Final answer:
Methotrexate interferes with the normal intracellular metabolism of folic acid.
Step-by-step explanation:
Methotrexate acts by inhibiting folic acid reductase, an enzyme responsible for the conversion of folic acid to tetrahydrofolic acid. This inhibition prevents the synthesis of coenzymes needed for one-carbon transfer reactions, which are essential for the synthesis of purines and pyrimidines.
Without these coenzymes, DNA synthesis is hindered and cell division cannot occur.