Chromatin is the complex of DNA and associated proteins that constitutes chromosomes in eukaryotic organisms. Located principally in the cell nucleus, it packages long DNA molecules into a restricted space while allowing regulated access to their sequences. Chromatin is therefore both a structural material and a participant in genetic regulation: its organization influences transcription, DNA copying, and chromosome transmission during cell division. Chromatin and chromosomes are not separate substances; a chromosome is an organized unit of chromatin whose degree of compaction changes with cellular activity. (genome.gov)
Molecular composition and nucleosomes
The basic repeating unit of chromatin is the nucleosome. Its core contains approximately 147 base pairs of DNA wrapped about 1.7 turns around an octamer of histone proteins, comprising two copies each of H2A, H2B, H3, and H4. Histones provide a surface around which DNA bends and establish numerous contacts that stabilize the particle. High-resolution X-ray crystallography revealed the organization of the histone octamer and its associated DNA in molecular detail. (nature.com)
Adjacent nucleosome cores are connected by linker DNA of variable length. Linker histone H1 associates near the DNA entry and exit regions and influences how nucleosomes connect and pack together. When sufficiently extended, nucleosome arrays resemble “beads on a string.” In intact nuclei, however, nucleosomes adopt diverse orientations and interactions rather than a single, uniformly repeating arrangement. Chromatin structure consequently depends on both the properties of individual particles and their local environment. (nature.com)
Histones also occur as variants that can give nucleosomes distinct properties. For example, structural studies of H2A.Z-containing nucleosomes identified localized differences in contacts between histone components. Such variants add compositional diversity to chromatin without requiring a change in the underlying DNA sequence. (nature.com)
Packing and nuclear organization
Chromatin must reconcile compaction with accessibility. Regular fibres approximately 30 nanometres wide can form in experimental preparations under particular ionic conditions. They are useful structural models, but should not be treated as a universal intermediate through which all nuclear chromatin passes. In situ cryo-electron tomography of human cells has instead revealed flexible, heterogeneous nucleosome arrangements, including short compact segments, without a uniform 30-nanometre fibre. (nature.com)
At larger scales, the genome is spatially organized rather than randomly mixed. Individual chromosomes occupy preferential nuclear regions called chromosome territories. Regions with similar functional properties also tend to associate, producing broad compartments enriched in relatively open or relatively closed chromatin. DNA regions widely separated along a chromosome can therefore lie close together in three-dimensional space. This organization provides a physical context for interactions among regulatory sequences and their target genes. (asc.ohio-state.edu)
Euchromatin and heterochromatin
Two traditional categories describe contrasting chromatin states. Euchromatin is generally less compact and more accessible to regulatory proteins. Heterochromatin is generally more compact and associated with reduced transcriptional activity. Accessibility affects whether transcription factors and the transcription machinery can engage DNA. These categories describe broad tendencies rather than an absolute division between active and inactive sequences. (ncbi.nlm.nih.gov)
Constitutive heterochromatin forms relatively persistent domains, commonly associated with repetitive DNA. Facultative heterochromatin describes regions whose repression depends on cell type or developmental state. Different domains combine characteristic histone modifications, DNA modifications, and binding proteins. Their organization helps explain how cells sharing substantially the same DNA sequence can maintain different patterns of gene expression. (ncbi.nlm.nih.gov)
Remodeling and chemical regulation
Chromatin is actively reorganized by chromatin-remodeling complexes. ATP-dependent remodelers use ATP to alter nucleosome organization, changing access to DNA without necessarily modifying its sequence. Remodeling is distinct from the chemical modification of histones, although the two processes often operate together. (ncbi.nlm.nih.gov)
Histones undergo reversible modifications, including acetylation, methylation, and phosphorylation. These can affect chromatin interactions and provide recognition sites for other proteins. Their effects depend on the residue modified and the surrounding molecular context: histone methylation, for example, cannot simply be equated with repression. DNA methylation supplies another regulatory layer that interacts with histone-based mechanisms. Together, these processes contribute to epigenetic regulation, through which cellular states can differ without changes to DNA sequence. (ncbi.nlm.nih.gov)
Replication and cellular memory
During DNA replication, nucleosomes ahead of the replication machinery are disrupted and chromatin is assembled on newly synthesized DNA. Parental histones are recycled, while additional histones supply the material needed to package the duplicated genome. Histone deposition occurs rapidly, but restoration of nucleosome organization and modification patterns can take longer. (nature.com)
Recycled histones can carry information about previous chromatin states. Experiments in mouse embryonic stem cells have shown that disturbing the balanced distribution of parental histones alters modification landscapes and developmental competence. Such inheritance supports cellular memory through mitosis, but maintenance is an active process rather than exact copying of every nucleosome. Chromatin-mediated memory consequently involves the coordinated transmission and reconstruction of regulatory information. (nature.com)
Experimental investigation
Chromatin is examined at complementary scales. Crystallography resolves nucleosome structure, while cryo-electron microscopy and tomography investigate molecular arrangements in cellular environments. ATAC-seq uses a transposase to insert sequencing adapters preferentially into accessible DNA, enabling genome-wide mapping of chromatin accessibility. Single-cell versions reveal differences that population measurements can obscure. (nature.com)
Hi-C combines proximity-based DNA ligation with DNA sequencing to measure contact frequencies between genomic regions. Its contact maps reveal chromosome territories and compartmentalization, but conventional population maps represent averages across many cells rather than a single fixed chromosome conformation. Combining structural, accessibility, and contact measurements allows researchers to distinguish local DNA packaging from larger-scale genome organization. (asc.ohio-state.edu)