Dense chromatin plates in metaphase chromosomes

Area of Science:

• Cell Biology
• Biophysics
• Chromatin Structure

Background:

• Previous work identified multilayered plate-like structures around denatured HeLa chromosomes.
• These structures were observed under metaphase ionic conditions.

Purpose of the Study:

• To extensively investigate the nature and properties of these plate-like structures.
• To determine if similar structures exist in other cell types.
• To elucidate the role of these plates in metaphase chromosome organization.

Main Methods:

• Atomic Force Microscopy (AFM) for imaging and mechanical property analysis.
• Cryo-electron microscopy and metal-shadowed imaging for structural details.
• Scanning and transmission electron microscopy for overall structure.

Main Results:

• Plate-like structures are also found in metaphase chicken lymphocyte chromosomes.
• AFM revealed plates are thin (approx. 6.5 nm/layer), compact, and mechanically resistant (Young's modulus ~0.2 GPa).
• Chromatin fibers emanate from uncrosslinked plates in low ionic strength, indicating tight tethering within plates; nucleosomes are tilted for interdigitation.

Conclusions:

• Chromatin possesses intrinsic properties enabling plate formation, as suggested by similarities between denatured plates and purified chromatin aggregates.
• These plates are the dominant component of compact chromatids.
• Metaphase chromosomes are proposed to be formed by stacked plates oriented perpendicular to the chromatid axis.