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Diapers, DNA and very few knots


By Hamish Johnston

Many important biological processes involve the packing and unpacking of long stringy molecules such as DNA into very dense structures.

One of the most amazing aspects of this little-understood process is that the stringy molecules don’t get all tied up into a mess of knots.

According to Alexander Grosberg of New York University — who was speaking about this knotty issue this morning — collapsing proteins are thought to form 19 distinct types of knots. Compare this to simple chains of the same length, which can get knotted up in about 3000 different ways.

“Evolution avoids or supresses knots in proteins”, he declared.

Grosberg argues that physicists need a new model for describing how biomolecules collapse. The key features, he says are the process being driven by pressure from the outside — and a mathematical way of avoiding knots.

One approach he has taken is to model proteins as rings instead of single strands. Why this seems to work was beyond me, but Grosberg seems to have made some progress in describing the collapse mathematically.

And what does this have do do with diapers? Well it seems that the water-hungry material in nappies undergoes a similar collapse — but as the photo above suggests, the actual process of compaction is unknown.

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