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Combined technique unravels DNA

05 Mar 2003

Scientists studying DNA combine optical trapping and single-molecule fluorescence for the first time.

Biologists will be able to study DNA in greater detail, thanks to a new three-laser instrument developed by scientists at Stanford University, US.

The combined optical trapping and single-molecule fluorescence technique devised by Matthew Lang and colleagues can be used to monitor chemical changes induced when two DNA strands are 'unzipped' (Journal of Biology 2003 2 6).

The optical trap unravels the DNA molecule while fluorescence signals reveal which particular parts of the DNA's structure are affected by this process.

Optical traps have been used before to unravel DNA molecules, but combining the method with sensitive fluorescence measurements has been impossible until now. This is because the light intensity of lasers used in an optical trap is so much greater than the light emitted by fluorescent labels attached to the DNA (known as fluorophores).

Lang and co-workers have got around this problem using spectral notch filters and carefully choosing the appropriate fluorophores.

In their setup, an Nd:YAG source emitting at 1064 nm is used for optical trapping; a second 827nm laser is used for position detection; and an argon-ion source emitting at 514nm excites the fluorescence. This excitation wavelength is coupled through a microscope and into the specimen as an evanescent wave (see diagram).

"Optical trapping and single-molecule fluorescence supply information beyond what can be learned using either technique alone," said the Stanford team in their paper. "Simultaneous fluorescence measurements can pinpoint the locations of specific structural changes."

One example of this is in the folding and unfolding of large molecules like nucleic acids and peptide chains, where structural changes could be assigned to rearrangements associated with enzyme reactions. These studies could help biologists understand the way in which certain molecules cause disease.

"We anticipate that this technique will have broad applicability to the study of fundamental biological questions," concludes the team.

Michael Hatcher is technology editor of Opto and Laser Europe magazine.

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