Solenoid Structure

A coiling electric conductor used for the generation of a magnetic field; by analogy, the coiled nucleosomal DNA fiber is frequently described as a solenoid, although it has no relation to electricity or magnetism. It merely resembles those coils. The DNA solenoids are about 30 nm in diameter, contain about six nucleosomes per turn, and are packed with a large number of structural and catalytic proteins. Actually two different models have been proposed for the 30 nm solenoid structure of the chromosome sub-fibers although neither of them has been fully and universally confirmed. One has been based on the crystal structure of a four-nucleosome core array lacking the linker histone and the other, far more compact structure, has been derived from the electron microscopic analysis of long nucleosome arrays containing the linker histone. The first model is of the two-start helix type, the second a one-start helix with interdigitated nucleosomes. Both of these models assume that the fiber...