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Folded DNA unravelled

Leiden physicist Maarten Kruithof has discovered how our DNA is infallibly able to fold itself and to unfold again. In his dissertation he demonstrates how DNA is folded in a single long, very flexible spiral.

Chromatin fibres

As this spiral shape is so flexible, not only can the DNA fold, it can at the same time also be read easily. Kruithof used magnetic tweezers to unravel so-called chromatin fibres. Our DNA is not loose within a cell, but is wrapped around proteins. The DNA and these proteins together are called nucleosomes. But these nucleosomes in turn also attract one another and form new structures: chromatin fibres.   

Spiral shape

Chromatin fibres
 

Kruithof discovered that these chromatin fibres have a single spiral as their basic structure. This spiral can also be stretched, like a spring. This ensures that the DNA is compact but can at the same time be read, which is crucial for copying information from the DNA.   

Magnetic tweezers

Kruithof not only looks at the shape of the chromatin fibre, but also at the forces that operate within the fibre. He placed small magnetic bearings on self-constructed chromatin fibres and pulled on them using magnetic tweezers. He used this method to examine the forces that are at work within the fibres. He used much smaller bearings than normal, which allowed a much more precise and more rapid measurement. This method can also have advantages for other research subjects. 

First crucial steps

The characteristics of cells are determined by the way in which the DNA in cells is folded: the DNA is always the same in every cell. The creation of nucleosomes and chromatins can therefore be traced back to the very start of our existence. Kruithof's discoveries go some way to demonstrating the first crucial steps in the life cycle of a cell. Maarten Kruithof obtained his PhD on 1 October at Leiden University, uner the supervision of Professor T. Schmidt. Kruithof's research is part of the Vidi project headed by John van Noort, who was awarded an NWO Vidi subsidy in 2003.

Link

DNA folded in compliant helix

Maarten Kruithof, Magnetic tweezers based force spectroscopy studies of the structure and dynamics of nucleosomes and chromatin. 

PhD defence: 1 October. 
Faculty: Science 
Supervisor: Professor T. Schmidt 

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