Question

Define a dyad axis. Imagine a nucleosome particle formed by wrapping 168 bp of DNA around an octameric disc of the four histones. If the DNA bps are numbered 1-168 what number would the dyad axis correspond to? What numbers would represent the entry site and exit site? In the structure, are the dyad and entry/exit sites proximal to each other?

Answer 1

Dyad axis is the axis of symmetry found nearly in the plane of nucleosome disk perpendicular to the superhelix bisecting the histone (H3-H4)2 tetramer through the bundle of four helix.the dyad axis is the region of the nucleosome where the highest density of histone-DNA interaction takes place. The dyad axis will correspond to 84 bp as the dyad axis divide nucleosome DNA by half into two-part short (83 bp) with one basepair fall on dyad and another half 84bp long part.