Most people have heard of genes, but few understand how they work. The textbooks say that the human genome has almost 30,000 genes and that a gene typically is used to create a protein molecule. But in fact it is difficult to speak of a gene as a thing. For example, proteins are usually not simply constructed from a given stretch of DNA that we would call a gene. Instead, proteins are constructed from several different stretches of DNA. In other words, our “genes” are often a collection of smaller segments that are separated in our DNA. These different DNA segments, which are given the uninteresting name of exons for “expressed regions,” are first copied, and then the copies are combined or appended to each other, into one single transcript. The job of combining exons is done by massive protein machines, and a great variety of different combinations are used, depending on what is needed at the moment. To summarize, a gene is often a collection of separate, smaller, DNA regions that can be combined in many different ways to produce different protein molecules. So those roughly 30,000 genes can produce a great many more different proteins. Read more