Which base does alkylated guanine pair with instead of cytosine?

Alkylated guanine undergoes structural changes that affect its base pairing properties. In normal DNA, guanine typically pairs with cytosine. However, when guanine is alkylated, it can create changes to the hydrogen bonding patterns, causing it to preferentially pair with thymine instead.

This abnormal pairing occurs because the alkylation can modify the functional groups of guanine, which might stabilize the interaction with thymine instead of cytosine, leading to base mispairing. Such mispairing can have implications for DNA replication and repair mechanisms, as it can introduce mutations.

In the context of the given options, thymine is the base that alkylated guanine pairs with instead of cytosine, making it the correct answer. This pairing is significant in discussions of mutagenesis and the consequences of DNA damage often seen with various chemical exposures in oncology.