Final answer:
Methotrexate inhibits the enzyme folic acid reductase, thereby preventing the conversion of folic acid to dihydrofolate and tetrahydrofolate, crucial for DNA synthesis and cell division. Trimethoprim also inhibits the conversion of dihydrofolate to tetrahydrofolate specifically in bacteria.
Step-by-step explanation:
The drug that inhibits both steps in the conversion of folic acid to dihydrofolate (DHF) and then to tetrahydrofolate (THF) is methotrexate. This drug functions as a competitive inhibitor by closely resembling the structure of folic acid, which leads to the inhibition of the enzyme folic acid reductase. This blockage prevents the transformation of folic acid into its active coenzyme form, THF, which is crucial for DNA synthesis because it's required for one-carbon transfer reactions. Consequently, cell division is affected as DNA synthesis cannot proceed without the production of THF. Additionally, trimethoprim is another drug that acts specifically at the step converting dihydrofolate to tetrahydrofolate, inhibiting the synthesis of the active form needed for DNA replication in bacteria.