Myron R. Melamed, MD

That nucleic acids can be measured spectrophotometrically in intact, fixed, or viable single cells was demonstrated in a series of publications by Caspersson in the 1930s and '40s, culminating in his now classic monograph [1]. In that paper, Caspersson demonstrated an increase in nucleic acid content in proliferating vs resting cells and malignant vs benign cells. However, although his measurements of ultraviolet absorption were specific for nucleic acids, the absorption maximums for DNA and RNA were too close to be distinguished. Probably, he was measuring differences in RNA, which are more striking than differences in DNA.