Three reasons to like junk DNA

ENCODE (Image: Ed Yong)

The recent dustup over the ENCODE project and its confusing finding that 80% of DNA is functional surprises me greatly. What surprises me especially is that people are surprised by junk DNA. Unfortunately this time the scientists are also culpable since, while the publicity surrounding ENCODE has been a media disaster, the 80% claim originated in the scientific papers themselves. There is no doubt that the project itself which represents a triumph of teamwork, dogged pursuit, technological mastery and first-rate science has produced enormously useful data, and there is no doubt it will continue to do so. What is in doubt is how long it will take for the public damage to be repaired.

Theres a lot written about the various misleading statements about the project made by both scientists and journalists and I cannot add much to it. All I can do is to point to some excellent articles:Larry Moran has waged a longstanding effort to spread the true wisdom about junk DNA for years on his blog. Ed Yong exhaustively summarizes a long list of opinions, links and analysis. T. Ryan Gregory has some great posts dispelling the myth of the myth of junk DNA. And John Timmer has the best popular account of the matter. The biggest mistake on the part of the scientists was to define functional so loosely that it could mean pretty much all of DNA. The second big mistake was not in clarifying what functional means to the public.

But what I found astonishing was why its so hard for people to accept that much of DNA must indeed be junk. Even to someone like me who is not an expert, the existence of junk DNA appeared perfectly normal. I think that junk DNA shouldnt shock us at all if we accept the standard evolutionary picture.

The standard evolutionary picture tells us that evolution is messy, incomplete and inefficient. DNA consists of many kinds of sequences. Some sequences have a bonafide biological function in that they are transcribed and then translated into proteins that have a clear physiological role. Then there are sequences which are only transcribed into RNA which doesnt do anything. There are also sequences which are only bound by DNA-binding proteins (which was one of the definitions of functional the ENCODE scientists subscribed to). Finally, there are sequences which dont do anything at all. Many of these sequences consist of pseudogenes and transposons and are defective and dysfunctional genes from viruses and other genetic flotsam, inserted into our genome through our long, imperfect and promiscuous genetic history. If we can appreciate that evolution is a flawed, piecemeal, inefficient and patchwork process, we should not be surprised to find this diversity of sequences with varying degrees of function or with no function in our genome.

The reason why most of these useless pieces have not been weeded out is simply because there was no need to. We should remember that evolution does not work toward a best possible outcome, it can only do the best with what it already has. Its too much of a risk and too much work to get rid of all these defective and non-functional sequences if they arent a burden; the work of simply duplicating these sequences is much lesser than that of getting rid of them. Thus the sequences hung around in our long evolutionary history and got passed on. The fact that they may not serve any function at all would be perfectively consistent with a haphazard natural mechanism depending on chance and the tacking on of non-functionality to useful functions simply as extra baggage.

There are two other facts in my view which should make it very easy for us to accept the existence of junk DNA. Consider that the salamander genome is ten times the size of the human genome. Now this implies two possibilities; either salamanders have ten times functional DNA than we do, or that the main difference between us and salamanders is that they have much more junk DNA. Wouldnt the complexity of salamander anatomy of physiology be vastly different if they really had so much more functional DNA? On the contrary, wouldnt the relative simplicity of salamanders compared to humans be much more consistent with just varying degrees of junk DNA? Which explanation sounds more plausible?

The third reason for accepting the reality of junk DNA is to simply think about mutational load. Our genomes, as of other organisms, have undergone lots of mutations during evolution. What would be the consequences if 90% of our genome were really functional and had undergone mutations? How would we have survived and flourished with such a high mutation rate? On the other hand, its much simpler to understand our survival if we assume that most mutations that happen in our genome happen in junk DNA.

As a summary then, we should be surprised to find someone who says they are surprised by junk DNA. Even someone like me who is not an expert can think of three reasons to like junk DNA:

1. The understanding that evolution is an inherently messy and inefficient process that often produces junk. This junk may be retained if its not causing trouble.

Link:

Three reasons to like junk DNA