2008!

IND-27 Composition of the Nucleosome

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 SDSpolyacrylamide 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. This exercise requires a microcentrifuge, electrophoresis buffers, stains, agarose and three 12 well polyacrylamide gels for 8 groups of students. These chemicals can be purchased separately (See below). The exercise provides a fine introduction to structure and composition of the nucleosome and typical results are shown on the DNA and protein gels at the right.