The fundamental unit of chromatin, the material of which eukaryotic chromosomes are made. It consists of a core of histone proteins around which are coiled about 160 base pairs of DNA. Consecutive nucleosomes, each 10 nm in diameter, are joined by uncoiled linker DNA, like beads on a string. This string is coiled into a 30 nm diameter solenoid, which undergoes further coiling in the fully condensed chromosome.