I am currently studying a textbook that presents the following definition of epigenetics:
Epigenetics is defined as heritable changes in gene expression without changes in the DNA sequence.
The authors then claim the following:
DNA methylation is a stable and inheritable epigenetic mark. This genetically programmed modification is almost exclusively found on the 5' position of the pyrimidine ring of cytosines (5mC) adjacent to a guanine.
Given these two excerpts, it seems somewhat contradictory to define epigenetics as heritable changes in gene expression without changes in the DNA sequence. After all, the DNA sequence is comprised of nucleotides, and it is these nucleotides that are modified during methylation (for instance, the addition of the methyl group on the 5' position of the pyrimidine ring of cytosines adjacent to a guanine), right? So, is it not technically true that the DNA sequence is changed? Or is what they're saying that, although the nucleotide is changed (such as in DNA methylation), the base pairs are unchanged (I'm unsure if this is true, just hypothesising)?
I would greatly appreciate it if people would please take the time to clarify this.