Staining of Ultraviolet Irradiated Chromosomes by the Gomori Alkaline Phosphatase Technique

When a chromosome segment is selectively irradiated with an ultraviolet micro-beam, the chromosome(s), which normally appear black by medium-dark phase-contrast microscopy, become “pale” in the irradiated region (decrease in refractive index). Previous ultraviolet absorption and Feulgen staining studies indicated that all or most of the deoxyribonucleic acid is lost in this region. After fixation, the irradiated area appears pale with most of the usual staining methods. The residual material in the paled spot, however, can be stained with the Comori alkaline phosphatase technique and is seen to be directly continuous with the nonirradiated segments. With the bright field microscope, there appears to be no decrease or increase in chromosome width. It is concluded that staining by the Gomori technique is independent of the presence or absence of deoxyribonucleic acid. Positive staining of chromosomes by the nonenzymatic peroxide method of Danielli indicated that staining was due to nonspecific precipitation of calcium phosphate rather than to enzymatic activity.