140 



CLELAND 



*m>MmHr^' 



under the microscope. Similar correlations have been established in varying 



degrees in many other organisms. 



All this has demonstrated beyond question that the genes are carried in 



the chromosomes and that the laws of the distribution of genes from parent 



to offspring are in reality the laws 

 governing the distribution of the chro- 

 mosomes from one generation to the 

 next. This localization of the genes 

 within the cell is the first important 

 step toward an analysis of the struc- 

 ture and chemical composition of the 

 gene. It enables us in our further stud- 

 ies to concentrate on the chromo- 

 somes. By studying their structure in 

 detail and analyzing them chemically, 

 it may be possible ultimately to un- 

 derstand the true nature of the genes 

 contained in the chromosomes. 



In undertaking this further explora- 

 tion, additional techniques are re- 

 quired, particularly the techniques of 

 biochemistry and histochemistry, sup- 

 plemented by the findings of elec- 

 tron microscopy and X-ray diffrac- 

 tion studies. Already much has been 

 learned by these methods, but this 

 constitutes only a bare beginning of 

 what must be learned before we can 

 have an adequate understanding of 

 the gene and its nature. As a result of 

 biochemical analyses, something is 

 now known of the chemical composi- 

 tion of the chromosomes, and con- 

 venient histochemical techniques have 

 been developed to test for the pres- 

 ence or absence of certain of these 

 compounds. These methods make it 

 possible to treat the chromosomes 

 with various reagents or with enzymes 

 and determine by microscopic obser- 

 vation the effect of such treatments 

 on the structure of the chromo- 

 somes. 



Fig. 5. Photograph of salivary chro- 

 mosome from Sciara, the mushroom fly. 

 Note that the bands vary in thickness 

 and structure. They can be recognized 

 and identified, and the whole sequence 

 mapped, so that, if a structural altera- 

 tion occurs, its presence can be detected 

 and its nature determined by micro- 

 scopic examination. {Courtesy of Dr. 

 Charles W. Metz.) 



