text: The nucleosome core particle (shown in the figure) consists of about 146 base pair of DNA wrapped in 1.67 left-handed superhelical turns around the histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4. Adjacent nucleosomes are joined by a stretch of free DNA termed linker DNA (which varies from 10 - 80 bp in length depending on species and tissue type).The whole structure generates a cylinder of diameter 11 nm and a height of 5.5 nm.
  Nucleosome core particles are observed when chromatin in interphase is treated to cause the chromatin to unfold partially. The resulting image, via an electron microscope, is "beads on a string". The string is the DNA, while each bead in the nucleosome is a core particle. The nucleosome core particle is composed of DNA and histone proteins.
  Partial DNAse digestion of chromatin reveals its nucleosome structure. Because DNA portions of nucleosome core particles are less accessible for DNAse than linking sections, DNA gets digested into fragments of lengths equal to multiplicity of distance between nucleosomes (180, 360, 540 base pairs etc.). Hence a very characteristic pattern similar to a ladder is visible during gel electrophoresis of that DNA. Such digestion can occur also under natural conditions during apoptosis ("cell suicide" or programmed cell death), because autodestruction of DNA typically is its role.