586 



CONTINUITY OF LIFE 



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Fig. 24-5. The chromosomes from the salivary glands of 

 Drosophila are extremely large and for that reason 

 show their constitution. Note the black discs arranged 

 on a non-staining strand. The genes are thought to 

 lie within the strand in a linear fashion. 



tics of each of the discs in their Hnear 

 arrangement. The most striking observation 

 is that they are remarkably similar to cor- 

 responding chromosomes of neighboring 

 cells. There is considerable evidence now 

 to show that each disc corresponds to a 

 group of genes with respect to location on 

 the chromosome. Whether or not these are 

 actually genes is open to question, although 

 pictures of genes have been published in 

 recent current scientific journals. 



In tlie early chapters of this booK we 

 discussed the gene in connection with the 

 origin of life on the earth. It was stated then 

 that the gene was a nucleoprotein and that 

 it was able to duplicate itself in a remark- 

 ably precise manner. Modern biologists 

 consider it the basic unit of life, and there 

 is considerable evidence to support this 

 view. Even though it is the basic unit of 

 all life, how can these tiny genes in sex 

 cells influence so profoundly the develop- 



ment of a billion-celled organism such as 

 a higher animal? Obviously it must come 

 about by some physico-chemical actions 

 that go on first within the nucleus, then the 

 cell, and ultimately the whole organism. 

 Some light has been tlirown on these prob- 

 lems in the last ten or fifteen years through 

 the study of microorganisms. 



Two American workers. Dr. G. W. Beadle 

 and Dr. E. L. Tatimi, while searching for an 

 organism that would lend itself to this kind 

 of work, came upon the common baker's 

 mold, Neurospora. This mold normally 

 thrives on sugar, salts, and one vitamin, 

 biotin, since it is able to synthesize all 

 otlier compounds that are essential in 

 manufacturing its protoplasm. Beadle and 

 Tatum found that by showering the spores 

 of this mold with x-rays they could produce 

 a large number of varieties of this mold 

 that required much more than the minimal 

 diet. Apparently the genes were changed in 

 some way (mutations) so that they could 

 no longer make possible tlie production of 

 a certain nutrient, say an amino acid or a 

 vitamin, and therefore required that sub- 

 stance in their diet. We know that synthesis 

 of proteins and other constituents of proto- 

 plasm is accomplished through the work of 

 enzymes; furthermore, we know that the 

 steps in synthesis are many and usually 

 complex. These men have been able to show, 

 by using their large numbers of strains of 

 Neiu-ospora, that each one was deficient in 

 its ability to synthesize one substance, that 

 each step in the production of a nutrient is 

 controlled by a single enzyme and that this 

 enzyme is directly controlled by a single 

 gene. Therefore, there is a one-to-one rela- 

 tionship between the gene, the enzyme, and 

 the ultimate biochemical reaction. 



On the basis of this information, let us 

 then explain tlie observations on hair color 

 in guinea pigs portrayed above. The black 

 guinea pig has a gene which is responsible 

 for his coat color. The black color is due to 

 a pigment, melanin, which is formed by the 

 oxidation of a complex chemical nicknamed 



