3.8 The Cell Cycle

3.8 The Cell Cycle

1. The cell cycle has three stages (represented by the small middle ring in the picture below)

   • Interphase
     • Occupies most of the cell cycle and is sometimes known as the resting phase because no division takes place
   • Nuclear Division
     • When the nucleus divides either into two (mitosis) or four (meiosis)
   • Cytokinesis
     • Follows nuclear division and is the process by which the cytoplasm divides to produce two new cells (mitosis) or four new cells (meiosis)

2. Cancer

   • Caused by a growth disorder in cells
     • The result of damage to the genes that regulate mitosis and the cell cycle
       • Leads to uncontrolled growth and division of cells
         • As a consequence to this, a group of abnormal cells forms, this is a tumour and it constantly develops and expands in size
           • A Tumour becomes cancerous if it becomes from BENIGN, to MALIGNANT
   • Benign Tumours - Grow more slowly, are more compact and are less likely to be life-threatening
   • Malignant Tumours - Grow rapidly, are less compact and are more likely to be life-threatening

3. Rate of Mitosis

   • Most cells divide by mitosis, either to increase the size of a tissue during development (growth) or to replace dead and worn out cells (repair)
     • The Rate of mitosis can be affected by the environment of the cell and by growth factors
       • It is also controlled by two types of gene
         • A mutation in one of these genes = uncontrolled mitosis
           • Mutated cell divide = tumour

4. Treatment on Cancer

   • Usually involves killing dividing cells by blocking a part of the cell cycle
     • Cell cycle disrupted = cell division and cancer growth ceases
   • Drugs used to treat cancer (chemotherapy) usually disrupt the cell cycle by ...
     • ... preventing DNA from replicating
     • ... Inhibiting the metaphase stage of mitosis by interfering with spindle formation
     • Drugs are more effective against rapidly dividing cells
       • Cancer cells = fast rate of division = they are damaged to a greater degree than normal cells
   • Problems?
     • These drug also effect normal cells
     • The drugs are more effective against fast dividing cells, this means that normal cells such as hair-producing cells are vulnerable to damage

5. The graph (left) shows the effect of a chemotherapy drug that kills dividing cells. It was given to a cancer patient once every three weeks starting at 0. This graph plots the changes in the number of healthy cells and cancer cell sin a tissue over the treatment period of 12 weeks

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