Chromosomes without a 30-nm chromatin fiber

Abstract

How is a long strand of genomic DNA packaged into a mitotic chromosome or nucleus? The nucleosome fiber (beads-on-a-string), in which DNA is wrapped around core histones, has long been assumed to be folded into a 30-nm chromatin fiber, and a further helically folded larger fiber. However, when frozen hydrated human mitotic cells were observed using cryoelectron microscopy, no higher-order structures that included 30-nm chromatin fibers were found. To investigate the bulk structure of mitotic chromosomes further, we performed small-angle X-ray scattering (SAXS), which can detect periodic structures in noncrystalline materials in solution. The results were striking: no structural feature larger than 11 nm was detected, even at 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.

Structure-driven homology pairing of chromatin fibers: the role of electrostatics and protein-induced bridging

“…Experimentally, many features of mutual interactions of chromatin fibers remain obscure, partly because of different chromatin forms that can potentially coexist in vivo [27]. Here, one should distinguish between the subtleties in interactions due to the complexity of the internal chromatin structure and due to different biophysical mechanisms likely to be involved.…”

Label-free imaging of the native, living cellular nanoarchitecture using partial-wave spectroscopic microscopy

“…Due to this complexity and the lack of existing optical techniques that can rapidly sample information below 200 nm, little is known about the higher-order chromatin structure at length scales between 10 and 200 nm or their dynamics in live cells (e.g., the folding structure of chromatin above that of mononucleosomes). Results from fixed-cell-imaging techniques, such as electron microscopy or SRM, have shown that chromatin between 20 and 200 nm is first organized into polynucleosomal 10-nm fibers, and in certain conditions, these fibers have been shown to assemble into 30-nm clusters (8)(9)(10), although the existence of the 30-nm fiber is a subject of an active debate. At length scales between 100 and 200 nm, recent work using SRM has shown a power-law (fractal) relation in the organization of chromatin, with domains of highly dense, inactive chromatin localizing within a few hundred nanometers of transcriptionally active sites (11).…”

Micro- and nanofluidic technologies for epigenetic profiling

“…Simple models such as the semi-flexible chain model based on the persistence length of the polymer may not be generally reliable. The reported persistence length spans a broad range from 30 nm to 300 nm, 41,42 reflecting the sensitivity of the system to the environmental variables previously discussed.…”

“…35,38 Furthermore, more recent evidence suggests that even the regular 30-nm fiber structure does not exist consistently in living mammalian cells. 37,41,42 Hence, our current understanding of chromatin structure suggests that the chromatin in a cell exists in a melt-like dynamic state, providing greater accessibility for biological purposes than the conventional static model. 41 Also on a primary structural level, there are small variations in aminoacid sequences even though core histones are highly conserved proteins across species.…”