Question

when does gene silencing occur?

Answer 1

You know that all of our cells pretty much have the same DNA.

So here's a question:
If all of our cells have the same DNA, then why do we have so many different kinds of cells? Like skin cells, and liver cells, and bone cells?
If all of our cell DNA is the same, shouldn't all our cells be the same too?

The answer is "gene silencing."

"Gene silencing" is where our cells "turn off" some of the genes, in our DNA,
so that these genes aren't expressed. They don't work.

Our genes are what decides what our cells look like.

Our skin cells and our bone cells are different,
because in each of those cells, different genes are turned on, and different genes are turned off.

Do you get it so far?

So, "gene silencing" happens when our stem cells need to "differentiate."
What that means is that it happens when our stem cells need to become some kind of special cell, like a skin cell, or a bone cell.