THE CHEMICAL NATURE OF GENETIC MATERIAL
There can be little doubt that the major share of the heredity of a strain is carried in the chromosomes of its germ cells. This conclusion is based firmly on the observations of the cytologist and the geneticist that specific mutations may be directly related to localized morphological changes in chromosomes. It serves as the starting point for one of the most thriving enterprises in modern biological research, namely, the identification and chemical description of genetic material. Progress has been rapid and we can now state with some assurance that the substance most directly associated with the storage and perpetuation of hereditary information is the deoxyribonucleic acid (DNA) of the chromosomal strands.
Aside from the circumstantial evidence furnished by the cytochemical localization of DNA in chromosomes, there are a number of other lines of evidence which give more explicit information bearing on this idea. It was demonstrated by Boivin, Vendrely, and Vendrely1 in 1948, for example, that the DNA content of somatic cells (diploid) was constant from tissue to tissue in a single species but that sperm cells (haploid) contained exactly half as much.