Percent satellite DNA as a function of tissue and age of mice

A selective loss of satellite DNA was found to occur to different extents as a function of tissue and age of mice using several common DNA extraction and purification procedures. This result emphasizes a serious problem that may be encountered in comparative studies of DNA structure and composition if selective loss of specific DNA sequences occurs. We have developed a DNA extraction and purification procedure that is simple and reliable and gives a high percent DNA yield, which substantially reduces the selective loss of heterochromatin DNA sequences. The method features a centrifugation step of a proteolytic digest of chromatin in 2.4 M CsCl. Percent DNA yield of 82-98% are routinely obtained with no apparent loss of satellite DNA sequences from different tissues or ages of mice. Utilizing this method, percent satellite DNA was found to remain essentially constant at 11 +/- 1% for spleen, kidney, and brain tissues obtained from mice of 10-780 days of age. However, for liver, percent satellite DNA remained at about 7-8% from 10 to 300 days of age and then increased to about 12-13% from 300 to 600 days of age. During this latter time interval (300-600 days), an increase of DNA per nucleus of about 3-fold occurred, due to the formation of tetra- and octaploid cell types. A steady loss in the total number of nuclei per gram of liver as a function of age was also found. These two opposing effects resulted in a nearly constant amount of DNA per gram and per organ for liver throughout the lifespan of the mouse.