Question

If the instructions are exactly the same, how does one cell become a brain cell and another a skin cell?

Answer 1

Each cell expresses, or turns on, only a fraction of its genes. The rest of the genes are repressed, or turned off.

Explanation:

The process of turning genes on and off is known as gene regulation. Gene regulation is an important part of normal development. (www.ghr.nlm.nih.gov)
The first way our genes get the big red or green light is through gene transcription. Then a protein called the transcription factor can cover up or reveal the genetic directions, thus determining whether the gene is on or off. (http://science.howstuffworks.com/life/genetic/genes-turned-off-on1.htm)