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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.

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A methylated nucleotide is the same nucleotide, for the purposes of base-pairing events. The methylated base will be paired with its Watson-Crick opposite after replication, for instance (and methylation will even persist after replication).

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The Methylation will change the individual nucleotide(s) and persist after replication. However the order of the base pairs does not change, so the information in the DNA will still remain , and the base pairs remain with their same partners.For an analogy consider a hardcover book as the DNA . Then the author of the book signs the cover. The signature is like the methylated nucleotide. There is now more information on the book cover but the message inside remains the same.

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