Figure 4.

Chromosome fold over is facilitated by strong centromere and telomere clustering and lamina adhesion. (A) Schematic of chromosomes showing the bend angles characterizing the fold-over: θ_centro and θ_arm. Folding over requires a low θ_centro and high θ_arm. (B) and (C) Probability distributions of the characteristic bend angles are plotted for various lengthwise compaction without any centromere or telomere adhesion. The bending of the chromosomes at the centromere as well as at the arms are in general counteracted by lengthwise compaction. (D) and (E) Probability density of the bend angles in the Rope-like (R) state, under various conditions such as centromere (or telomere) phase separation, and adhesion with uniform or polar lamina. Polar lamina produces the strongest fold-over signal. (F) Contact maps of the genome with five Rope-like chromosomes with strong centromere and telomere adhesion (χ_C = χ_T = −0.3) along with their uniform adhesion to the lamina () shown in the lower triangles, while the upper traingle corresponds to a polar lamina configuration (). The black arrows show the region where there is loss of contacts in the polar case. (G) Chromosome contact probability curves corresponding to the polar and uniform cases in the contact map. (H) and (I) Simulation snapshots showing one chromosome with centromeres as big red spheres and telomeres as big blue spheres, and the two chromosome arms are shown as dark green and cyan tubes. For the uniform lamina case (H), the lamina beads are shown as small gray spheres, while for the polar case, the lamina beads interacting favorably with the centromere are shown as small red spheres and that with the telomere are shown as small blue spheres.