"Frozen" chromatin: insights on its structuring from cryo-EM tomography

Chromatin is a DNA-protein polymer that ensures the safekeeping and realization of genetic information in eukaryotic cells. The genome of higher eukaryotes is packed into a relatively small nuclear volume and organized in a non-random yet non-deterministic manner, exhibiting a high degree of complexity and flexibility. Despite the dense packing of chromatin, the cell effectively controls transcription, replication, and DNA repair. Genomic approaches, direct visualization, and computer modeling have allowed to accumulate a large amount of data on the structure and behavior of chromatin in various functional states. However, many of these techniques rely on chemical cross-linking, membrane permeabilization, or altered ionic conditions, which may perturb native chromatin structure. This has led to contradictory views on chromatin organization in situ. Cryo-electron microscopy (Cryo-EM) ensures preservation of the cellular structures in a close to native hydrated state and provides high resolving power, thus becoming a tool of choice for studying organization of chromatin at the nucleosomal and supranucleosomal scales. This review summarizes how cryo-EM has advanced our understanding of chromatin organization and function, considers the limitations of the method, and discusses future directions and perspectives in the field.