At least 75 per cent of human genome is junk DNA, or useless but harmless DNA, suggests a new research.

The functional portion of the human genome probably falls between 10 and 15 per cent, with an upper limit of 25 per cent, according to the study published online in the journal Genome Biology and Evolution.

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The researchers from University of Houston in the US took a deceptively simple approach to determining how much of the genome is functional, using the deleterious mutation rate — that is, the rate at which harmful mutations occur — and the replacement fertility rate.

Both genome size and the rate of deleterious mutations in functional parts of the genome have previously been determined, and historical data documents human population levels.

With that information, the researchers developed a model to calculate the decrease in reproductive success induced by harmful mutations, known as the "mutational load", in relation to the portion of the genome that is functional.

"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," said Dan Graur, evolutionary biologist at the University of Houston.

"There is no need to sequence everything under the sun. We need only to sequence the sections we know are functional," Graur added.