What is Junk DNA?

Paulo Amaral, from the Gurdon Institute in Cambridge, explains what is junk DNA...

Paulo - Junk DNA is a term that appeared in the 1970s to describe the part of our DNA that does not have any information to make proteins. To understand why it has been termed junk DNA, we have to briefly consider how we found out how the functions of the main biological molecules. Proteins are very important molecules that make up most of the enzymes and building blocks in our body, including our skin and hair, and have been the focus of biochemistry for over a century. Since the 1950s and '60s, we recognise that DNA is the genetic material inherited from our parents and that it has information to make proteins. We discovered these based on the study of bacteria which are relatively simple, easy to grow in the lab, and in which most of the DNA is used to make proteins. However, there are many fundamental differences between bacteria and us. For example, it turns out that humans and other complex organisms have DNAs that are very big. In fact, millions of times bigger than that of bacteria. This was a surprise to many. Most of our DNA does not encode proteins. About 98 per cent is non-coding and only 2 per cent has information to make proteins. At the time of this discovery, it seemed that all this extra DNA was disorganised and repetitive and that some consider it to be junk. This has stuck for a long time. After decades of research, we yet don't know what proportion of our DNA has no function at all. If you consider that original concept of junk DNA, we could simply not survive without it. We now know that some of this DNA is actually very important. For example, for embryonic development and that many diseases including cancers are caused by small defects in the non-coding DNA. This is now a very active area of research and we are just beginning to understand the functions of non-coding DNA. For example, some of these non-coding DNAs act as switches that control when genes should be turned on or off in different parts of the body. Others make several thousands of molecules called RNAs which have a vast array of functions on their own and are not used to make proteins. Interestingly, the part of our DNA that makes proteins are essentially the same in all animals. What changes the non-coding DNA which roughly increases in proportion in more complex animals such as mammals, compared to more simple ones such as worms or marine sponges. For most of the information that makes the animal species different from each other, including humans from other animals, seems to be in the non-coding DNA. So, if you're asking me, what would happen if you could remove all the junk DNA from us and survive, I would say that we would all look like baker's yeast at best.

Kat - Thanks to listener Richard Thornley for that question, and also to Paulo Amaral from the Gurdon Institute in Cambridge and Harriet Johnson. And if you've got any questions about genes, DNA and genetics, just email them to me atgenetics@thenakedscientists.com.