Frozen-hydrated chromatin from metaphase chromosomes has an interdigitated multilayer structure

Area of Science:

• Structural biology
• Molecular genetics
• Cell biology

Background:

• Understanding chromatin folding is crucial for comprehending chromosome structure and function.
• Metaphase chromosomes represent a highly condensed state of chromatin, yet the precise folding mechanisms remain incompletely understood.

Purpose of the Study:

• To investigate the three-dimensional folding of chromatin within metaphase chromosomes.
• To elucidate the structural organization of chromatin at the nanoscale.

Main Methods:

• Cryo-electron tomography (cryo-ET) was employed to generate high-resolution 3D reconstructions of chromatin.
• Small-angle X-ray scattering (SAXS) was utilized to analyze the overall structure of whole chromosomes.

Main Results:

• Cryo-ET revealed that chromatin forms planar, multilayered plates with a layer thickness of approximately 7.5 nm, consistent with a tilted nucleosome monolayer.
• These layers are densely packed, with interdigitating nucleosomes, leading to a combined thickness of ~13 nm for two adjacent layers.
• SAXS data showed a prominent peak at ~6 nm, correlating with the spacing between layers and interdigitating nucleosomes.

Conclusions:

• Metaphase chromosomes are organized into stacked chromatin layers, challenging previous models of uniform folding.
• The interdigitation of nucleosomes within layers suggests a highly efficient packing mechanism.
• This layered model provides new insights into the structural basis of chromosome condensation.