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The Cell Cycle


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The Cell Cycle

  1. 1. The Cell Cycle How Cells Divide and Grow
  2. 2. Humble Beginnings  Every living thing on earth starts as a single cell
  3. 3. All Cells Have a “Life Cycle”• Includes periods of growth, rest & replication• Can be short (minutes) or long (years)• Different cell types have very different cycles
  4. 4. Cells Divide for Different Reasons• To replace old/worn out cells (red blood cells)• To repair injured tissues or combat illness• To reproduce (create another entire organism)
  5. 5. Three Types of Cell Division Binary Fission • Used by prokaryotes (unicellular: bacteria) • Fast & simple: every 20 – 30 min. Mitosis • Used by eukaryotic, somatic cells (body) • Much more complex • Highly regulated Meiosis • Even more complicated (cells divide twice) • Used by eukaryotic germ cells (reproductive)
  6. 6. Binary Fission in Prokaryotes Bacteria only have 1 chromosome • A single strand of DNA • Circular in shape
  7. 7. Binary Fission in Prokaryotes DNA duplicates • Two circular DNA strands move apart • Cell doubles in size
  8. 8. Binary Fission in Prokaryotes DNA moves to opposite ends of cell Cell membrane grows inward Cell wall grows from membrane materials & pinches cell in two
  9. 9. Binary Fission in Prokaryotes Two new cells are formed • Identical genetically (same DNA) • Cytoplasm contents vary
  10. 10. Types of Cell Division Binary Fission • Used by prokaryotes (unicellular: bacteria) • Fast & simple: every 20 – 30 min. Mitosis • Used by eukaryotic, somatic cells (body) • Much more complex • Highly regulated Meiosis • Even more complicated • Used by eukaryotic germ cells (reproductive)
  11. 11. Cell Cycle in Eukaryotes Interphase • Longest stage • 90% of cell’s life • G - no division 0 • 3 checkpoints: • G – gap 1 1 • S - synthesis • G – gap 2 2 Mitosis • 4 stages • Cytokinesis completes the cycle
  12. 12. Interphase – G1 (1st Gap or Growth phase)  Cell Growth • Organelles replicate • Cell volume doubles  Cell does its job • Normal metabolic processes  Longest phase  Some cells stay here permanently  G1 checkpoint • Is cell big enough? • Environment good?
  13. 13. Interphase – S (Synthesis phase)  Chromosome Replication • Loose bundles of chromatin • All DNA in nucleus copied resulting in… • Two identical strands of each chromosome • Sister chromatids joined by centromere
  14. 14. Interphase – G2 (2nd Gap or Growth phase)  More growth • Cell volume increases  G2 Checkpoint • DNA mistakes? • Cell volume OK? • Environment good?  Shortest phase
  15. 15. Mitosis4 Stages: • Prophase • Metaphase • Anaphase • Telophase
  16. 16. Mitosis - Prophase DNA coils & condenses (like thread on a spool) to form distinct chromosomes Nuclear membrane disappears Spindle fibers form out of centrosomes (2 centrioles ea.)
  17. 17. Metaphase Spindle fibers attach to kinetochores on the centromeres Chromosomes line up at the equatorial plane (middle) of the cell Mitotic spindle checkpoint • Are all chromosomes lined up? • Are they all attached to spindle fibers?
  18. 18. Anaphase Spindle fibers shorten, pulling sister chromatids apart Chromatids are drawn to opposite ends (poles) of the cell Centromere connections are cut
  19. 19. Mitotic Spindle Microtubules of the spindle attach to kinetochores on ? chromosomes Sister chromatids are pulled apart by ? spindle fibers Other unattached spindle fibers push against one another to elongate (stretch out) the cell
  20. 20. Telophase Chromosomes reach the poles New nuclear membranes form around chromatids Chromosomes unwind to become thread-like chromatin again Nucleolus forms
  21. 21. Cytokinesis The cytoplasm is evenly divided New cell membrane forms in animal cells New cell wall is built across for plant cells
  22. 22. Cytokinesis in Animal Cells Microfilaments assemble around cell middle Actin& myosin microfilaments form a ring around cell ATP (energy) used to squeeze cell in half
  23. 23. Cytokinesis in Plant Cells Rigid cell wall can’t just be squeezed in half Golgi apparatus sends vesicles filled with cell wall building materials to center Cell plate forms at equatorial plane Cell wall fills in gaps, cleaves cell in two
  24. 24. Interphase – G0 (pre-growth or resting phase) Mitosiscreates two identical daughter cells that go on to enter Interphase • Some enter G 1 and divide again • Some enter G0 - a rest phase where they do their job
  25. 25. Mitosis summarizedEukaryotic(plant &animal)Somatic (non-reproductive)Regulated bycheckpointsPMAT:ProphaseMetaphaseAnaphaseTelophase
  26. 26. Week 11 Lab ReviewMicroscope Lab: onion root tip mitosis
  27. 27. Cell Cycles vary by Type of Cell  Nerve, liver & muscle cells • Most stay in G0 (never divide) • Some divide only after years, if there are injuries • Brain & spinal chord injuries are often permanent because of this Skin, hair, eye, cheek & intestinal lining cells • Exposed to more wear & tear than other cells • Replaced every 1 - 35  Red blood cells (hemoglobin) days • Most rapidly dividing • Formed from bone marrow stem cells cells in humans • Never divide once formed • Die & replaced after ~ 4 months
  28. 28. Cancer - Cell Cycle out of Control Cancer cells • Tumors result from rapid, out of control cells dividing • Bypass normal G1, G2 and/or mitotic spindle checkpoints • Sloppy cell division = big trouble  Lack of quality controls • More mutations • Disorganized/deformed cells • Signal proteins lost/changed • Contact inhibition lost
  29. 29. Your Turn… What is binary fission? What type of cells use this process? Draw and label the cell cycle…include G1, S, G2, M and cytokinesis…what happens at each stage? What are the stages of Mitosis? What disease is associated with unregulated mitosis?