On any given day we’re likely to read about the new discovery of a gene for… such and such a disease, or trait. So genes do things, but how do they do things, and just what is a gene actually?
Well, it turns out that’s a hard question to answer, and the definition is constantly changing.
A good starting point is knowing that a gene is transcribed. Think of a court stenographer writing down everything everyone says. It’s written verbatim, but in a different form. In a similar way, a gene is transcribed from DNA into RNA, which is basically the same thing but RNA has an extra oxygen molecule on each of it’s nucleotides. Remember DNA is a string of nucleotides, sometimes millions of nucleotides long. From this very very long string of DNA, a relatively short string of RNA is transcribed, sometimes as short as a couple hundred nucleotides. This is happening at many locations on DNA at the same time. The reason for this is that those transcribed regions are genes that are going to make proteins. For that to happen, the RNA acts as a messenger for DNA, so we call it mRNA. The mRNA interacts with the ribosome, which is like a little machine that reads the mRNA and translates that message into a protein. So first the DNA is transcribed into a message that is given over for translation. Going from mRNA to a protein is like taking the stenographers notes and translating them into another language.
Originally, geneticists thought that each gene would produce a single protein. But we now know that each gene can produce many many proteins because sometimes the message is missing pieces. Imagine the stenographer misses a few sentences, on purpose, and the transcription looks familiar, but there are obvious differences. This is completely natural, the proteins are slightly different versions of each other.
That is the most straight forward description of what a gene is, in the physical sense. Scientists also used to think that only genes were transcribed, which is a very small portion of our DNA, something like 2%. But in recent years we have come to appreciate that much of our DNA is transcribed and that many of this RNA is not acting as a messenger to make proteins, but is actually the functional end product. No proteins needed, RNA can do functional work in the cell all by itself. Those regions are also called genes, so the definition has been expanded.
So when we talk about a gene, what we really mean is some piece of DNA that produces a product. That’s the simplest definition, but of course, its not complete. That product could be RNA or a protein, but it should be functional. Uncovering the function of all those gene products is still a difficult task, taking years of study.
When people say “the gene for such-and-such” what they mean is that the product of that gene is involved in some particular biological function. But these days, scientists don’t really describe genes in those terms, partly because every biological function involves so many genes, and their individual actions may not be well described.
