Chromatin structure revealed by X-ray scattering analysis and computational modeling.

Area of Science:

• Molecular Biology
• Genetics
• Biophysics

Background:

• The organization of 2 meters of human genomic DNA within each cell is not fully understood.
• Classical models propose DNA is packaged into an 11-nm nucleosome fiber, then further folded into a 30-nm chromatin fiber, potentially forming larger helical structures.

Purpose of the Study:

• To investigate the bulk structure of interphase chromatin and mitotic chromosomes.
• To determine if the proposed 30-nm chromatin fiber structure exists in vivo.

Main Methods:

• Small-angle X-ray scattering (SAXS) was employed to analyze the structural features of chromatin.
• Computational analysis of "in silico condensed chromatin" was used to interpret SAXS data.

Main Results:

• SAXS analysis detected no structural features larger than the 11-nm nucleosome fiber.
• The results suggest that chromatin in interphase nuclei and mitotic chromosomes is composed of irregularly folded nucleosome fibers.
• The classical 30-nm chromatin fiber structure was not detected.

Conclusions:

• The textbook model of chromatin organization, including the 30-nm fiber, may not accurately represent the in vivo structure.
• Chromatin appears to exist primarily as irregularly folded 11-nm nucleosome fibers.
• Further discussion on alternative methods for chromatin structure investigation is provided.