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Lecture 5

Lecture 5 (Textbook; Cell Division) - February 5, 2013.docx

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McMaster University
Joe Kim

BIO 2C03 2013 Chapter 2 – Cell Division Mitosis Interphase –period of growth and development between cell divisions; synthesize DNA, produce RNA and proteins, biochemical reactions  G1 (gap 1) – cell grows, cell division protein synthesis, ~10 hours o Cells may enter G0 in response to regulatory signals o G0 – non-dividing phase; stable state during which cells maintain constant size; remain indefinitely  G1/S Checkpoint – end of G1; checks if cell has all enzymes necessary for DNA replication  S (DNA synthesis) – chromosomes duplicate; one chromatid  sister chromatids; ~9 hours  G2 (gap 2) – biochemical events for cell division; ~4 hours  G2/M Checkpoint – end of G2; checks if DNA is undamaged M (Mitotic) Phase (mitosis and cytokinesis) – separation of sister chromatids; cell division  Prophase o Chromosomes condense o Chromosomes consist of two chromatids (because of DNA replication in interphase) o Formation of mitotic spindle (organized array of microtubules) out of centrosomes that migrate to opposite sides of the cell and contain centrioles  Prometaphase o Nuclear membrane disintegrates o Spindle microtubules enter nuclear region o Ends of microtubules from centrosomes at both poles anchor to kinetochore of each sister chromatid  Metaphase o Chromosomes become arranged on the metaphase plate o Centrosomes at opposite poles with microtubules radiating out attached to sister chromatids o Spindle assembly checkpoint - ensures that each chromosome is aligned on metaphase plate and attached to spindle fibers from opposite poles  Anaphase o Sister chromatids separate and move toward opposite spindle poles o Molecular motors disassemble tubulin molecules from spindle and generate forces that pull the chromosome toward the spindle pole  Telophase o Chromosomes arrive at opposite poles  Each separated sister chromatid is now considered a single chromosome o Nuclear membrane reforms around each set of chromosomes – produces 2 nuclei inside cell o Chromosomes relax and lengthen  Cytokinesis – cytoplasmic division (sometimes occurs simultaneously with telophase) Meiosis Interphase  G1  S  G2 Meiosis I – reduction phase  Prophase I o Leptotene – chromosomes contract and become visible o Zygotene – chromosomes continue to condense; homologous chromosomes pair up and begin synapsis (bivalent/tetrad) o Pachytene – chromosomes become shorter and thicker; synaptonemal complex develops between homologous chromosomes; crossing over (chromosomes exchange genetic information) o Diplotene – centromere of paired chromosomes move apart; two homologs remain attached at each chiasma (Result of crossing over) o Diakinesis – chromosome condensation; chiasmata move toward ends of chromosomes as the strands slip apart; homologs remain paired at tips o Nuclear membrane breaks down; spindle forms  Metaphase I o Homologous pairs of chromosomes align along the metaphase plate o Microtubules attach to chromosomes  Anaphase I o Homologous pairs separate – pulled to opposite poles  Telophase I o Chromosomes arrive a poles o Cytoplasm divides Interkinesis – nuclear membrane reforms around the chromosomes clustered at each pole; spindle breaks down, chromosomes relax Meiosis II  Prophase II o Chromosomes recondense o Spindle reforms 1 BIO 2C03 2013
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