142 CLELAND 



tute, however, a relatively small proportion of the chromosome, the nucleo- 

 proteins accounting for the bulk of its material. 



The question of first importance in this study of chromosome chemistry 

 is what part of the chromosomal structure constitutes the gene? Genes show 

 great diversity in their functional activities. Not only are there thousands 

 of different kinds of genes in a single organism, doing thousands of different 

 things, but the genes in different organisms differ, at least in part, from each 



Fig. 7. Diagram to illustrate the Watson and Crick 

 hypothesis of nucleic acid structure. Two nucleic acid 

 molecules twist around each other with the sugar and 

 base portion of their nucleotides extending horizontally 

 inward. The nucleotides of opposite nucleic acid mole- 

 cules are attached to each other at their inner ends in 

 a very precise manner, thus constituting the "rungs" of 

 the twisted ladder. 



other. The total number of different genes in all the species of plants, ani- 

 mals, and microorganisms is undoubtedly very great. What part of the chro- 

 mosome is capable of existing in such myriad forms? Apparently not the 

 histones, for they are relatively simple structures; probably not the globular 

 proteins, which constitute such a small proportion of the chromosome. The 

 suspicion falls on the nucleic acids which recent students have shown are 

 capable of an enormous variety in their structure. This surmise is strength- 

 ened by the finding that self-duplicating bodies in the cell seem all to contain 

 nucleic acid: since one of the chief characteristics of the gene is its ability to 

 reproduce itself, this suggests that nucleic acid is tied in with its structure 

 in some way. It is probable, however, that not all nucleic acid is genie in 

 character. Some organisms, such as members of the lily family, for instance, 

 have relatively enormous chromosomes, and the chromosome set has a rela- 

 tively huge amount of nucleic acid compared with such organisms as the fruit 

 fly or man. It is not likely, however, that the lily has more genie material 



