epigenetics

Think of your DNA as a cookbook and epigenetics as the sticky notes and bookmarks placed on top of it. The recipes never change, but the notes decide which dishes get cooked today and which stay closed. Epigenetics is the study of those notes: changes in how genes are switched on or off that can be passed to daughter cells without altering the underlying DNA sequence.

More precisely, epigenetics covers heritable patterns of gene activity that arise from chemical marks on the DNA or on the proteins it wraps around, rather than from changes in the letters of the genetic code itself. Two cells with identical DNA, such as a neuron and a skin cell, behave very differently because different epigenetic marks keep different genes accessible or shut away.

These marks include the addition of methyl groups to DNA, chemical tags on histone proteins, and the repackaging of chromatin into open or closed states. Some marks are stable across many cell divisions; others shift in response to development, age, or environment.

The word can be used loosely, so it helps to be precise. Strictly, epigenetics refers to states that are heritable through cell division; more broadly the term is sometimes stretched to any gene-regulation mechanism that does not change the DNA sequence. Honest scientists keep the two senses apart.