromosomes are not visible chromosomes are duplicated to prepare for cell division MITOSIS (division) – process of cell division in which the cell’s nuclear material divides 4 phases 1. Prophase longest phase chromatin condenses into chromosomes – shaped like an X each part of the X is known as a sister chromatids two small structures called centrioles appear the centrioles are contained in the centrosome spindle fibers (made of microtubules) connect to the centrioles the nuclear membrane disappears centrioles migrate to opposite ends of the cell and spindle forms between them. Many plant cells do not contain centrioles. 2. Metaphase short sister chromatids line up in middle (M) (middle = equator) of the spindle the spindle is football shaped structure connecting the two centrioles. each chromatid attaches to a spindle fiber at end the chromatids are pulled apart 3. Anaphase chromatids pulled to opposite poles (pulled Apart) the phase ends when each set of chromosomes reaches its centriole 4. Telophase (Two cells) spindle fibers disappear each set of chromosomes becomes enclosed in a nuclear membrane new cells begin independent existence CYTOKINESIS – cytoplasm divides and is pinched off to form 2 new cells each new daughter cell begins independent existence the daughter cells are identical with identical chromosomes Plant cells cannot pinch in half because the cell walls are too thick so a cell plate forms plant cells do not have centrioles Cancer – uncontrolled growth and cell division. Crowd out normal cells resulting in loss of tissue function Cancer cells spend less time in interphase so they grow and divide unrestrained. Caused by genetic change or damage Carcinogens – substances that are known to cause cancer Apoptosis – programmed cell death – shrink and shrivel in a controlled process. Stem cells – unspecialized cells that can develop into specialized cells under the right condition. Embryonic stem cells - After fertilization, the resulting mass of cells divides repeatedly until there are about 100–150 cells. These cells have not become specialized. Adult stem cells - Found in various tissues in the body and might be used to maintain and repair the same kind of tissue o Less controversial because the adult stem cells can be obtained with the consent of their donor