Stretchy DNA shows off its elastic qualities

YOGA fans take note. DNA can stretch to nearly twice its length without breaking. The discovery could help to develop anti-cancer drugs. It also points to a more prosaic role for the blueprint of life, as a reference material for calibrating machines that measure the tiniest of forces.

Despite its immense importance, there is still a lot we don't understand about DNA. One outstanding puzzle concerns its stretchiness. Like many polymers, the twisty molecule stretches easily under an initial, small force but gets progressively more difficult to stretch as the force increases. At 65 piconewtons, however, DNA suddenly loosens up, becoming easy to stretch again, until it reaches around 170 per cent of its original length.

The cause of this sudden transition has long been a mystery. "You'd think it would be easy to answer 'what happens to DNA when you stretch it?'," says Mark Williams, a biophysicist at Northeastern University in Boston, Massachusetts, who was not involved with the work. "But this is something that people have debated for years. You can't just take a picture of the structure."

One option is that the added length comes from untwisting DNA's double helix, leaving a straight structure that looks more like a ladder than a spiral staircase. However, no such structure has ever been observed.

A competing explanation says that when enough force is applied, some of the bonds between the paired strands come apart, causing the strands to lose their double-helical structure and stretch out easily (see diagram). In 2009, an experiment using dye that only binds to sections of DNA in which the two strands are bonded together favoured this option.

In the experiment, separated regions seemed only to arise near nicks in the strand, suggesting the bonds between the strands may not break under force alone but require an initial tear. Now Thomas Perkins of the US National Institute of Standards and Technology and Hern Paik of the University of Colorado, both in Boulder, show this isn't so.

After checking that a length of DNA had no nicks or tears, they attached one end to a stable surface and the other to a tiny bead that could be pushed with laser light. Activating the laser caused the bead to move, gradually stepping up the force on the DNA, which overstretched at 65 piconewtons.

The pair repeated the experiment with strands that had one and two nicks, and found that the DNA still overstretched at 65 piconewtons. They conclude that nicks are not required for overstretching, and that the phenomenon must therefore be due to force-induced breaking of bonds between the two strands, leading to elongation of the helix (Journal of the American Chemical Society, DOI: 10.1021/ja108952v).

If further tests confirm this, DNA could find a new use. Its sudden switch to stretchiness means that highly sensitive instruments, such as atomic force microscopes, could be calibrated by tugging on DNA and recording exactly when it overstretches.

Understanding how far DNA can stretch before breaking could also help with the design of anti-cancer drugs that aim to disrupt or break apart maliciously replicating DNA, Williams suggests.