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MITOSIS To be answered ----THINK How many cells are you composed of? When an organism grows bigger, do you get more cells or just bigger cells or both? When do your cells divide the fastest? Slowest? Do all cells continuously divide? Why or Why n’t? Are all cells capable of division and replacement? Why would a cell divide? When would a cell divide? ANSWERS How many cells are you composed of?- Many cells When an organism grows bigger, do you get more cells or just bigger cells or both? – More cells and bigger cells also When do your cells divide the fastest? Slowest? - Fastest and slowest Do all cells continuously divide? Why or Why n’t? – YES1. Some cells do continuously divide, such as epithelial (skin) cells, cells in the gastrointestinal tract and blood cells in the bone marrow. 2. The cells that keep on dividing, even though they are not supposed to, are cancer cells. They eat up all your nutrients and form tumors etc. NO- There are some cells that are extremely specialized such as neurons, heart cells, and skeletal muscle cells, which will not divide. Are all cells capable of division and replacement? NO Permanent cells are cells that are incapable of reproduction as adults. This includes brain cells, heart cells, and skeletal muscle cells. Stable cells are cells that multiply only when needed. They spend most of the time in the quiescent G0 phase of the cell cycle, but can be stimulated to enter the cell cycle when needed. Examples include: the liver, the proximal tubules of the kidney, and endocrine glands. labile cells are cells that multiply constantly throughout life. They spend little or no time in the quiescent G0 phase of the cell cycle, but regularly perform cell division. Why does a cell divide? - As cells absorb nutrients and get larger, the volume of the cell increases faster than the surface area. • This means that a cell can no longer absorb nutrients and get rid of wastes fast enough to support its demands (volume) • So what’s a cell to do? – Solution: divide in 2! Surface area for exchange not great enough to support cell’s needs When would a cell divide? • Growth • Repair or Replacement • Cancer Different cells divide at different rates: Most mammalian cells = 12-24 hours Some bacterial cells = 20-30 minutes Yeast cell – 2 hours Amoeba – a few days Human embryo cell – 15-20 minutes Human adult cell – 8 hours to 100 days How does cell division change over a lifetime? All cells die after a certain number of divisions (programmed cell death). At any given time some cells are dividing and some cells are dying. Example: • Childhood = cell division > cell death • Adulthood = cell division = cell death • The Later Years (Aging) = cell division < cell death MITOSIS • Mitosis is the process in which the nucleus divides to form two new nuclei. (OR) • Equal distribution of the 2 sets of DNA amongst the 2 daughter cells. • Mitosis: Division of somatic (body) cells • Mitosis: Asexual Reproduction MITOTIC PHASES 4 Stages: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Interphase occurs before mitosis begins • Chromosomes are copied (# doubles) • Chromosomes appear as threadlike coils (chromatin) at the start, but each chromosome and its copy(sister chromosome) change to sister chromatids at end of this phase Nucleus CELL MEMBRANE Cytoplasm As cell enters mitosis from interphase it has 2 complete sets of chromosomes because of replication in the S phase. Each set must be re-arranged and distributed into the 2 new daughter nuclei. This is mitosis. Prophase Chromosome pair up! 1. Chromosomes thicken and shorten -become visible -2 chromatids joined by a centromere 2. Centrioles move to the opposite sides of the nucleus 3. Spindle fibers form between the poles. 4. Nucleolus disappears 5. Nuclear membrane disintegrate Sister chromatids Centrioles Spindle fibers Prophase Animal Cell Plant Cell Spindle fibers Centrioles Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm METAPHASE • Chromosomes meet in the middle! 1. Chromosomes arrange at equator of cell 2. Become attached to spindle fibres by centromeres Metaphase Animal Cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm Plant Cell Anaphase • Chromosomes get pulled apart 1. Spindle fibres contract pulling chromatids to the opposite poles of the cell Anaphase Animal Cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm Plant Cell Telophase • 1. 2. 3. 4. 5. Now there are two! Chromosomes uncoil Spindle fibres disintegrate Centrioles replicate Nucleur membrane forms Cell divides Telophase Animal Cell Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm Plant Cell Animal Mitosis -- Review Interphase Prophase Metaphase Anaphase Telophase Interphase Plant Mitosis -- Review Interphase Prophase Metaphase Anaphase Telophase Interphase The Guarantee of Mitosis… Why is this so important? The 2 daughter cells formed are identical to each other and identical to the mother cell. 2n = 46 4n = 92 2n = 46 In Mitosis, each daughter cell is exactly the same as the original mother cell. What Happens After Mitosis? The cell returns to interphase Chromosomes uncoil back into chromatin The cycle repeats itself over & over… Bacteria Reproduce via Binary Fission Mitosis is also an ASEXUAL form of reproduction. These are other examples of the uses of mitosis to create new organisms asexually: Propogation of plants by cuttings Runners from plants like strawberries Budding of Yeast