Achemical mutagen that is structurally similar to a nucleotide but has different base-pairing rules is called a

Nucleoside analog

Explanation:

Nucleoside analogs are the mutagenic chemicals that have a structure that is similar to the normal bases of the nucleotide and thus can be incorporated in the DNA during the process of replication, however as it has different base-pairing rules than the normal nucleotide base it can lead to mutations.An example of a nucleoside analog is acyclovir which an analog of guanosine. 

The gens present in the dna in the nucleus of the cell in the body