This is a vast topic but, In general, cellular signaling molecules transported into the nucleus causes genes to be turned on or off.
Oops. Should be “Cause genes to be ...”
I understand. But how do the signaling molecules know where they are in the body? The human starts as a single cell then keeps dividing. Somewhere, cells start to specialize. How does a cell know to become a liver cell, or nerve cell etc?
Why are the appropriate gene expressions happening where they’re suppose to?
Probably a dumb question but the idea fascinates me to no end.