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Mitosis Why do cells divide? (Purpose of Mitosis) • Growth • Repair and replace dead cells • asexual reproduction What cells divide often? • • • • • • • Skin Stomach lining Red Blood cells Embryo Plant roots Hair Nails What cells rarely/never divide? • Nervous System (nerve cells) • Liver (liver cells) Interesting Fact: Why do we age? • Eventually cells stop being replaced • “Apoptosis” – Cell death • “We die because our cells die.” – William R. Clark “C” Terms • Chromosomes – Long threads of genetic material – Found in nucleus • Chromatid – One side of a duplicated chromosome “C” Terms • Centromere – Structures that hold sister chromatids together • NOTE – 2 sister chromatids = 1 duplicated chromosome • DNA is broken into long strands and wrapped around proteins to form structures called chromosomes. “C” Terms… • Centrioles – Small protein bodies – In cytoplasm – Animal cells only – Spindle fibres extend from centrioles. Micrograph of a Chromosome Cell Reproduction • Cells must divide to increase the number of body cells (not egg or sperm) in an organism • There are two parts to it: – mitosis - the division of the nuclear material – cytokinesis - the division of the cytoplasm and organelles Cell Division in a Nutshell • Before: – Chromosome duplicates = 2 sister chromatids (DNA replication ) • During: – Sister chromatids separate • After: – 2 “daughter” cells – Genetically identical Mitosis Phases • • • • • Interphase Prophase Metaphase Anaphase Telophase Stages of Mitosis: Interphase • resting phase (period between divisions) • centrioles are just outside the nuclear membrane • nucleolus is present and DNA appears as fine threads called chromatin • DNA replicates and appears as double chromosomes and centrioles begin to move apart near the end of this phase Interphase membrane Stages of Mitosis: Prophase • nucleolus disappears • centrioles move to opposite sides of nucleus • asters and spindle fibers form around each centriole • nuclear membrane disappears • chromosomes are visible as two sister chromatids held together at the centromere Prophase Spindle fiber membrane Stages of Mitosis: Metaphase • chromosomes line up along equator (middle) of cell • spindle fibers extend from centrioles (poles) to centromeres of the paired chromatids Metaphase Stages of Mitosis: Anaphase • chromatids separate and move toward opposite poles Anaphase Stages of Mitosis: Telophase • chromosomes disappear; chromatin reappears • spindle fibers and asters disappear • nucleolus reappears and nuclear membrane reforms • cell membrane pinches inward to split cytoplasm (cytokinesis) Telophase & Cytokinesis Cytokinesis… Concept Map Mitosis Animation Mitosis in Plants • Mitosis in plants is very similar to animals with 2 exceptions: – plant cells lack centrioles – plant cells form a cell plate between the two daughter cells in telophase. Cytokinesis in Plants Cytokinesis • Why would it occur differently in animal and plant cells? • Plant cells have a rigid cell wall! Cytokinesis Animal Cells • Cell membrane pinches inward • Creates cleavage furrow • Think: – Pull a string around a balloon Plant Cells • Cell Plate forms between two new nuclei • Becomes cell wall What phases do you see? A C B D What phase? Interphase What phase? Prophase What phase? Metaphase What phase? Anaphase What phase? Telophase What phase? Interphase Understanding Concepts 1) Go to page 90 in BIOLOGY 11 BOOK - Answer questions 2, 3, 4, 5, 6, 7, 9, 10 2) Complete Mitosis Review Worksheet