By preventing the synthesis of DNA halting cell growth.
Explanation:
Fluorouracil and methotraxate prevent the synthesis of the neucleoside Thymidine thus preventing DNA replication and elongation. Methotraxate has a structure analogous to Folic acid which is important for thymidine synthesis. Thus, it acts as a competitive inhibitor on dihdrofolate reductase an enzyme that is essential for tetrahydrofolate formation, a folic acid derivative.
Fluorouracil acts by inhibiting thymidylate synthase which catalyses an essential step in Thymidine synthesis.
