IS3 Cell Cycle

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  • 1. Cell Cycle
  • 2. Questions
    Why are you bigger now than you were born?
    Why do cells divide instead of growing?
    What needs to happen for you to grow?
    Prokaryotes x Eukaryotes:
    How do you think their cells divide?
  • 3. Prokaryotes x Eukaryotes
    Prokaryotes contain 1 single DNA molecule (chromosome)
    They divide by binary fission: DNA duplicates +cell splits in 2
    Eukaryotes have more DNA molecules (chromosomes
    They are enclosed in the nucleus
    A complete set is necessary for the cell to function properly
    Cell division more complex: mitosis
  • 4. What is the purpose of cell division?
    Growth (example: baby  adult)
    Reproduction (asexual reproduction in single celled organisms)
    Replacement of dead/damaged/infected (example: skin/red blood cells/bone cells)
    Gamete formation in multi-cellular organisms (a special cell division process called meiosis)
  • 5. Why are cells small?
    Surface area to volume ratio limits cell size
    Rate of heat production/waste/resource consumption – volume
    Rate or exchange material/energy – surface area
    As cell size increases, the surface area to volume ratio decreases
    Metabolic rates increase faster than the surface area’s ability to exchange nutrients, hence a maximum size is reached.
    Cell size, therefore, remains small
  • 6. Chromosome Structure
    In eukaryotes: made up of DNA and proteins
    At different times, proteins cause the DNA to:
    be spread out like spaghetti in a bowl
    be tightly condensed into the X-shaped (these we can see in the microscope)
    Region where 2 molecules are attached: centromere
    that serves as an attachment point for the spindle fibers during mitosis.
  • 7.
  • 8. How often do cells divide?
    • Some cells divide constantly (e.g. skin) while others rarely or never divide (e.g. neurons)
    • 9. Cycle: Interphase – Mitosis – Interphase – Mitosis
    • 10. Interphase = period between 2 cell divisions. Cell increases in size, but the chromosomes are invisible (long and thin)
  • Mitosis
    Mitosis is the division of the eukaryote nucleus, which goes on throughout life in all parts of the body.
    Organelles can be randomly separated into the daughter cells but chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA.
    Every human cell has the same 46 chromosomes
    Mitosis is usually divided into 4 phases:
    Prophase (P)
    Metaphase (M)
    Anaphase (A)
    Telophase (T)
  • 11.
  • 12. End of Telophase: Cytokinesis
    The organelles get divided up into the 2 daughter cells passively: they go with whichever cell they find themselves in.
    Plant x Animal cells:
    Plants: a new cell wall made of cellulose forms between the 2 new nuclei (cell plate)
    Animals: a ring of actin fibers (microfilaments) forms around the cell equator, pinching the cell in half.
  • 13. Summary of Mitosis
    Chromosomes condense
    Nuclear envelope disappears
    Centrosomes move to opposite sides of the cell
    Spindle fibers form and attach to centromeres on the chromosomes
    Chromosomes are lined up on equator of cell
    Centrioles are at opposite ends of cell
    Centromeres divide: each 2-chromatid chromosome becomes two 1-chromatid chromosomes
    Chromatids pulled to opposite poles by the spindle fibers
    Chromosomes decondense
    Nuclear envelope reappears
    Cytokinesis: the cytoplasm is divided into 2 cells
  • 14.
  • 15. What happens when there is no control of cell division?
    Tumors = result of uncontrolled cell division
    Tumors can occur in any organ or tissue, though are most common after exposure to carcinogens (e.g. tobacco smoke) or in particularly active tissues (e.g. breast, skin)
    Angiogenesis: tumor recruits blood vessels and grows larger
    Metastasis: part of the tumor invades the blood vessel, travels through the blood and starts to forma a tumor in another part of the body
  • 16.
  • 17.
  • 18. Cancer Treatment
    • Most common treatments: surgery, radiation or chemotherapy
    • 19. It is hard to remove all the tumor cells. Tumors often lack sharp boundaries for easy removal, and metastatic tumors can be very small and anywhere in the body.
    • 20. Radiation and chemotherapy are aimed at killing actively dividing cells, but killing all dividing cells is lethal: you must make new blood cells, skin cells, etc. So treatment must be carefully balanced to avoid killing the patient.
    • 21. Chemotherapy also has the problem of natural selection within the tumor. If any of the tumor cells are resistant to the chemical, they will survive and multiply. The cancer seems to have disappeared, but it comes back a few years later in a form that is resistant to chemotherapy. Using multiple drugs can decrease the risk of relapse: it’s hard for a cell to develop resistance to several drugs at the same time.