Mononucleosomes

A mononucleosome is the basic unit of chromatin consisting of a single nucleosome particle. In its canonical form, it comprises approximately 147 base pairs of DNA wrapped around a histone octamer formed by two copies each of histones H2A, H2B, H3, and H4.

The histone core is the octamer around which the DNA is wrapped about 1.65 turns. The linker histone H1 is not part of the core particle but can associate with the nucleosome to form higher-order structures; when H1 is bound to a single nucleosome, the particle is sometimes referred to as a chromatosome.

Isolation and use: In experiments, mononucleosomes are generated by mild micrococcal nuclease digestion of chromatin, which preferentially cuts linker DNA and yields DNA fragments of about 147 bp corresponding to a single nucleosome. Mononucleosomes can be purified by electrophoresis or density gradient methods and are used in chromatin immunoprecipitation (ChIP), nucleosome mapping by sequencing (MNase-Seq), and various structural analyses.

Biological significance: The position and composition of mononucleosomes influence DNA accessibility for transcription, replication, and repair. Post-translational modifications on histone tails, such as acetylation and methylation, mark active or repressed regions, and histone variants like H2A.Z can alter stability and remodeling. Chromatin remodelers can reposition or slide nucleosomes, affecting gene expression. The presence of H1-containing chromatosomes contributes to higher-order chromatin folding and compaction.

Reconstitution and research relevance: Mononucleosomes can be reconstituted in vitro from recombinant histones and defined DNA sequences for biophysical and biochemical studies of nucleosome dynamics and histone-DNA interactions.