Large-scale chromatin structure is the higher-order folding of chromatin on the scale of several hundred kilobases. Little is known about this folding, but the structures must influence transcription, replication and repair of DNA. To learn more about these structures, we previously examined transcriptionally active domains of 400 - 2000 kb by light microscopy. We found that several different domains exhibit similar structures in light microscope images, namely a series of adjacent puncta or """"""""beads"""""""" approximately 0.5 microns in diameter. When these same domains are transcriptionally inactive, only one punctum or """"""""bead"""""""" is detected. Thus transcription induces a decondensation from single beaded to multi-beaded structures. To observe these structures at higher resolution we are exploring the applicability of x-ray microscopy. This imaging modality enables visualization of whole, unsectioned mammalian cells at 25 nm resolution. Detection of chromatin relies on the natural contrast afforded by the absorption of organic matter relative to water. Our goal is to determine if chromatin structure can be detected by this approach and if so, then to provide a 3D image of nuclear chromatin at 25 nm resolution. Our current results suggest that there is not sufficient contrast to visualize chromatin, at least by transmission x-ray microscopy. However, we have obtained strikingly detailed images of the cytoplasm by the same approach. We can now clearly visualize all membrane structures in the cell by x-ray microscopy down to 30 nm resolution. The advantage of this new approach is that the method achieves this resolution for intact, unstained 3D specimens, thereby providing an unprecedented 3D view of cellular architecture. We are presently considering ways to improve contrast of chromatin based structures, including development of phase contrast x-ray microscopy or imaging by standard absorption x-ray microscopy of frozen nuclear sections.
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