Blogging the Human Genome


Junk DNA—or, as scientists call it nowadays, noncoding DNA—remains a mystery: No one knows how much of it is essential for life. As one scientist mused, “Is the genome a trash novel, from which you can remove a hundred pages and it doesn’t matter, or is it more like a Hemingway, where if you remove a page, the story line is lost?” However enigmatic, though, noncoding DNA has proved mighty useful for scientists in one way—it’s great for tracking evolution, through so-called DNA clocks.


DNA clocks take advantage of the fact that DNA mutates at a constant rate: Every so many years, a new mutation should pop up along a stretch of DNA. So in examining the natural history of two related species—which once had the same DNA sequence—a scientist can count the number of different mutations that have accumulated along a stretch, and estimate from that how many years have passed since the species started drifting apart. Except it’s not quite that simple. Mutations can arise anywhere in the genome, in gene DNA and noncoding DNA alike. But mutations to genes have bigger consequences: They can disable proteins and kill a creature. As a result, mutations within genes often get weeded out and don’t get passed on to future generations. Noncoding DNA faces fewer constraints—it can mutate more freely without causing problems when it’s passed along. Counting mutations in noncoding DNA therefore provides more accurate estimates in many cases because the timer there isn’t getting reset.

Written By: Sam Kean
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  1. Is it possible to “edit out” the junk DNA, and still get a viable organism?  Is the technology for gene manipulation up to this task yet?  Is anyone trying?  Just wondering, would it lead to organisms that are a bit more energy-efficient, need less food, replicate faster….  I’ve not heard of this being done, not even in science fiction. (References please if you know of any).  Thanks

  2. How do you go about telling coding DNA apart from non-coding DNA?

    What would be the survival disadvantage of carrying a fatter load of non-coding DNA?

    I will hazard a guess that something like eyebrows take “effort” to preserve. As soon as you loose the need of them, they will disappear.  But perhaps junk DNA requires effort to get rid of it, like cleaning the attic, so the task is endlessly procrastinated until the mess becomes pathological, speaking with violent hand-waving..

  3. I hope I’m not answering a rhetorical question, but the answer is yes. Research into figuring out how they work and manipulating them has been going on for years. You can search PubMed or just Google this to find a wealth of references. However, I don’t believe transcription factors really qualify as “junk”, since they have known functions.

  4. Interesting idea…you are aware that a significant part of the human genome is already considered to be remnants of past retroviral infections? I recall one older estimate (quoted by Griffiths, Genome Biology 2001, 2(6):reviews1017.1–1017.5) putting the percentage of viral DNA in the human genome at 8%.

  5.  Copying DNA takes some effort.  If a significant proportion of an organism’s DNA was junk that could be removed without ill effects, it would seem – on the face of it – that the organism would need less energy, resources, food and/or time to support the DNA copying that goes on during its life, making the organism a little more efficient, perhaps enough to improve its survival rates in marginal environments.

    Contrarywise, if there was a penalty associated with retaining the junk, we should expect it to be selected against, and tidy attics would be the norm.  So perhaps the cluttered attic has some advantage as yet unexplained, or at least is neutral.

    Anyone care to speculate on the advantages of a cluttered attic?  I know mine is full of junk, I just want  a handy excuse not to clean it out.

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