Biochemistry textbooks merely said that there are two kinds of nucleic acids, "animal" and "vegetal." Thymonucleic acid, a typical animal nucleic acid, had a queer sugar residue, which makes it a DNA. Plant nucleic acids, like zymonucleic acid from yeast, contained a pentose residue, a characteristic of our RNAs. Both nucleic acids, as indicated by their names, were located in the cell nuclei—DNA in animal cells, RNA in plant cells. Their function was unknown, but one could exclude a genetic role in view of their low molecular weight. Chemical embryologists displayed limited interest in nucleic acids: Joseph Needham devoted only 14 of 1,724 pages to them in his definitive Chemical Embryology (1931).
R. Feulgen had found in 1924 that mildly hydrolyzed DNA gives a classical aldehyde color reaction, forming a violet compound with fuchsine sulfurous acid. He applied this reaction (the Feulgen reaction) to histological sections and discovered that all ...
