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). When the DNA isolated from these nuclei is electrophoresed on agarose gels, the DNA exhibits a ladder pattern with the length of the DNA in each band representing a multiple of the nucleosomal unit as shown in the Figure below. With this background information, students first examine nucleosomal DNA from calf thymus that is given in this chemical package. They then prepare nuclei from wheat germ, incubate them with micrococcal nuclease and examine the ladder pattern in the DNA by electrophoresis. This exercise requires two 2-3 hour laboratory periods.