How does fluorouracil inhibit pyrimidine synthesis?
Your cells produce thymidine by converting uridine using an enzyme called Thymidylate Synthase. 5-Fluorouracil irreversibly inhibits this enzyme.

Here is the structure of 5-Fluorouracil: 

Here is the mechanism the enzyme uses to make thymidine:


It looks like the enzyme forms a bond to the uridine ring through a sulfur. 5-Fluorouracil probably inhibits the reaction at the next step because fluorine is much more electronegative than the proton it replaced. This makes it harder for the electrons on the double bond in step 2 to attack the folate. Might also inhibit step 3 because the fluorine won't want to give up its electrons to reform the double bond. Because 5-fluorouracil is stuck to the enzyme's active site, the enzyme can't perform its given duty of converting uridine to thymidine. Because you can't use uridine in your DNA, your cells don't have enough thymidine to copy the DNA and replication is slowed. If thymidine supplies are low enough, cells might die.