PAGE 3 OF 15 Nucleic Acids Research, 2007, Vol. 35, No. 16 e106 Figure 1. Basic structure and folding of the nucleosomal ﬁlament. Panel I: (A) A nucleosomal ﬁlament with ﬁve nucleosomes. (B) Core particle. (C) Histone H1, (D) A nucleosomal repeat length (NRL) of 200 bp consisting of 146 bp of nucleosomal DNA (red) plus 54 bp of linker DNA (black). (E) The angle (a) between the entering and exiting linkers, the change in direction of the linkers (d = 1808 a) and the dyad axis (ad). (F) The rotational angle (b) between the ﬂat faces of the nucleosomes. (G) The slope of the DNA (g) and the angle (Z1) between the projection of the linkers into a plane through the axis of symmetry of the nucleosome (as) and the dyad axis, bisecting a. (H) The projection (a0) of a in a chromatin ﬁber into a plane perpendicular to the ﬁber axis. Panel II: Unfolding the nucleosomal ﬁlament from a = 08 to a = 1808. Panel III: The eﬀect of b on the direction and coiling of the ﬁlament. Rotation of the terminal nucleosomes of a trinucleosome of a polygon ﬁber (A) and a star ﬁber (B). The trinucleosomes are oriented with the dyad axis of the central nucleosome (yellow) in the viewing direction. Arrows indicate the direction of the ﬁlament from nucleosome No. 1!3. (C) Side-on view of a dinucleosomes showing the pitch (p) of the dinucleosomes, as deﬁned by the height diﬀerence between the entry and exit sites of the terminal linkers in a plane perpendicular to the interconnecting linker. (D) Dinucleosomes oriented with the interconnecting linker in the viewing direction and nucleosome no. 1 in front. Clockwise and counter clockwise rotations are indicated by (+) and ( ) and the pitch of the dinucleosomes is indicated by arrows. White and black asterisks indicate the entry and exit sites of the terminal linkers. Panel IV: Possible eﬀects of binding of histone H1 on the geometry of the dinucleosome, as shown for NRL = 200 bp. (A and B) The entry and exit sites of the linkers on the nucleosomes approach each other, corresponding to two full coils of DNA around the core particles, while the size