Wednesday, January 05, 2005

Junk DNA?

Science magazine's # 5 story of 2004 is about the growing realization that much of the so called "junk DNA" which doesn't directly code for proteins (some 95% of the genome), does in fact provide important gene expression regulation functions, as well as code for transposable sequences that can jump around the genome, and for tiny RNA molecules (i.e. ncRNA - noncoding RNA, instead of messenger RNA for example), among other things. In fact, the complexity of an organism doesn't appear strongly linked to the number of genes it has (we're down to ~ 20,000-25,000), but in fact is linked to the percentage of the DNA that is noncoding (humans - highly complex mammals - also have a relatively high percentage of non-coding DNA). See for example: Taft and Mattick's paper, from the Quantitative Biology section on arxiv. There's a website dedicated to the subject too: Noncodingdna.com. The tiny RNA molecule coding sequences are particularily amusing - one of the theories for the origin of life is the RNA World hypothesis, which holds that RNA strands were the beginning point for life, since they can both self-replicate and also catalyze some chemical reactions like proteins. Perhaps these tiny RNA molecules are a 3 billion year old link to the origins of life on earth.

I've also had a pet theory for a while now that the non-coding DNA sections are important in accounting for all the subtle differences between people. When it is not being actively replicated or translated into RNA, DNA is coiled up tightly and wrapped around spool-like histone proteins. Thus the idea is that the differences in the A-T and C-G hydrogen bond strengths and the frequencies with which these base pairs occur in the junk DNA surrounding a given gene will subtly affect how easily the DNA is uncoiled and thus slightly affect the expression rate for that particular gene. This could then lead to, say, all the subtle variations in facial structures between different people (which our brains are so primed to detect), or perhaps partially account for differences in the inherent affinities of different brains. Presumably this could become testable after we gain more understanding in designing DNA and in protein folding and function.

Comments:
Excellent discussion on dna testing. I bookmarked your blog. I have my own dna testing blog if you want to take a look.
 
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