There are 20,000 genes in the human genome, but only a small fraction of them are active in any given cell. This video from Nature explains with beautiful clarity the system that activity, turning genes on and off. It's called the epigenome, and it's incredibly important. Now you can understand how it works, too.
Back when DNA sequencing was still in its infancy, scientists thought that having a complete sequence of human DNA would unlock all its secrets. Nope — it turned out to be a lot of more complicated. One of those big complications is the epigenome, which is a map of the activity in any given genome at any given time. Like every other part of your body, your genome is constantly in motion and constantly changing — you can't understand it without watching its behavior in real time.
Today, scientists funded by the National Institutes of Health published results from years of research on mapping the epigenomes of 100 different cell and tissue types. A skin cell is different from a lung cell, which is different from a brain cell, yet all of them started with the same sequences of DNA. Their epigenomes give them their unique identities. In recent years, the epigenome has gotten a lot of attention because habits like smoking and diet can spur epigenetic changes—changes that may even be passed onto your children. There are also connections between the behavior of the epigenome and conditions like Alzheimers.
At last, the epigenome is opening its secrets to us.