28-8 THE YEAST CELL 



circuitous reasoning. The one -gene, one -enzyme h3rpothesis was the 

 outcome of this theoretical background. It seems simpler and more 

 reasonable to consider the filamentous chromosome as a structure 

 on which catalysts are placed in an orderly sequence so that reactions 

 can occur with one step following the other in its proper order. The 

 fact that the nucleus is set off from the rest of the cell in a non -re- 

 acting hyaloplasm, with the nucleolus also in this isolated region, 

 makes it possible for the specificities of the enzymes to be deter- 

 mined before they are released into the mixed cytoplasm. The ap- 

 parent random positions of "genes" along the chromosome is probably 

 due to insufficient data, and further study should reveal sequences 

 along the chromosome, such that one synthetic step follows another 

 in regular order. 



According to this view, the chromosomes, like so many other 

 biological structures, do not serve a single purpose, but insure both 

 the precise partitioning of the hereditary apparatus at each cell di- 

 vision and the orderly sjmtheses of cellular enzymes. 



UNEAR DIFFERENTIATION AND 

 POSITION EFFECT 



Goldschmidt pointed out that the failure of the classical theory 

 of the gene to account for position effect revealed a fundamental 

 inadequacy of the concept. He argiTed that the chromosome is not 

 divided into corpusular genes, but is fimctional along its entire length 

 and that the chromosome operates as a unit and not as a separate 

 series of discrete, independent genes. His proposal that the chro- 

 mosome rather than independent genes is the functional unit forms 

 the basis for a theory which fits the facts with regard to position 

 effect. The fact that it is possible to demonstrate numerous highly 

 specific loci on the chromosome arranged in linear order is the 

 basis for most of the opposition to Groldschmidt's concept. If the 

 region devoted to the production of a single gene -product were an 

 area through which crossovers seldom occurred and on which a 

 series of transferrable components was located, losses of regions 

 which might produce position effects would occur whenever a slight- 

 ly unequal exchange took place. The deficiency which might result 

 from exchange could be corrected by an exchange of chromosome - 

 components in the heterozygote without involving crossing-over. 

 However, in unequal exchanges involving an inversion between non- 

 homologous chromosomes, repair would not be so readily achieved. 

 This is the condition most frequently associated with position effect 

 and suggests that the transfer of chromogenes occurs at synapsis. 



