FIGURE 1. Packaging DNA.

From the following article:

Controlling the double helix

Gary Felsenfeld and Mark Groudine

Nature 421, 448-453(23 January 2003)



a, The organization of DNA within the chromatin structure. The lowest level of organization is the nucleosome, in which two superhelical turns of DNA (a total of 165 base pairs) are wound around the outside of a histone octamer. Nucleosomes are connected to one another by short stretches of linker DNA. At the next level of organization the string of nucleosomes is folded into a fibre about 30 nm in diameter, and these fibres are then further folded into higher-order structures. At levels of structure beyond the nucleosome the details of folding are still uncertain. (Redrawn from ref. 41, with permission). b, The structure of the nucleosome core particle was uncovered by X-ray diffraction, to a resolution of 2.8Å (ref. 42). It shows the DNA double helix wound around the central histone octamer. Hydrogen bonds and electrostatic interactions with the histones hold the DNA in place.