The primary level of chromosome structure in eukaryotes occurs when the DNA molecule is wrapped around histone proteins into particles called nucleosomes. Evidence for this “beads on a string” model is derived from nuclease digestion studies. When nuclei are incubated with micrococcal nuclease, the enzyme cleaves the linker DNA between nucleosomes (the string) but not the nucleosomal core DNA (the beads). With this background information, students isolate nuclei from chicken erythrocytes and then prepare nucleosomes from the nuclei by using a simple procedure that employs micrococcal nuclease. They then analyze the resulting nucleosome DNA fragments by agarose gel electrophoresis. They also determine the molecular weights of the five histone proteins associated with the isolated nucleosomes by comparing their migration on SDS-polyacrylamide gels to the migration of prestained standard proteins of known size. Sufficient chemicals are provided so that this experiment can be performed twice by 8 groups of students.