THE NUCLEIC ACID CONTENT OF TISSUES AND CELLS 15 



PN/DNA-P (i.e., the average protein content of the cells) are significantly 

 greater in males than females. 



Whatever the agents directly influencing metabolism in the male and fe- 

 male, the possibility exists that these differences between species are re- 

 lated to differences in the nature of the sex-linked chromosomes, between, 

 say, Drosophila (females 2X, males X large Y) on the one hand, and rat 

 (females XX, males X small y) on the other. Although Callan^^ found no 

 change in PNA concentration in females with an extra Y chromosome, it 

 can be seen in Table IV that the concentration of PNA-P per unit of fresh 

 tissue does not show the same consistent trend that is found for the PNA-P/ 

 DNA-P ratio in different sexes. 



IV. Embryonic Development and Postnatal Growth 



Embryonic blood and liver cells of the chick have much higher concen- 

 trations of total nucleotide than the corresponding adult cells, ^^ and in 

 sheep^ tissue the total nucleic acid concentration per unit dry weight is 

 always higher for embryonic than for adult tissue. ^^ Although these facts 

 have been taken to indicate that nucleic acids are associated \Wth the rapid 

 cellular proliferation and protein synthesis characteristic of embryonic tis- 

 sues, changing concentrations during growth are to some extent deceptive.^^ 

 In terms of amounts per cell, it can be seen in Table V^^' ^- that in chick 

 embryo liver and muscle the PNA-P content of the cell actually increases 

 or remains unchanged during a period when the concentration per unit 

 fresh weight is diminishing. The fall in nucleic acid concentration in the 

 cytoplasm consequently results from dilution of PNA by protein or other 

 cell constituents. 



When measurements are made on such a complex system as the whole 

 embryo, interpretations are even more difficult. It has been suggested that, 

 in the developing chick embryo and mouse embryo, the coincidence of high 

 PNA and DNA concentrations with high concentrations of protein support 

 the view that nucleic acids are involved in protein synthesis.^*' *^ Although 

 higher concentrations of PNA are often found in embryonic tissues, this is 

 not invariable (Table V,*' ^^), and they may not always accompany the 

 phase of rapid accumulation of protein. ^^' ^^ 



" H. G. Callan, Nature 161, 440 (1948). 

 92 H. Herrmann, Ann. N. Y. Acad. Sci. 55, 99 (1952). 

 " I. Geschwind and C. H. U,J. Biol. Chem. 180, 467 (1949). 

 ^* T. Caspersson and B. Thorell, Chromosoma 2, 132 (1941). 

 96 J. N. Davidson and C. Waymouth, Biochem. J. 38, 39 (1944). 

 96 A. B. Novikoff and V. R. Potter, /. Biol. Chem. 173, 233 (1948). 

 " D. V. N. Reddy, M. E. Lombardo, and L. R. Cerecedo, J. Biol. Chem. 198, 267 

 (1952). 



98 J. B. Flexner and L. B. Flexner, /. Cellular Comp. Physiol. 38, 1 (1951). 



99 H. Herrmann and J. S. Nicholas, J. Exptl. Zool. 112, 341 (1949). 



