High resolution ray-traced model of a nucleosome, isolated on black.

A nucleosome is the basic unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound in sequence around four histone protein cores. This structure is often compared to thread wrapped around a spool.

Nucleosomes form the fundamental repeating units of eukaryotic chromatin, which is used to pack the large eukaryotic genomes into the nucleus while still ensuring appropriate access to it. In mammalian cells approximately 2 m of linear DNA have to be packed into a nucleus of roughly 10 µm diameter.   

Nucleosomes are folded through a series of successively higher order structures to eventually form a chromosome; this compacts DNA and ensures correct gene expression. 

(Nucleosome - Wikipedia)
 

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