31
Jan
09

Interesting myth

I had a page hit earlier today for this search: “only 3%of dna does anything.” Perhaps I can shed a little light on this myth. A few years ago, when the entire human genome was sequenced (not really, they missed the highly methylated regions and telomeres, so more like 92%). It was stated that only 3% of the genome encoded proteins. This does not mean the rest of the DNA does nothing. This only means that 3% of the genome contains an open reading frame (ORF) for protein. The genome is not JUST about encoding proteins, though. There are sequences responsible for encoding ribosomes, sequences which promote genes, regulate transcription, regulate genome structure, and possibly many other functions. Other segments are integrated retroviruses (endogenous retroviruses (another 8% of the genome) and retrotransposons) and other transposable elements (transposons). SINES make up about 13% of the genome. At this point, we’re up to 25% of the genome not including promoters, enhancers, suppressors, and silencing sequences. This also doesn’t include telomeres or LTRs either. The genome has quite a few functional elements which make up far more than 3%.

In any event, you cannot equate “no known function” to “functionless.” Just because we do not know what a sequence does only makes the function of it unknown, not absent. In the same light, some of these sequences can be completely removed from other mammals with no obvious effect on the organism. These regions are likely conserved functionless areas, however, these regions may have regained function in some way (either as enhancers or distant promoters).

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4 Responses to “Interesting myth”


  1. 1 Colloquy
    January 31, 2009 at 1:18 pm

    Ihank you for that! Iz likes lernin stuff! 🙂

  2. 2 The Science Pundit
    February 14, 2009 at 2:03 pm

    I went to a Darwin Day symposium yesterday and Thursday. One of the speakers talked about her work in the evolution of lactose tolerance. It turns out that LT has independentally evolved at least five times in human populations (four times in subsaharan Africa and once in North Africa/Europe). Here’s the interesting part: none of the five know mutations that can cause LT are located in the gene that codes for the enzyme lactate dehydrogenase. They’re all in nearby sequences.

    The way it works is that all mammals can digest the sugar lactose when they’re infants (lactate dehydrogenase breaks lactose down into glucose and galactose), but the gene which produces lactate dehydrogenase gets switched off some time shortly after weening. Some human populations have acquired mutations that prevent that gene from being shut off, and that phenotype is naturally selected for. These mutations are in non-coding but regulatory parts of the genome.

    In other words, the non-coding part of the genome contains a ton of important functionality.

  3. 3 The Science Pundit
    February 14, 2009 at 2:09 pm

    Correction: the enzyme I was talking about is lactase. Lactate dehydrogenase is something else (sports physiologists can tell you all about it). That’s what I get for not taking notes. 😦

  4. 4 jaredcormier
    February 14, 2009 at 3:23 pm

    Yea, I wrote a paper on lactase persistence (the continued production of lactose into adulthood), it’s actually more than that, I’ll have to put up the paper one day.


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