If I understand correctly, as stem cells divide, they become more and more specialized. The very first (fertilized) cell still can divide into every other cell in the body, but as they divide further, each cell can divide only into a smaller subset of different cell types.
However, it seems that (other than gametes) each cell contains the full genome, an identical copy of the entire DNA. So if the DNA does not get altered as cells specialize, how does a cell keep track of which parts of that DNA it is supposed to read - and how does it tell its daughter cells to change their specialization when it divides?