Chromatin structure outside and inside the nucleus.

Area of Science:

• Molecular Biology
• Genetics
• Biophysics

Background:

• The 30-nm chromatin fiber has been a central model in understanding DNA packaging.
• Its structure has been investigated using various biophysical and microscopic techniques.

Purpose of the Study:

• To review the evidence for the 30-nm chromatin fiber's structure and its in vivo relevance.
• To assess the role of the 30-nm fiber in transcriptional regulation and chromosome folding.

Main Methods:

• Review of electron microscopy studies.
• Analysis of biophysical data (X-ray scattering, neutron scattering, sedimentation, light scattering, electric dichroism).
• Examination of chromatin folding analyses within the nucleus.

Main Results:

• The 30-nm fiber structure is supported by multiple biophysical methods.
• Reconstituted chromatin fibers mimic native fiber features.
• No simple correlation exists between the 30-nm fiber and transcriptionally silent chromatin in genome-wide studies.
• Regular extended compact structures of the 30-nm fiber are not generally detected in vivo, with exceptions in specific cell types and transcribed regions.

Conclusions:

• The 30-nm chromatin fiber's role as a universal in vivo structure for silent chromatin is unlikely.
• 30-nm fiber-like structures may exist in localized nuclear domains, such as constitutive heterochromatin or at chromosome domain surfaces.