Junk DNA

Only 8.2% of our DNA is actually useful, the rest is ‘junk’ apparently

It’s been only a decade since the Human Genome Project finished its task of mapping all the code that makes up our DNA. The hard part came later, though – identifying what each piece of code does or, oddly enough, does not. According to the most recent estimate for instance, only 8.2% of the code embedded in the human genome is actually useful, in the sense that it performs a function whether activating a gene, regulating it, and so on. The rest is what scientists class as “junk DNA”.

Junk DNA

Image: Institute for Creation Research

Genomes are like biological books, written in genetic letters known as bases; the human genome contains about 3.2 billion bases. The study led by researchers at University of Oxford in England found only 250 million of these letters are functional, and 2 billion or so are not. This might not seem like a really tiny amount, but it’s still considerably higher than a previous estimates which had the figure hovering between 3 and 5 percent. That’s not to say that previous findings were wrong. Instead, it has more to do with how each group classes functional DNA. For instance, a research published by the Encyclopedia of DNA Elements Project (ENCODE) found 80% of our DNA is ‘useful’. But the researchers counted all pieces of DNA on which some protein activity occurred, whether or not this was useful to the cell. The problem is that protein activity occurs all the time. When a cell divides, the DNA replicates and causes protein activity, but this doesn’t mean that particular code is useful.

“Whether people like it or not, the vast majority of our genome is junk,” said Dan Graur for LiveScience, a professor of molecular evolutionary biology at the University of Houston in Texas, who was not involved with the new study. “We know that because we have so many organisms that have much smaller genomes than we do and organisms that have much larger genomes than we have. The size of your genome is not really what matters.”

In this new study, the researchers used an evolutionary model to estimate which part of the DNA is useful or otherwise. This consideration was applied not only to the human genome, but others as well like cattle, ferrets, rabbits or pandas. This sort of novel analysis looks at how DNA has changed since all these mammals, including us humans, split from a common ancestor some 100 million years ago. The reasoning is that by looking at how many genes suffered the least amount of mutations you can count the useful ones – the less mutations the better the chance that the gene in question is useful. Like humans, just 8.2 percent of the DNA in each of these animals is functional, the findings suggest.

Even more interesting, just 1% of our genes code proteins that handle our day to day bodily functions, while the rest of 7% regulate these genes by turning on and off – which is just as vital, granted.

Some might be surprised by these kind of findings. How can it be possible such a thing as “junk DNA” exists? Wouldn’t nature find a way to eliminate redundancy? Why clutter DNA if there’s no use to it? Nobody knows for sure why, but the simplest answer is that nature isn’t perfect and life can be a mess. It’s enough to look at other organisms to understand that more DNA doesn’t equal more complexity. An onion’s genome is five times larger; the wheat’s is eleven times larger. Over millions of years, the human genome has spontaneously gotten bigger, swelling with useless copies of genes and new transposable elements. Our ancestors tolerated all that extra baggage because it wasn’t actually all that heavy. This is at least one side of the story.

7 thoughts on “Only 8.2% of our DNA is actually useful, the rest is ‘junk’ apparently

  1. Kenneth Tyner

    Then that means we need to start finding a purpose for the remainder of our DNA. Maybe we should get busy with the 10% of our brain!

  2. Koroleva

    These studies can be helpful to understand certain things but, they do not consider that these studies are done on the matter which is detached from an actual body; therefor some very important part is missing. Not only this, but they do not consider the possibility of activating DNA, repairing DNA and why some of our DNA is not “being used” at the moment they are studying it.

  3. JoeCoder

    The junk DNA argument from that paper is (my paraphrase):

    1. Humans have 8.2% of their DNA in common with some other mammals.
    2. Therefore that 8.2% of DNA must have existed in the last common ancestor of humans and those mammals.
    3. They’re using neutral theory (standard in population genetics) so they assume that in time since those lineages diverged, evolution couldn’t have created much more function than that, so only around 8.2% of human DNA is functional.

    You can read it yourself and see that’s actually what they’re arguing. Pasting the relevant bits:

    “we address these issues by identifying and characterising sequence that has been constrained with respect to insertions and deletions for pairs of eutherian genomes over a range of divergences… From extrapolations we estimate that 8.2% (7.1–9.2%) of the human genome is presently subject to negative selection and thus is likely to be functional… Our estimate that 7.1%–9.2% of the human genome is functional is around ten-fold lower than the quantity of sequence covered by the ENCODE defined elements… This indicates that a large fraction of the sequence comprised by elements identified by ENCODE as having biochemical activity can be deleted without impacting on fitness… Nucleotide substitution rates in AR [ancient repetitive] sequences are very similar to estimates of the synonymous substitution rate (dS), hence our results appear insensitive to the choice of neutral sequence standard.”

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