One histone protein octamer has about 147 base pairs of DNA wrapped around it. This DNA–protein unit is called a nucleosome core particle, the basic packaging unit of eukaryotic chromatin.
The octamer contains eight core histones: two copies each of H2A, H2B, H3, and H4. The DNA makes roughly 1.67 left-handed turns around this positively charged protein surface, while negatively charged DNA is attracted to lysine- and arginine-rich histone regions.
The wrapped segment is often reported as approximately 146 or 147 base pairs because experimental descriptions and counting conventions differ slightly. The frequently taught canonical figure is 147 bp. Linker DNA connects neighboring nucleosomes and adds to the total DNA associated with each repeating unit, which is why chromatin spacing is often closer to roughly DNA 160–240 bp per nucleosome depending on cell type.
Nucleosomes compact enormous genomes, but they also regulate access: remodeling complexes and histone modifications can make particular DNA regions easier or harder for transcription, replication, and repair proteins to reach.