In the early 1970s, a variety of experiments led to the hypothesis that the cell cycle is driven by specific signaling molecules present in the cytoplasm. Experiment 1 Experiment 2 S SS G1G1 G1G1 M MM EXPERIMENT RESULTS When a cell in the S phase was fused with a cell in G 1, the G 1 nucleus immediately entered the S phase—DNA was synthesized. When a cell in the M phase was fused with a cell in G 1, the G 1 nucleus immediately began mitosis—a spindle formed and chromatin condensed, even though the chromosome had not been duplicated.

Cell Cycle Control  Cell cycle control system: a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle.  The cell cycle is regulated at certain checkpoints by both internal and external signals.  A checkpoint is a control point in the cell cycle where stop and go-ahead signals can regulate the cycle.

Cyclins and Cyclin- Dependent Kinases  Cell cycle regulatory molecules are mainly proteins of two types: kinases and cyclins.  Kinases are enzymes that activate or inactivate other proteins by phosphorylating them.  Most cell cycle kinases stay at a constant concentration in the cell.  Cell cycle cyclins have fluctuating concentrations in the cell.  To be active, cell cycle kinases must be attached to a cyclin (cyclin-dependent kinase or Cdk).

MPF as a Cdk

Internal Checkpoints ProteinLocationPurpose Ras cyclinG1 checkpointChecks that cells are big enough to enter the next part of cell cycle. P53G1 checkpointInspects chromosomes for damage. ATM/NibrinS checkpointInspects copied DNA for mistakes. MPFG2 checkpointPhosphorylates proteins, which initiates mitosis. MAD1MetaphaseChecks that spindle fibers are attached to chromatids.

External Checkpoints  Growth factor: protein releases by certain cells that stimulates other cells to divide.  Density-dependent inhibition: crowded cells stop dividing.  Binding of a cell-surface protein to an adjoining cell sends a cell division-inhibiting signal.  Anchorage dependence: cells must be attached to substratum to divide.

Loss of Cell Cycle Controls in Cancer Cells  Cancer cells do not stop dividing when growth factors are depleted.  Hypothesis: cancer cells do not need growth factors in their culture medium to grow.  Hypothesis: cancer cells have an abnormal cell cycle control system.  The underlying basis of the abnormality is almost always a change in one or more genes that alters the function of their protein products, resulting in faulty cell cycle control. HeLa cells (green microtubles and orange Golgi)

 If a cancer cell stops dividing, it occurs at a random point in the cycle.  Cancer cells can go on dividing indefinitely in culture with a continual supply of nutrients.  Cells are said to have undergone transformation.  Cancer cells evade the normal controls that trigger a cell to undergo apoptosis when something is wrong. Lung cancer cell dividing.

Development of Cancer Cancer develops only after a cell experiences ~6 key mutations (“hits”).  Unlimited Growth Turn on growth promoter genes.  Ignore Checkpoints Turn off tumor suppressor genes (p53).  Escape Apoptosis Turn off suicide genes.  Immortality = unlimited divisions Turn on chromosome maintenance genes.  Promotes blood vessel growth Turn on blood vessel growth genes.  Overcome anchor and density dependence Turn off touch-sensor gene. It ’ s like an out of control car!

p53 Mutation  Controls G1 phase; tells cell to kill itself (apoptosis) if DNA damage cannot be repaired.  Prevents development of mutated cells.  Mutation in gene allows cancer cells to continue dividing. p53 is the Cell Cycle Enforcer

DNA damage is caused by heat, radiation, or chemicals. p53 allows cells with repaired DNA to divide. Step 1 DNA damage is caused by heat, radiation, or chemicals. Step 1 Step 2 Damaged cells continue to divide. If other damage accumulates, the cell can turn cancerous. Step 3 p53 triggers the destruction of cells damaged beyond repair. ABNORMAL p53 NORMAL p53 abnormal p53 protein cancer cell Step 3 The p53 protein fails to stop cell division and repair DNA. Cell divides without repair to damaged DNA. Cell division stops, and p53 triggers enzymes to repair damaged region. Step 2 DNA repair enzyme p53 protein p53 protein p53: Master Regulator Gene

How does cancer happen?  Benign tumor: abnormal cells remain at the original site because they have too few genetic and cellular changes to survive at another site.  Malignant tumor: cells whose genetic and cellular changes enable them to spread to new tissues and impair functions of one or more organs. Breast cancer cell