Chromatin structure: does the 30-nm fibre exist in vivo?

A long strand of DNA is wrapped around the core histone and forms a nucleosome. Although the nucleosome has long been assumed to be folded into 30-nm chromatin fibres, their structural details and how such fibres are organised into a nucleus or mitotic chromosome remain unclear. When we observed frozen hydrated (vitrified) human mitotic cells using cryo-electron microscopy, which enables direct high-resolution imaging of the cellular structures in a close-to-native state, we found no higher order structures including 30-nm chromatin fibres in the chromosome. Therefore, we propose that the nucleosome fibres exist in a highly disordered, interdigitated state like a 'polymer melt' that undergoes dynamic movement. We postulate that a similar state exists in active interphase nuclei, resulting in several advantages in the transcription and DNA replication processes.