Researchers have taken a major step forward understanding how enzymes 'edit' genes, which has paved the way for correcting genetic diseases in patients.

Researchers at the Universities of Bristol, Munster and the Lithuanian Institute of Biotechnology have observed the process by which a class of enzymes called CRISPR - pronounced 'crisper' - bind and alter the structure of DNA.

The results have provided a vital piece of the puzzle if these genome editing tools are ultimately going to be used to correct genetic diseases in humans.

CRISPR enzymes were first discovered in bacteria in the 1980s as an immune defence used by bacteria against invading viruses. Scientists have more recently shown that one type of CRISPR enzyme - Cas9 - can be used to edit the human genome - the complete set of genetic information for humans.

These enzymes have been tailored to accurately target a single combination of letters within the three billion base pairs of the DNA molecule. This is the equivalent of correcting a single misspelt word in a 23-volume encyclopaedia.To find this needle in a haystack, CRISPR enzymes use a molecule of RNA - a nucleic acid similar in structure to DNA. The targeting process requires the CRISPR enzymes to pull apart the DNA strands and insert the RNA to form a sequence-specific structure called an 'R-loop'.

The global team tested the R-loop model using specially modified microscopes in which single DNA molecules are stretched in a magnetic field. By altering the twisting force on the DNA, the researchers could directly monitor R-loop formation events by individual CRISPR enzymes.

This allowed them to reveal previously hidden steps in the process and to probe the influence of the sequence of DNA bases.

The results have been published in the Proceedings of the National Academy of Sciences (PNAS).