Schematic representation of the spatial relationships between the nucleosome, the DNA molecule and RNA Pol II. (a) The nucleosome histone core (grey) is positioned on the DNA molecule (blue) with the first three minor groove-histone contact points containing RR dinucleotides (red). The RNA Pol II complex (gold) is shown here without its associated co-factors for clarity. (b) The same as in (a) but a side view, showing the RR dinucleotide in intimate contact with the histones. (c) If the nucleosome is shifted 5 bases closer to the RNA Pol II, it must rotate in space by 5 × 36° = 180° around the helical axis with respect to RNA pol II in order to preserve the contacts between the histones and the minor groove. (d) The same as in (c) but a side view, showing how the RR dinucleotides are now facing outwards and how the RNA Pol II 'sees' the first nucleosome from an entirely different angle.