The mitotic cell cycle in dividing cells is very carefully controlled. Some of the stages have been completed for you. At various points during the mitotic cell cycle, checks are made. A cell goes through cell death (apoptosis) if errors occur that cannot be repaired. This makes sure that the daughter cells produced are genetically identical to each other and to the original cell. Drugs have been developed that can inhibit the mitotic cell cycle and cause the cell to carry out apoptosis. These drugs are used in the treatment of cancer. Vincristine and 5-fluorouracil are chemical compounds that act as cell cycle inhibitors and can lead to apoptosis. • Vincristine binds to spindle microtubules and prevents the spindle from carrying out its function. • 5-fluorouracil prevents the synthesis of thymine nucleotides. Vincristine has been used in the treatment of certain types of leukaemia. Fig. 6.1 is a photomicrograph of a blood smear of a person with one form of leukaemia, which affects lymphocytes.