Nucleosome
A nucleosome is the basic repeating unit of chromatin in eukaryotic cells. It packages DNA by wrapping it around histone proteins, helping long genomes fit inside nuclei while shaping access to genes.
What a nucleosome is
A nucleosome is a compact structure formed when DNA wraps around a core of histone proteins. Many nucleosomes arranged along DNA create the bead-like first level of chromatin organization. This packaging is essential in eukaryotic cells, where long DNA molecules must fit inside the nucleus.
The histone core
The nucleosome core particle is built around a histone octamer: two copies each of histones H2A, H2B, H3, and H4. These proteins provide a spool-like surface for DNA. Histone H1 is not part of the core octamer, but it can bind linker DNA and help stabilize higher-order chromatin structure.
DNA wrapping
Roughly 147 base pairs of DNA wrap around the histone core in the nucleosome core particle. Neighboring nucleosomes are connected by stretches of linker DNA. This arrangement shortens and organizes DNA while still allowing chromatin to change shape when genes need to be copied or repaired.
Beads on a string
A common textbook image compares nucleosomes to beads on a string: histone cores are the beads, and DNA is the string. The metaphor is useful, but living chromatin is more dynamic. Nucleosomes can shift, loosen, be removed, or be rebuilt by cellular machinery.
Nucleosome positioning
Where nucleosomes sit along DNA matters. A nucleosome covering a promoter or regulatory site can make it harder for transcription factors or RNA polymerase to bind. When nucleosomes are repositioned or removed, the same DNA sequence can become more accessible.
Histone marks and remodeling
Histone tails can carry chemical modifications such as acetylation or methylation. Chromatin-remodeling complexes can also move or restructure nucleosomes. Together, these processes help cells manage gene regulation, DNA repair, replication, and cell identity without changing the DNA sequence.
How scientists study nucleosomes
Researchers study nucleosomes with structural biology, nuclease digestion, sequencing-based chromatin assays, and microscopy. These methods can reveal nucleosome positions, histone modifications, accessibility patterns, and how chromatin organization changes across cell types or conditions.
Why it matters
Nucleosomes are small, repeated structures with large consequences. They make genome packaging possible and provide a physical layer of gene regulation. Understanding nucleosomes helps explain chromatin, epigenetics, chromosome structure, and why DNA access is as important as DNA sequence.