system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: All cells in a living organisms share the same DNA but develop into different types of cells. What mechanism tells these cells which type of cell to become? How does it work?
gpt
It all goes back to when you were conceived. The fertilized egg begins to divide. The cells start out undifferentiated, they could turn into anything. Then as the cells work through the genetic instructions they start designating certain cells for performing certain functions, making them "differentiated". Once differentiated those cells basically turn off all the parts of their DNA that doesnt work towards fulfilling their new role. So a muscle cell and a nerve cell may have a copy of the same instruction manual but each is working from different chapters.

As an adult a stem cell divides creating a new undifferentiated cell. That cell then responds to signals being sent out from surrounding cells. Skin for example. It is made of many layers of skin cells which are being produced on the bottom layer,then gradually get pushed up by newer cells and eventually reach the surface, die and flake off.

The stem cell is like your mom or dad, and they just made you and they tell you "you can be anything you want" and then the other skin cells are life and they say "no you're going to be a skin cell", so you become a skin cell.