30 Nature of the Genetic Material 



of this enzyme. But later work did not lead to such simple results 

 (see discussion by Chevrcmont and Firket, 1953). Also, a DNAase 

 was located in the nucleus (but also in the cytoplasm), which again 

 is not very helpful. 



Mazia's conclusions about the problem which is foremost in the 

 mind of the geneticist, the nature of the genetical material in a 

 biochemical sense, are as follows: "The 'physical basis of heredity' 

 is something in the chromosome which may or may not be DNA, 

 but which follows DNA for all the practical purposes of cytogenetics. 

 We cannot be sure that the question 'what is the genetic constituent 

 of the chromosome?' is a meaningful one. The structure of the 

 chromosome as a whole is so astonishingly reproducible that the 

 skillful cytologist can always recognize a given one . . . The regu- 

 larities of chromosome structure may reflect regularities of inter- 

 molecular relationships so decisive that it may prove to be misleading 

 to think in ternis of molecules at all. . . . The fractions that we 

 isolate may be extremely artificial entities, telling us a little about 

 the weak points in the fabric but nothing about the associations as 

 they actually function." 



Mazia discusses the possibility that an unknown fraction (other 

 than the known basic protein) may be combined with DNA for 

 genie function and finds that the quantities needed would easily 

 escape analysis. However, it remains a fact (according to studies 

 with tagged P and N) that DNA is very stable and incorporates 

 these atoms only during mitosis. After reviewing the biochemical 

 evidence, Mazia concludes: "It would then seem that DNA meets 

 a number of our specifications for genetic material. However, it 

 must be remembered that the specifications themselves were neces- 

 sarily extremely crude, simply because we are not yet able to formu- 

 late in chemical language the more sophisticated questions that the 

 science of genetics asks of the gene. If we recall how poorly charac- 

 terized are the chemical constituents of the nucleus and their as- 

 sociations, we will admit that we are scarcely in a position to formu- 

 late the alternatives, much less to propose that one of the known 

 fractions is the genetic fraction." 



It is not probable that these conclusions will have to be changed 

 after Mazia's new work (1954). He succeeded in breaking up the 

 sperm-head chromosomes into tiny segments of nucleoprotein, each 

 about 4,000 A in size. This was done without splitting peptide bonds, 

 rather by treatment with agents (versene or citric acid and distilled 

 water) which do not break ordinary bonds but separate units held 



