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Mitosis Chapter 9 Cell Division • process by which new identical cells are produced from existing cells – stomach and intestine cells every 2-3 days – skin cells every 20 days – liver cells every 200 days – brain cells reproduce every 30-50 years Chromosomes • short, stringy structures of DNA • carriers of genetic material • usually chromatin: long strands of DNA which are wrapped around histone proteins • just before cell division, chromatin compacts and reorganizes into chromosomes – each chromosome internally duplicated – consists of two identical DNA chains (sister chromatids) attached together at a single point (centromere) Chromosome Structure Mitosis in Animal Cells • outside nucleus – centrosome (microtubule organizing center) – organizes mitotic spindle •contains many fibers each composed of a bundle of microtubules – in animals, contains two centrioles •oriented at right angles to each other •each with 9 triplets of microtubules arranged in a cylinder • Centrosome was also replicated in S-phase, so now two centrosomes Mitosis Phase One: Prophase • longest phase of mitosis • chromatin has condensed (visible in microscope) • nuclear envelope and nucleolus disintegrate • centrioles move to ends of cell (in animals) – form microtubules in star-like arrays called asters • spindle forms – football-shaped, cagelike structure make up of microtubules – in plants, spindle does not need centrioles Pictures of Prophase Prometaphase • centromeres develop two kinetochores – specialized protein complex – 1 over each sister chromatid •physically hook sister chromatids up with specialized microtubules (kinetochore fibers) •connect sisters to opposite poles of mother cell Mitosis Phase Two: Metaphase • spindle fibers attach to chromosomes at kinetochores • chromosomes are lined up across equatorial plane – metaphase plate (looks like glass) shows where mother cell will be divided Mitosis Phase Three: Anaphase • centromeres dissolve • daughter chromosomes separate and are pulled to opposite ends of cell by the shortening of the kinetochore fibers Mitosis Phase Four: Telophase • begins as daughter chromosomes reach opposite ends • chromosomes unwind to become chromatin • spindle breaks down • nucleolus reappears; nuclear envelope forms Cytokinesis • cytoplasm divides to form two separate, identical cells • animal cytokinesis: – A cleavage furrow appears between daughter nuclei – Formed by a contractile ring of actin filaments – Like pulling on a draw string – Eventually pinches mother cell in two Cytokinesis in Animals Cytokinesis in Plants • cell walls do not permit furrowing • begins with formation of a cell plate – many small membrane-bounded vesicles – eventually fuse into one thin vesicle extending across the mother cell – membranes of the cell plate become the plasma membrane between the daughter cells •contents of vesicles become the middle lamella between the two daughter cells •daughter cells later secrete primary cell walls on opposite sides of middle lamella Cytokinesis in Plants Animation of Stages of Mitosis Mitosis takes 30 minutes to 1 ½ hours depending on the type of cell Remember: I P MAT C Results of Mitosis • guarantees genetic continuity with two identical cells • unicellular organisms simply multiply into two organisms; they do not become multicellular organisms • multicellular organisms form tissues, organs, and organ systems as a result of mitosis Identify which of the following examples in the video clip are actually mitosis The End