How do some cells become brain cells and others become skin cells, when the dna in all the cells is exactly the same?

It has the ability to turn off genes.

Hope it helps, Good luck! (:

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature