Researchers at the Institut Laue-Langevin have used neutron scattering to work out how stretchy DNA is,
after many previous studies produced very different answers. The
results can help to explain how DNA can bend and split in order to
establish traits in living organisms and then pass these on from
generation to generation.
DNA's characteristic double helix is constantly twisted, bent
and stretched when it's inside a cell. How the DNA responds to this
pressure is what determines two of its biological functions --
replication and transcription.
Previous measurements of DNA stretchiness have varied by an
order of magnitude, between 0.3 to 3 newtons per metre. And even
these figures are lower than other's produced from sound velocity
measurements. The neutron scattering revealed that DNA has a
flexibility of 83 newtons per metre -- roughly the same as
Researchers took wet-spun samples of DNA, which they wound onto a bobbin. These were then
placed into a neutron scattering spectrometer. The team measured
how the frequency of sound waves running along the double helix
structure changed with their wavelength. Professor Mark Johnson, an
ILL physicist, explained: "We are essentially measuring the speed
of sound in DNA, which gives you a direct measurement of its
By using computer simulations of the DNA stretching, they could
then account for the whole range of previously reported values.
They noticed that the solution used to suspend the DNA -- composed
of charged ions like lithium and sodium in water to mimic cell
conditions -- can influence the stiffness of the DNA. If
DNA is pulled sharply, it tends to block -- much like a seatbelt --
because the ions in the solution get between DNA atoms. However if
it is pulled slowly it stretches.
The stretchiness that researchers at ILL determined closely
supports recent work on the overstretching of DNA. The DNA's
structure has been shown to unwind and unzip -- starting the
replication process that drives genetic inheritance -- in response
to tension inside the cell because the structural stiffness
prevents the double-strand DNA molecule from extending.
Dr Lambert van Eijck, one of the lead authors, from Delft
University of Technology, The Netherlands: "Our findings support
increasing evidence that suggests DNA's stiffness is an important
factor in how it is stored in cells and then unzips, starting the
replication process that drives genetic inheritance.
The research is published in Physical