Structural study of human chromosomes by synchrotron radiation beam

How is 2-m-long genomic DNA organized into a mitotic chromosome or nucleus? The nucleosome fiber has long been assumed to be folded into a 30-nm chromatin fiber, and further helically folded larger fiber. To investigate the bulk structure of mitotic chromosomes, we performed small-angle X-ray scattering (SAXS) at SPring-8. No structural feature larger than 11 nm was detected, even on a chromosome-diameter scale (∼1 μm). We also found a similar scattering pattern in interphase nuclei of HeLa cells in the range up to ∼275 nm. Our findings suggest a common structural feature in interphase and mitotic chromatins: compact and irregular folding of nucleosome fibers occurs without a 30-nm chromatin structure. (author)