Revealing chromatin organization in metaphase chromosomes

Abstract

The local structural organization of chromatin in mitotic chromosomes is not well understood. A new cryo‐electron tomography study from the Daban laboratory (Chicano et al, 2019) reveals that mitotic chromatin isolated from human cells can assume a plate‐like fine structure, containing layers of interdigitated nucleosomes. Such a multilayered organization suggests a possible model for mitotic chromatin compaction.

Figure 1. Models of mitotic chromatin organization

(A) In the scaffold/radial loop model, a core scaffold of condensin I and II complexes together with topoisomerase IIα compacts chromosomes by extrusion of nested chromatin loops, including long 400‐kb outer and 80‐kb inner loops (Gibcus et al, 2018). The chromatin fibers are locally disordered, forming a polymer melt‐like state containing interdigitated fibers of stacked nucleosomes. (B) In the multilayered plate model of mitotic chromatin organization, individual chromatin loops are organized into dense plates of nucleosomes (Chicano et al, 2019). Plates can interdigitate at contact sites and interact through nucleosome stacking interactions.