In genetics, the term junk DNA refers to regions of DNA that are noncoding.
DNA contains instructions (coding) that are used to create proteins in the cell. However, the amount of DNA contained inside each cell is vast and not all of the genetic sequences present within a DNA molecule actually code for a protein.
Some of this noncoding DNA is used to produce non-coding RNA components such as transfer RNA, regulatory RNA and ribosomal RNA. However, other DNA regions are not transcribed into proteins, nor are they used to produce RNA molecules and their function is unknown.
The proportion of coding versus noncoding DNA varies significantly between species. In the human genome for example, almost all (98%) of the DNA is noncoding, while in bacteria, only 2% of the genetic material does not code for anything.
The term Junk DNA
The term “junk DNA” was first used in the 1960s, but was formalized by Susumu Ohno in 1972. Ohno noticed that the amount of mutation occurring as a result of deleterious mutations set a limit for the amount of functional loci that could be expected when a normal mutation rate was considered. In a Nature review published in the 1980, Leslie Orgel and Francis Crick stated that junk DNA “had little specificity and conveys little or no selective advantage to the organism."
However, over the years, researchers have found evidence to suggest that junk DNA may provide some form of functional activity. Some lines of evidence suggest that fragments of what were originally non-functional DNA have undergone the process of exaptation throughout evolution. Exaptation refers to the acquisition of a function through means other than natural selection.
In 2012, a research program called the ENCODE project concluded that around three quarters of the noncoding DNA in the human genome did undergo transcription and that almost 50% of the genome was available to the proteins involved in genetic regulation such as transcription factors.
However, these findings have been criticized by other scientists who claim that the accessibility of these genomic segments to transcription factors does not mean they necessarily have any biochemical function or that transcription of the segments is in any way advantageous in terms of evolution.