The Department of Justice and the Definition of Junk DNA

In drafting an amicus brief in Maryland v. King, the case in which the Supreme Court is reviewing the constitutionality of routine collection of DNA before conviction, I decided it is important to clarify the term "junk DNA" if only because it gets tossed around in so many court opinions and briefs. The Department of Justice defines “junk DNA” as “[s]tretches of DNA that do not code for genes.” U.S. Dep’t of Justice, Nat’l Institute of Justice, DNA Initiative Training for Officers of the Court, Glossary, http://www.dna.gov/glossary/ (last visited Dec. 17, 2012). In scientific discourse, however, DNA does not “code for genes.” Rather, parts of genes encode proteins and RNAs. "Junk DNA" is not a synonym for the rest of the genome. It is a provocative and deprecated term for that "fraction of DNA that has little or no adaptive advantage for the organism." Sean R. Eddy, The C-value Paradox, Junk DNA and ENCODE, 22 Current Biology R898 (2012). Some of what NIJ thinks is "junk DNA" is important to fitness. It is not "junk."

NIJ's sloppy treatment of terms like "genes" and "junk" is unfortunate, but in the end I decided the awkward definition was not important enough to snipe at in the brief. On a blog, however, one can be more snippy.