Selective digestion of mouse chromosomes with restriction endonucleases. II. X-ray microanalysis of HaeIII-treated chromosomes.

We used X-ray microanalysis to study the changes induced in mouse metaphase chromosomes as a result of digestion with the restriction endonuclease HaeIII. The phosphorus X-ray signal was used as a marker for DNA and the sulfur signal for protein. Calcium, iron, copper, and zinc were also detected. HaeIII induced a loss of phosphorus from both the centromeres and chromosome arms, but the losses in the arms were much greater. These changes were accompanied by an increase in the electron density of the centromeres and a reduction in that of the arms. No reduction in the sulfur signal in either arms or centromeres occurred as a result of HaeIII digestion. Except for calcium, which showed only a moderate reduction, the inorganic ions exhibited very large losses as a result of HaeIII digestion. The differentiation of chromosome arms and centromeres as a result of HaeIII digestion is therefore not simply due to differential loss of DNA but also involves structural reorganization of the chromatin, as shown by electron microscopy. This reorganization does not involve loss of proteins but may be correlated with changes in the amounts of inorganic ions known to be involved in chromatin condensation.