Noncoding DNA - Junk DNA

Junk DNA

"Junk DNA" is a term that was introduced in 1972 by Susumu Ohno, who noted that the mutational load from deleterious mutations placed an upper limit on the number of functional loci that could be expected given a typical mutation rate. Ohno predicted that mammal genomes could not have more than 30,000 loci under selection before the "cost" from the mutational load would cause an inescapable decline in fitness, and eventually extinction. This prediction remains robust, with the human genome containing approximately 20,000 genes.

Junk DNA remains a label for the portions of a genome sequence for which no discernible function had been identified. According to a 1980 review in Nature by Leslie Orgel and Francis Crick, junk DNA has "little specificity and conveys little or no selective advantage to the organism". The term is used mainly in popular science and in a colloquial way in scientific publications. "Scientific American" claims that its connotations may have slowed research into the biological functions of noncoding DNA.

Several lines of evidence indicate that some "junk DNA" sequences are likely to have unidentified functional activity, and other sequences may have had functions in the past. Recently junk DNA sequences were artificially expressed resulting in the synthesis of functional proteins. This new approach is called junkomics. In 2012, the ENCODE project, a research program supported by the National Human Genome Research Institute, reported that 76% of the human genome's noncoding DNA sequences were transcribed and that nearly half of the genome was in some way accessible to genetic regulatory proteins such as transcription factors. This, however, does not necessarily mean that all of these segments have true biochemical function.

Still, a significant amount of the sequence of the genomes of eukaryotic organisms currently appears to fall under no existing classification other than "junk". For example, one experiment removed 0.1% of the mouse genome with no detectable effect on the phenotype. This result suggests that the removed DNA was largely nonfunctional. In addition, these sequences are enriched for the heterochromatic histone modification H3K9me3.