chromatin has organization across overlapping length scales, and each assay measures a particular aspect rather than a universal “openness”. Local DNA exposure, histone occupancy, chemical state, long-range proximity and position within the nucleus are related but distinct. A strong experimental design chooses the relevant scale and combines a genomic assay with an independent structural or functional measurement.
At the molecular scale, DNA wraps around a histone octamer to form a nucleosome core, with roughly 147 base pairs protected; linker DNA connects neighbouring units, and linker histones can affect their organization. The core contains two copies each of H2A, H2B, H3 and H4. Crystallography established the detailed DNA–histone arrangement using a defined nucleosome core particle.[ref]
Crystal structure of the nucleosome core particle at 2.8 Å resolution by Karolin Luger and others. Atomic structure of the reconstituted core particle
Such structures explain local contacts, but a purified core does not specify the arrangement of an entire chromosome in a living nucleus. histone variants, tail modifications and ATP-dependent remodelling change stability and access without requiring DNA-sequence changes.For local accessibility, ATAC-seq uses transposase-mediated insertion of sequencing adapters into accessible DNA. It maps accessible promoters and regulatory elements, and its fragment-length distribution can contain signatures of nucleosome-free regions and neighbouring nucleosomes. The original assay combined rapid accessibility mapping with information about nucleosome organization.[ref]
Transposition of native chromatin for fast and sensitive epigenomic profiling of open chromatin, DNA-binding proteins and nucleosome position by Jason D. Buenrostro and others. ATAC-seq development and accessible-region profiling
DNase I hypersensitivity provides a related cleavage-based measure. Both have sequence bias and depend on preparation and enzyme exposure; a footprint can reflect enzyme preferences or protected binding, and is not a direct identification of the occupying transcription factor. Compare biological replicates, enzyme-dose controls, appropriate background and biological cell composition. Accessibility does not prove that an element regulates a particular gene.nucleosome mapping by MNase-seq addresses occupancy and spacing more directly: partial micrococcal-nuclease digestion preferentially removes exposed linker DNA, and sequencing protected fragments maps nucleosome-associated regions. Digestion titration is essential because over-digestion can preferentially lose fragile nucleosomes and create apparent depletion; MNase also has sequence preferences and can recover other protected particles. Compare fragment sizes, histone enrichment and an independent accessibility assay. Chemical mapping or defined in vitro reconstitution can refine particular nucleosome positions, while single-molecule measurements can expose heterogeneity hidden in a population average.
Chemical states require different assays. histone acetylation and methylation at particular residues can correlate with distinct regulatory states; they are not interchangeable with physical accessibility. chromatin immunoprecipitation followed by sequencing, or targeted CUT&RUN, maps association of a histone modification or protein with genomic regions. antibody specificity, histone abundance, input/background and calibrated normalization matter, especially when the modification changes globally. In CUT&RUN, an antibody recruits a nuclease to release nearby DNA, reducing some backgrounds associated with bulk chromatin fragmentation.[ref]
An efficient targeted nuclease strategy for high-resolution mapping of DNA binding sites by Peter J. Skene and Steven Henikoff. Principle and validation of targeted chromatin cleavage
DNA methylation can be measured by appropriately controlled bisulfite-based or direct-read approaches; standard bisulfite measurements do not automatically distinguish every modified cytosine species. These assays locate a chemical mark, not the geometric folding of the fibre.At the level of nucleosome arrays, regular compact fibres can form in some reconstituted or specialized conditions. A universal hierarchy in which every interphase chromosome passes through a uniform 30-nm fibre is not supported. electron tomography and related imaging can examine the distribution and packing of chromatin directly. ChromEMT combined DNA-selective staining with electron tomography and revealed irregular chromatin chains and variable compaction in biological cells.[ref]
ChromEMT: Visualizing 3D chromatin structure and compaction in interphase and mitotic cells by Horng D. Ou and others. In situ chromatin ultrastructure and compaction
Fixation, staining, sectioning and limited sampled volumes must be considered; cryogenic and super-resolution approaches provide complementary views. Dense staining also need not imply a distinct biochemical chromatin state.Longer-range folding produces loops and domains of enhanced contact. chromosome conformation capture crosslinks, digests and ligates spatially associated fragments; targeted variants test selected loci, whereas Hi-C samples contacts genome-wide. Contact matrices reveal local domains and broad A/B compartmentalization. High-resolution in situ Hi-C mapped chromatin loops and their association with CTCF orientation and cohesin.[ref]
A 3D map of the human genome at kilobase resolution reveals principles of chromatin looping by Suhas S. P. Rao and others. In situ contact maps, loop anchors and domain structure
A topologically associating domain is a region with preferential internal contacts under a stated analysis, not necessarily a stable, sharply bounded object in every biological cell. Contact frequency is neither an absolute distance nor proof that two sites directly interact. Restriction-site distribution, crosslinking, ligation, coverage and bin size affect resolution and interpretation.Perturbation can distinguish mechanisms underlying similar-looking domains. Acute cohesin loss eliminated loop domains while compartment organization persisted, showing that local loop organization and large-scale compartmental segregation are separable.[ref]
Cohesin loss eliminates all loop domains by Suhas S. P. Rao and others. Acute depletion, loss of loops and persistence of compartments
Imaging by DNA fluorescence in situ hybridization can measure distance distributions between chosen loci and test heterogeneity; live labels track dynamics but can perturb loci. Agreement between contact enrichment and a shift in distance distributions is stronger than either assay alone, although the observables still differ.At nuclear scale, chromosomes occupy territories and regions associate preferentially with the nuclear lamina or nucleolus. DNA imaging maps territories and radial positions, while lamina-association assays such as DamID map sequences near a labelled nuclear compartment over an integration interval. The original DamID approach tethered a DNA adenine methyltransferase to a chromatin protein to record nearby genomic regions.[ref]
Identification of in vivo DNA targets of chromatin proteins using tethered Dam methyltransferase by Bas van Steensel and Steven Henikoff. Targeted methylation as a proximity record
Such maps record proximity and history rather than a single fixed structure. Mitotic condensation is a distinct organization with additional constraints and chromosome-shaping machinery, not simply maximally closed interphase chromatin. The appropriate pairing is exposure assays for local access, protected-fragment assays for nucleosomes, protein/mark profiling for composition, contact assays for genomic proximity and imaging for spatial organization, with perturbations to connect organization to function.