Every cultured person today has heard of DNA, RNA and protein synthesis. But nucleic acids were not so well-known in 1927, when Albert Dalcq asked me to study the localization of "thymonucleic acid" in ovarian eggs (ovocytes).
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).
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