One way to fight viral infections is to prevent a virus from replicating its DNA. Without DNA replication, the virus cannot multiply. Some viral drug therapies cause the introduction of fake nucleotides into cells. When the virus uses one of these fake nucleotides in an attempt to replicate its DNA, the fake nucleotide doesn’t work and viral DNA replication is halted. For example, azidothymidine (AZT), a drug used to fight the human immunodeficiency virus (HIV) that causes AIDS, results in the introduction of the following fake thymine-containing nucleotide into cells. Examine the structures of the real nucleotide and the AZT fake nucleotide. Propose a mechanism for how this fake nucleotide might halt DNA replication.

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Thymine CH₂ HC HN CH -CH₂ NH AZT nucleotide CH CH3 Thymine CH₂ HC HN O=L CH -CH₂ CH- OH Actual nucleotide CH3