Guo-Liang Chew
PhD student
Harvard University

Andy, AK, CA

I've learned how DNA contains instructions for building specific proteins, but what about the next step? In other words, how does the protein control what colour a person's hair will be, or how tall she will be? Also, how does the protein control how the body is constructed? Does it define that my kneebone is connected to my thighbone, or that my heart has four chambers? Thanks!

Guo-Liang Chew
HHMI predoctoral fellow,
PhD student,
Molecular and Cellular Biology,
Harvard University

You’re absolutely right. The DNA in our cells does provide the instructions for how to make the proteins in our body. It’s like a recipe, telling you what to mix together (different amino acids), and more importantly, the order in which you combine them. Note that all cells in our body have the same DNA (with a few exceptions: e.g., red blood cells lose their DNA as they develop). However, the instruction manual of the genome tells you a lot more than just how to build proteins, that is, it tells you when and where to build them. As you can imagine, this information is very important. Of the 3 billion letters that make up the human genome, only 1.5% of them instruct how to build a protein; the rest covers the when and where.

Some proteins have direct effects. For example, the protein that forms your hair (keratin) is black, blond, brown, or red because that’s how its recipe is written. The many proteins that help digest food act directly on food to break it down. However, other proteins act indirectly. For instance, some proteins control when digestive proteins are made, that is, proteins can sense the presence of food, and tell the cells in your intestines to start making proteins (digestive enzymes) that break down this food.

The process of how a body is built—how an organism goes from one cell, to many cells, to having an inside and outside, to having different tissues such as bone and skin, to growing different organs—is a lot more complicated. Furthermore, scientists don’t yet know how it all works.

Complicating things further, the instructions for proteins in the genome are also read and interpreted by other proteins, which may in turn make proteins that read and interpret other parts of the genome. This creates a very complicated cascade of instructions, which gives you an idea of why it takes such a long time for an animal to develop.

08/08/12 12:16