Twisting in a crowd

The scenery of the nucleus The nucleoplasm is no less dense, even though it contains neither membrane nor rigid filament. However, it contains one remarkable type of filament, the DNA, which, for a mammalian cell with a small nucleus, can account for close to 10% (100 mg m1-1) of the nuclear mass (including water). This is a very high concentration much higher than that generally used for experiments in vitro (e.g. 200 Bg ml-1). It is close to the concentration at which pure DNA in solution starts to show strong colligative behaviour, resulting in the formation of large organized complexes such as liquid crystals (for example, cholesteric crystals form at 200 mg m1-1)1, 2. The nucleus also contains histones (amounting to roughly the same mass as the DNA), various polymerases, topoisomerases and other enzymes, different kinds of RNAs, and smaller molecules and ions. Altogether, these components account for 20-30% of the dry weight of the nucleus. At a magnification of 100000 times, the nucleus would appear less than 1 m in diameter, but the DNA it contained would be 0.2 mm thick and 200 km long! Goodsell 3 has drawn a picturesque representation of this crowded environment.