At least three-quarters of the human genome is made up of non-functional (unused or unimportant) ‘junk DNA’, and the actual proportion is probably higher, according to the findings of the study carried out by researcher Dan Graur, from the University of Houston (USA) and which includes the journal Genome Biology and Evolution.
Ever since James Watson and Francis Crick discovered in the 1950s the famous double helix structure , the model of DNA that we know and use today, scientists have been debating which degree of the genome is responsible for making us such and as we are.
Graur estimates that the functional portion of the human genome probably makes up only 10-15% of our total DNA, with an upper limit of 25%. The rest of our genome – in somewhere between 75 and 90% of our DNA- is what is called junk DNA: No genetic material necessarily harmful or toxic, but nucleotide sequences and fragmented unreadable are nonfunctional protein coding terms that stimulate important chemical reactions in our bodies.
The rationale behind Graur’s model is based on the way mutations creep into DNA, and how we, as a species, eliminate these mutations for the benefit of all.
These types of genetic variants appear in our genome over time, subtly changing or rearranging the four chemical bases that make up DNA into parts of our genetic code.
About mutations
When mutations occur in junk DNA, they are considered neutral , since that genetic code does not cause any change, but when mutations occur in our functional DNA, the one that defines us, these mutations can often be harmful and even lethal.
On this basis, the lower the percentage of functional DNA, the less chance of genetic mutation.
In Graur’s calculations, the limit of functional DNA has to be very low; otherwise, dangerous mutations would keep piling up, meaning that we would have to produce an incredibly large number of offspring for the small percentage of healthy specimens to survive.
“For 80% of the human genome to be functional, each couple in the world would have to father an average of 15 children and all but two would have to die,” says Graur. “It is more likely then that about 10-25% is not junk DNA.”
“We need to know the functional fraction of the human genome to focus biomedical research on the parts that can be used to prevent and cure diseases. There is no need to sequence everything. We only need to sequence the sections that we know are functional,” concludes the evolutionary biologist.
This theory contradicts other scientific approaches linked to the ENCODE project, which state that 80% of the genome is functional. It is clear that the last word on this issue has not yet been said.
Reference: An upper limit on the functional fraction of the human genome. Dan Graur. 2017. Genome Biology and Evolution. doi.org/10.1093/gbe/evx121
