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The longer the DNA molecule, the faster it moves

Delft University Of Technology : 19 November, 2006  (Technical Article)
In extremely small, liquid-filled channels, long DNA molecules flow faster than shorter ones. This observation could lead to new methods for sorting and separating biomolecules, like DNA, by length. TU Delft and the Foundation for Fundamental Research on Matter, published these research results in the Proceedings of the National Academy of Sciences.
The ways in which biomolecules like DNA move in small, liquid-filled channels are vitally important for modern, bioanalytical applications. 'Labs-on-a-chip' work remarkably efficiently, by analysing biomolecules flowing through such small, liquid-filled channels (nanochannels). It is now possible to create liquid-filled channels that are as small as a single DNA molecule.

Those involved in this research are post-doctoral researcher Derek Stein, undergraduate Wiepke Koopmans, PhD student Frank van der Heijden, and research group leader Cees Dekker of TU Delft's Kavli Institute of Nanoscience, and the Foundation for Fundamental Research on Matter. They use pressure differences to make DNA molecules flow through rectangular liquid-filled channels only a few micrometers deep. The speed at which DNA molecules flow through the channels appears to depend on the length of the molecule in question. The longer the molecule, the faster it moves.

This effect can be explained by the fact that a flexible polymer like DNA can adopt a huge number of possible shapes. The random thermal movement of a polymer causes it to take on the shape of a loosely rolled-up ball of wool. On average, the longer the molecule, the larger the size of the ball. Large molecules (balls) are more often found flowing down the centre of the channel than smaller molecules (balls). Because there is a faster rate of flow in the centre of the channel than at its edges, longer DNA molecules attain higher speeds than shorter molecules.

Because the DNA molecules' speeds are determined by their lengths, nanochannels can help sort the molecules by length. Molecular sorting usually involves gel electrophoresis, however, the rectangular channels used by the TU Delft researchers offer a much simpler and more easily adjustable method. This method enables single molecules to be separated, without the need for extra molecular labels. Moreover, researchers can now separate extremely long molecules, something that had previously been very difficult using the gel electrophoresis method.

When the dimensions of the channels were reduced, the DNA molecules started to behave strangely. In channels that are less than one micrometer deep, molecules of all different lengths ultimately attain the same speed. This can be explained by the extremely close proximity of the walls, which limits the number of shapes a molecule can take. Consequently, all molecules move together like compressed, flattened balls. Delft researchers gain high-honours for low-noise amplifier design by M&C.
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