The term "junk DNA" has been under attack lately, but a pair of geneticists are now making the case for this much maligned and misunderstood concept. There are significant chunks of our DNA, they argue, that really are utterly useless.
By the end of the Human Genome Project, biologists had discovered that over 98% of our DNA has no apparent biological function. Apparent being the keyword, because by the end of the 2012 ENCODE project — a kind of encyclopedia of human DNA — geneticists learned that 80% of what was thought to be junk, or noncoding, DNA did in fact have a purpose. Since then, many researchers and science writers have shunned the term, regarding it as a kind of silly placeholder for parts of the genome that we still don't understand (leading some to call it the "dark matter" of DNA).
But a new PLoS study prepared by Alexander Palazzo and T. Ryan Gregory claims that we've gone too far. They say that junk DNA is very real, a notion that can be supported by genetic principles that are both decades old and by new ones that are still emerging.
They admit that many segments of noncoding DNA do have apparent functions, such as coding exons, sequences that are transcribed into functional RNA molecules, regulatory regions that control gene expression, and repeats that play structural roles at the chromosomal level.
But they highlight a number of other sequences that deserve the moniker "junk." They include:
- Transposable elements (including retroelements, retroviruses, and so-called cut-and-paste transposons)
- Highly repetitive DNA
- Introns (nucleotide sequences that are removed by RNA splicing)
- Conserved sequences
Palazzo and Gregory go on to describe the evolutionary and biological reasons for these elements, concluding that
evolutionary considerations, information regarding genome size diversity, and knowledge about the origins and features of genomic components do not support the notion that all of the DNA must have a function by virtue of its mere existence. Nothing in the recent research or commentary on the subject has challenged these observations.
Read the entire study at PLoS: "The Case for Junk DNA."
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