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75 percent of human genome is junk DNA

24 July 2017
Appeared in BioNews 910

Almost 75 percent of the human genome is 'junk DNA', suggests a new study.

The findings contradict the results of a similar study published in 2012 by a consortium of scientists from the ENCODE (Encyclopedia of DNA Elements) project, which claimed 80 percent of the human genome to be functional (see BioNews 672).

'We need to know the functional fraction of the human genome in order to focus biomedical research on the parts that can be used to prevent and cure disease,' explained Professor Dan Graur of the University of Houston, Texas, who led the study. 'There is no need to sequence everything under the sun. We need only to sequence the sections we know are functional.'

Early studies on the human genome around the 1950s had assumed that almost the entire DNA sequence was involved in coding proteins, but this assumption was gradually shed as studies revealed some DNA appeared to have no function. The term 'junk DNA' was first coined in 1972.

While ENCODE defined DNA which showed any 'biochemical activity' as being functional, Professor Graur defined functional DNA as having evolved to do something useful, and so mutations in these regions would be likely to be detrimental (deleterious).

Deleterious mutations are normally weeded out of a species through the process of natural selection. Using the accepted mutation rate for the functional part of the human genome, the team developed a model to calculate the decrease in overall survival in relation to the proportion of functional DNA.

Past historical population records show that each human couple per generation needed to produce slightly more than two children to maintain a stable population size. Yet the team found that if 80 percent of the human genome were functional, the accumulation of deleterious mutations in offspring would require an unrealistically high birth rate to maintain a stable population – each couple per generation would need to bear at least 15 offspring, in order for two to survive and reproduce.

Instead, the team conclude that only 10 - 15 percent or a maximum of 25 percent of the human genome is functional. These findings compare favourably with a 2014 estimate of eight percent (see BioNews 765).

Speaking to New Scientist, Dr Ryan Gregory of the University of Guelph, Ontorio agreed that most DNA is probably junk, but cautioned it is not yet known how much DNA has a non-sequence-related function, and that some regions of DNA are useful without having an important sequence.

The results were published in the journal Genome Biology and Evolution.

An upper limit on the functional fraction of the human genome
Genome Biology and Evolution |  11 July 2017
At least 75 per cent of our DNA really is useless junk after all
New Scientist |  17 July 2017
Most of Human Genome Nonfunctional: Study
The Scientist |  17 July 2017
New limits to functional portion of human genome reported
Science Daily |  14 July 2017
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