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Mitosis and meiosis


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Mitosis and meiosis

  1. 1. Why cells divide• Cells divide because: – Demands the cell places on its DNA – Moving enough nutrients and wastes across the cell membrane• Cell division – process forming two “daughter” cell
  2. 2. Cell Division• 1st stage – mitosis (division of the nucleus)• 2nd stage – cytokinesis – division of cytoplasm• Chromosomes – contain DNA and proteins• Before cell division, each chromosome replicates and consists of two “sister” chromatids
  3. 3. Cont.• Each pair of chromatids is attached at the centromere, which will separate during cell division and each new cell will have identical copies of the old cell• Cell cycle: – Cell grows – Prepares for division (replicates DNA) – Divides into two daughter cells – Cycle begins again
  4. 4. Mitosis• 4 phases:• Prophase – chromatin condense into chromosomes, centriole separates and a spindle begins to form, the nuclear envelope breaks down• Metaphase – chromosomes line up across center of cell, connected to spindle fiber (helps separate chromosomes) at centromere
  5. 5. Cont.• Anaphase – mitosis: centromeres that join the sister chromatids split, chromatids separate and become individual chromosomes (two groups)• Telophase – chromosomes disperse into tangle of dense material, nuclear envelope re-forms around chromosome clusters, spindle breaks apart, nucleolus is visible in daughter nucleus.
  6. 6. Cont.• Cytokinesis – division of cytoplasm in the M phase, cell membrane folds into cell pinching cell into two equal parts containing nucleus and cytoplasmic organelles
  7. 7. Cell regulating• Cyclin – (protein) regulates the timing of the cell cycle in eukaryotic cells• Regulation – internal (replication) and external (growth) are controlled by proteins• Cancer – uncontrolled cell growth (disease of the cell cycle)
  8. 8. Meiosis• Homologous – two sets of chromosomes (set from female, set from male), two complete sets of chromosomes and two complete sets of genes• Diploid – “two sets” cell that contains homologous chromosomes (2N=8)• Haploid – “one set” (N=4)
  9. 9. Phases of Meiosis• Meiosis – process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell• Divided into two distinct phases• Meiosis I and II
  10. 10. Meiosis I• Prophase I – each chromosome pairs with its corresponding homologous chromosome to form a tetrad (4 chromatids)• Metaphase I – spindle fibers attach to the chromosomes• Anaphase I – fibers pull the homologous chromosomes toward opposite ends of the cells
  11. 11. Cont.• Telophase I and cytokinesis – nuclear membrane form and the cell separates into two haploid (N) daughter cells, each with half the number of chromosomes as the original cells• Crossing-over – exchange of chromatids which results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles
  12. 12. Meiosis II• Prophase II - two haploid (N) daughter cells, each with half the number of chromosomes as the original cells• Metaphase II – chromosomes line up in a similar way to the metaphase stage of mitosis• Anaphase – sister chromatids separate and move toward opposite ends of the cell
  13. 13. Cont.• Telophase and cytokinesis – Meiosis II results in 4 haploid (N) daughter cells (2 chromosomes each)
  14. 14. Gamete Formation• Males – haploid gamete is the sperm• Females – only one haploid gamete, egg• Comparing mitosis and meiosis: – Mitosis produces 2 genetically identical diploid (2N) daughter cells: allows the body to grow and replace cells – Meiosis produces four genetically different haploid cells: used in sexual reproduction
  15. 15. Mitosis MeiosisAsexual Reproduction Sexual ReproductionProduces two identical cells Produces four different cellsNew cell contains a full set New cells contain half the(diploid) of chromosomes number of chromosomes (haploid)DNA replication once during DNA replication once duringinterphase interphase before Meiosis 1One nuclear division Two nuclear divisionsNo crossing over – no genetic Crossing over during prophasevariation 1 to ensure genetic variationSister chromatids pulled to Homologous chromosomesopposite poles during move to opposite poles duringanaphase resulting in 2 anaphase 1. Centromeres holdidentical cells. The centromere sister chromatids togethersplits ensuring that identical resulting in non-identical cellschromatids are pulled to theopposite polesOccurs in all types of body Occurs only in sex cellscells (egg & sperm)Purpose: cell growth and Purpose: produce sex cellsrepair