Genetics and Epigenetics — Nature's Pen-and-Pencil Set

The sequence of the four nucleotides of the genetic code is like an indelible ink that, with rare exceptions, is faithfully transcribed from cell to cell and from generation to generation. But on top of this code lies another one, literally “epigenetic,” which is represented by methyl groups added to the DNA base cytosine, as well as covalent changes in histone proteins around which the DNA is coiled. This epigenetic information is more like a code written in pencil in the margins around the DNA. Although the genome largely distinguishes one person from another, the epigenome, or epigenetic information, distinguishes . . .