A long-standing debate about the structural transition that DNA undergoes when it is stretched seems to have been resolved

Researchers in Europe have literally unravelled a mystery that has been puzzling scientists for years: what happens to a molecule of DNA when it is stretched to its breaking point. The question is important because DNA is subjected to a range of mechanical manipulations within the cell: it can be folded, unfolded, coiled and uncoiled, unzipped and zipped up again. A detailed understanding of the elastic properties of DNA can give scientists key insights into interactions of DNA and the proteins that carry out these manipulations.

Almost two decades ago it was shown that when a molecule of double-stranded DNA is pulled from either end, it undergoes a peculiar transition. Initially the molecule resists stretching. Then, at a force of 65 piconewtons, the polymer suddenly surrenders and stretches to 1.7 times its original length with little additional force. It then becomes resistant to stretching once more.

Two competing ideas arose to explain this behaviour. The first suggested that the DNA remains intact, but that at 65 pN the helix unwinds to form a straight ladder and that the base pairs - the ’rungs’ of the ladder - tilt. The second idea was that the strands of DNA come apart - that the rungs of the ladder break, forming lengths of single-stranded DNA.

Now Erwin Peterman of Vrije University in Amsterdam and colleagues from France and Sweden appear to have answered the question. The researchers carried out a well-established DNA-stretching experiment by attaching one end of each strand of a length of double-stranded DNA to a polystyrene bead. The beads can be pulled apart with a controlled and measurable force by lasers - a process called optical tweezing.

However, in a novel approach the team exposed the DNA to two different fluorescent tags. One binds exclusively to single-stranded DNA and the other to double-stranded DNA. In this way it is possible directly to view under a microscope if the DNA breaks into single strands or remains intact.

’Pulling the DNA from both ends is a bit like pulling on a jacket that has a double-ended zipper,’ says Peterman. ’What we found was that at the transition force the zipper starts to come apart at either end, but remains zipped in the middle. In other words, the DNA comes apart at either end, and under tension this single-stranded DNA is 70 per cent longer than double-stranded DNA.’

Peterman adds, ’I think this work will make it possible to understand in really great detail the thermodynamics and energetics of what happens to DNA when it is stretched, and this will help us make good models of the interactions of DNA with proteins.’

Sarah Harris, who models DNA stretching at the University of Leeds in the UK, is thrilled by the study. ’It is an absolutely gorgeous piece of work. It tells us about the way the structure of a single molecule is changing under tension, something we have not been able to see before,’ she says. ’It does look as though they have solved the question - but as always these issues are very complicated so this might not be the end of the story.’

Simon Hadlington