Final answer:
Nitrous acid is the mutagen that removes amine groups from bases in DNA, leading to base substitutions after DNA replication, such as converting cytosine to uracil and adenine to hypoxanthine.
Step-by-step explanation:
The mutagen that acts by removing amine groups from the bases in DNA is nitrous acid. Nitrous acid deaminates nucleotide bases such as cytosine and adenine, resulting in base substitutions after DNA replication. Specifically, deamination by nitrous acid converts cytosine to uracil, which then base pairs with adenine, leading to a GC to AT base pair substitution. Furthermore, it converts adenine to hypoxanthine, which base pairs with cytosine, thus altering the original base pairing.