Chapter 14 

 THE NATURE OF AND CHANGES IN GENES 



Some Properties of Genes 



Since the early days in the study of genetics there has been 

 much speculation upon the physical and chemical properties of 

 genes. Various suggestions have been offered as to their nature, 

 and approximations of their size and chemical constitution have 

 been hypothesized from different lines of evidence. In pre- 

 vious chapters certain generally recognized facts have been 

 brought out. Some additional facts in regard to the nature and 

 behavior of genes are discussed in Chapters 15 through 22. 



A prevalent view of the relation of genes to chromosomes has 

 been well stated by Demerec. The fundamental part of the 

 chromosome is the long fiber-like chromonema. This "thread," 

 which extends the entire length of the chromosome, appears to 

 be the same chemically and physically throughout its length. 

 Furthermore, the chromonema of one chromosome is considered 

 to be identical in nature with the chromonemata of the other 

 chromosomes in the same cell. Since a chromonema appears to 

 be homogeneous, it must be concluded that the genes do not 

 form part of the chromonema. Although our knowledge of genes 

 and chromosomes is still in a rather hazy state, the chromonema 

 can be visualized as the backbone of the chromosome, but it 

 can be pictured as having a large number of side "branches" 

 protruding from it and approximately at right angles to it 

 throughout its length. Chemical radicals appear to attach them- 

 selves to these "branches" of the chromonema, and these at- 

 tached radicals may be different chemically from one another. 

 This attached material, therefore, differentiates various sections 

 of the chromosome into different units. These units, which are 

 different from one another chemically and are attached to the 

 chromonemata, appear to be the genes. It is interesting in this 

 connection to note that when the chromosomes of Drosophila 

 are bombarded with X-rays they may be broken at various 



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