KINDS or CITROMOMERES AND GENES 



itself, that is to say, of collecting together and organizing materials 

 in an arrangement like its own. This activity is scarcely conceivable 

 except as involving the organizing of related arrangements which 

 will pass into the cytoplasm. A more complex gene would be a 

 larger structure, its essential unity depending on the insufficiency 

 of its parts to produce fully organized products, although these 

 parts themselves might be simple genes in a reproductive sense. 

 The minimum qualification for a gene, in this complex sense, then 

 shifts from reproduction to action. 



On this view, simpler genes might be the source of materials for 

 more complex genes, which would arise through their components 

 being brought together by structural change in an order or arrange- 

 ment suited for efficient action in building something useful for the 

 cell and for the organism. 



Kinds of Chromomeres and Genes 



What is the evidence for an organization of this kind ? Let us 

 begin on the cy tological side. Chromomeres, which appear according 

 to Caspersson to be units of activity, units of protein production, 

 are of different sizes. Those in the heterochromatin (when they are 

 free) are smaller and are engaged in organizing smaller proteins 

 of histone type. Those in the euchromatin are larger and are pro- 

 ducing larger proteins of globulin type. The smaller chromomeres 

 are less specific and less powerful in their attractions, both in the 

 polytene and the pachytene stages. This may even be the cause as 

 well as the consequence of extensive reduplication of identical 

 genes. Such reduplication, in turn, will scarcely have deleterious 

 effects since the action of heterochromatin is o£ a lower specificity 

 than that of euchromatin. An extra Y chromosome has little 

 specificity of effect in Drosopliila, one of its most noticeable con- 

 sequences being, it is said, the presence of extra ribose nucleic 

 acid in the egg. Similarly, supernumerary chromosomes in maize, 

 which are heterochromatic, may be present to the number of 

 20 or 30 without producing any specific abnormality. One or two 

 such chromosomes, if euchromatic, would on the other hand cause 

 a visible disturbance of growth. Heterochromatin and euchromatin 

 therefore appear to represent two levels of gene integration, two 

 visible stages in the growth of the gene. 



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