A nucleosome is the basic repeating unit of eukaryotic chromatin. It consists of about 146 base pairs of DNA wrapped 1.65 times around a histone octamer (a core of 8 histone proteins: 2 copies each of H2A, H2B, H3, and H4). The structure looks like DNA wound around a spool.
The human genome contains ~3 billion base pairs of DNA. If stretched out, this DNA would be about 2 meters long — yet it must fit into a nucleus only ~10 micrometers in diameter. The solution is hierarchical packaging: DNA wraps around histone proteins to form nucleosomes, nucleosomes coil into chromatin fibers, and chromatin compacts further into chromosomes. This packaging is so efficient that you can compress 2 meters into a microscopic space.
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By wrapping DNA around a histone core, the effective length is reduced by about 7-fold. One nucleosome (146 bp + linker ~20 bp = ~166 bp total) occupies the space of stretched DNA of ~166 × 0.34 nm = ~56 nm. Wrapping reduces this to about 11 nm. Multiple levels of coiling (nucleosomes → 30 nm fiber → higher-order loops) achieve massive compaction.
Histone H1 binds to linker DNA between nucleosomes and helps compact nucleosomes into the 30 nm chromatin fiber. H1 is sometimes called the 'linker histone.' It stabilizes higher-order chromatin structure and is important for gene silencing in heterochromatin.
Euchromatin is loosely packed chromatin (open structure) that is transcriptionally active — genes here are expressed. Heterochromatin is tightly packed chromatin (condensed structure) that is transcriptionally silent — genes here are not expressed. The difference is partly due to histone modifications and DNA methylation.
Histone modifications (acetylation, methylation, phosphorylation) can either promote or suppress transcription. Histone acetylation generally loosens chromatin (euchromatin, active genes). Histone methylation can promote or suppress, depending on which residue is methylated. These 'epigenetic marks' are reversible and heritable through cell division, allowing dynamic gene regulation.
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