Final answer:
Antiviral drugs like acyclovir, famciclovir, and valacyclovir inhibit nucleic acid biosynthesis in viruses, specifically by causing chain termination during DNA replication. These drugs are activated by viral enzymes and have higher affinity for viral DNA polymerase than host cell DNA polymerase.
Step-by-step explanation:
Antiviral drugs such as acyclovir, famciclovir, and valacyclovir work by inhibiting nucleic acid biosynthesis in viruses. Specifically, acyclovir is a synthetic analog of the nucleoside guanosine, which is activated by a viral enzyme called thymidine kinase. When added to a replicating DNA strand, acyclovir causes chain termination, preventing the virus from replicating its DNA. This drug's specificity for virus-infected cells comes from the need for a viral enzyme to activate it and its higher affinity for viral DNA polymerase compared to host cell DNA polymerase.