the deoxyribonucleic acid content of the nucleus 171 



2. Possible Variations of the DNA Content of the Nucleus 

 a. DNA and Polyteny 



Schrader and Leuchtenberger^^ have reported in the plant Tradescantia 

 different vakies for the DNA content, determined by photometry, of the 

 interphase nuclei of the root tip (5.5 in arbitrary units), leaf (9.0), and bud 

 tapetum (12.0), although the number of chromosomes is the same for all 

 these tissues. This surprising result can be explained, as these authors point 

 out, by the occurrence of polyteny in such tissues. The chromosomes are 

 composed of a number of threads (chromonemata) ; if each thread carries a 

 definite amount of DNA, the reduplication of these chromonemata (poly- 

 teny) must lead to an increase of the DNA content of the nucleus without 

 any increase in the number of chromosomes. The results of Swift" suggest 

 that a process of synchronous reduplication in all the chromosomes is con- 

 cerned (endomitosis), for the values found are exact multiples of the diploid 

 amount in nondividing tissues. But, from the figures of Schrader and Leuch- 

 tenberger^^ and of Bryan^" on Tradescantia, and of Huskins and Steinitz^^ 

 on Rhoeo, it appears that the process involved in plants must be a partial 

 reduplication of the chromosomal set. 



The process of reduplication of chromonemata occurs very strikingly in 

 the well-known giant chromosomes of the salivary gland of Drosophila. 

 Kurnick and Herskowitz^^ have found in Drosophila salivary gland nuclei 

 DNA values which vary progressively from 0.56 to 71.2 pg. The limb anlage 

 cell nucleus selected as a suitable diploid nucleus for the determination of 

 the base value in Drosophila contains about 0.17 pg. of DNA. These results 

 have been obtained by photometry using methyl green for the determina- 

 tion of DNA. This 420-fold increase in DNA content in the largest salivary 

 gland nuclei would represent 840 chromomonemata per double salivary 

 chromosome.*^ 



The findings of Moses** on Paramecium caudatum give precise information 

 on the chemical nature of the two nuclei of these Protozoa: the macronu- 

 cleus and the micronucleus contain similar concentrations of DNA, PNA, 

 total protein, and nonhistone protein, but the macronucleus carries a mul- 

 tiple (of the order of 40X) of the DNA of the micronucleus, which would 

 contain the diploid amount of genetic elements. 



'8 F. Schrader and C. Leuchtenberger, Proc. Natl. Acad. Sci. U. S. 35, 464 (1949). 

 '^ H. H. Swift, Proc. Natl. Acad. Sci. U. S. 36, 643 (1950). 



80 J. H. D. Bryan. Chromosoma 4, 369 (1951). 



81 G. L. Huskins and L. M. Steinitz, J. Heredity 39, 34 (1948). 



82 N. B. Kurnick and I. H. Herskowitz, /. Cellular Comp. Physiol. 39, 281 (1952). 



8' E. K. Petterson and M. E. Dackerman have adapted a micromethod for the 

 chemical study of the DNA of Drosophila salivary glands; the amount which they 

 found per cell was 284 pg. DNA. [Arch. Biochem. and Biophys. 36, 97 (1952)]. 



8^ M. J. Moses, /. Morphol. 87, 493 (1950). 



