GENETICS: THE HERITABLE TRANSMISSION OF CATALYSTS 163 



arginine synthesis the genetic study shows a modification of one gene." 

 Tryptophane, an amino acid essential for humans, is made by 

 Neurospora by condensing serine with indole. The catalyst deficien- 

 cies of certain Neurospora mutants prevent them from making indole; 

 but if indole or tryptophane is supplied, they form in the medium 

 anthranilic acid, the precursor of indole. Still another mutant is 

 unable to synthesize anthranilic acid, but can convert it into indole. 

 "Each strain differs from the original wild-type Neurospora by a single 

 gene, and the two mutant genes concerned can be shown to lie in dif- 

 ferent positions in the chromosomes. ... In the future we may 



Scale f- 



-lo/x- 



•£^V<>V*.>--4* ••'•'tfv-*!-' 





Figure 23. Salivary gland chromosome from Drosophila melanogaster. (From 

 paper in "Colloid Chemistry," Vol. 5, by J. Alexander, Reinhold Publishing Corp., 



N. Y.) 



expect to see increasing use of genetics this way by biochemists." 

 Choline, a carrier of methyl groups to other molecules, and a com- 

 ponent of the important protoplasmic constituent lecithin, is made by 

 first synthesizing a precursor, monomethylethanolamine and then add- 

 ing two additional methyl groups. 



The Salivary Gland Chromosomes 



In 1935 Professor T. S. Painter of the University of Texas ex- 

 amined the cells in the salivary glands of the tiny grub repre- 

 senting the larval stage of Drosophila, and made the important 

 discovery that in these cells the chromosomes are swollen from 

 175 to 200 times their length in ordinary cells. In the extended 

 chromosomes there are great numbers of characteristic bands, 

 which vary with and correspond to the genetic constitution of the 

 individual grub, and therefore constitute a "genetic spectrum." 



