Friday, 22 July 2011

Researchers discover "7th and 8th bases of DNA"? Hardly. Here's your daily dose of science journalism fail.

Making the rounds on the blogosphere and the news sites today is the announcement that researchers have discovered the "7th and 8th bases of DNA". This announcement comes from a paper published online on Science's pre-print server1, Science Express by researchers at the University of North Carolina School of Medicine, and most of the news reports seem to be based on an article posted to Science Daily. The article reads:
"For decades, scientists have known that DNA consists of four basic units -- adenine, guanine, thymine and cytosine. Those four bases have been taught in science textbooks and have formed the basis of the growing knowledge regarding how genes code for life. Yet in recent history, scientists have expanded that list from four to six. Now, with a finding published online in the July 21, 2011, issue of the journal Science, researchers from the UNC School of Medicine have discovered the seventh and eighth bases of DNA."
Oooh! Exciting! What are these bases, exactly?
"These last two bases -- called 5-formylcytosine and 5 carboxylcytosine -- are actually versions of cytosine that have been modified by Tet proteins, molecular entities thought to play a role in DNA demethylation and stem cell reprogramming."
So, wait a second. These "new" bases are only modified forms of cytosine? So what? This is no big deal at all. There are well over a dozen known modified bases. Here, let me list a few:
  • 5-hydroxymethylcytosine
  • 5-hydroxymethyluracil
  • N4-methylcytosine
  • 7-methylguanine
  • N6-methylcytosine
  • β-D-hydroxymethyluracil
Need I go on? If we're counting modified bases, then there are perhaps two dozen or more known bases. Why do 5-formylcytosine and 5-carboxylcytosine get the elevated status as the '7th and 8th' bases, when there are so many more modified bases that seem to have gone ignored (and who, for that matter, gave 5-methylcytosine and 5-hydroxymethylcytosine the distinction of being the 5th and 6th)?

Is the discovery of 5-formylcytosine and 5 carboxylcytosine interesting and exciting? Yes, most definitely. Are they the "7th and 8th" bases of DNA? Nope, not at all.

Ito, S., Shen, L., Dai, Q., Wu, S.C., Collins, L.B., Swenberg, J.A., He, C., and Zhang, Y.  Tet Proteins Can Convert 5-Methylcytosine to 5-Formylcytosine and 5-Carboxylcytosine. 2011.  Science Published Online 21 July 2011 doi:10.1126/science.1210597

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