12 I. LESLIE 



gations of Brachet^^ and Caspersson,^^ ^nd it is discussed at length in Chap- 

 ter 28. A recent report that the PNA/DNA ratio does not vary during the 

 secretory cycle of the pancreas^^ apparently complicates the situation. 



Because DNA is located in the nucleus and is present in constant amount 

 in each diploid cell, it follows that the concentration of DNA-P per unit of 

 fresh weight will be high in tissues with little cytoplasm and extracellular 

 material and low when the proportion of cytoplasmic substance is relatively 

 large. On this basis, the proportion of chromosomal material in the cells can 

 be seen to decrease according to the following scale: thymus (about 250 

 /xg. DNA-P/100 mg.), lymphoid tissue (180 /xg.), rat bone marrow (140 ng.), 

 small intestine (130 )ug.), spleen (100 ng.), leucocyte (95 ng.), lung (75 ng.), 

 submaxillary gland (50 /xg.), pancreas (45 ng.), liver and kidney (30 ng.), 

 heart, testis, and seminal vesicles (25 /xg.), and brain and skeletal muscle 

 (10 Mg- or less). The thymolymphatic system forms a distinct group charac- 

 terized by high concentrations of DNA (above 90 Mg-), whereas tissues 

 which are actively functioning in a synthetic, mechanical, or nervous capac- 

 ity all have low concentrations of DNA-P (50 Mg- or less) and relatively 

 large amounts of cytoplasm. 



The concentrations of PNA-P in the tissue per unit of fresh weight are 

 distributed in a way which cuts across one of the above divisions. In this 

 case the group wdth high PNA-P concentrations (i.e., above 50 Mg- PNA-P/ 

 100 mg. fresh weight) includes liver, pancreas, small intestine, and the cells 

 of the reticulo-endothelial system. Denuc^** has shown that the concentra- 

 tion of PNA is much higher in the secretory than in the storage portion of 

 the silk glands of Bomhyx mori. The tissues with the lowest PNA-P con- 

 centrations include the group of active "mechanical" organs, namely, kid- 

 ney, heart, and skeletal muscle. Within this last group Mandel reports that 

 heart tissue from a variety of species always has a higher PNA concentra- 

 tion than the corresponding, and less active, skeletal muscle.^^ Mandel and 

 his colleagues have also reported values for the concentration of nucleic 

 acids in milk, blood, and pi asma.*^^ ■''•*' 



Any interpretations on the basis of concentration per unit weight of tis- 

 sue are complicated by the differences in the distribution and activity of 

 intracellular PNA. In rat liver cells, 85 to 90 % of the PNA is located in the 

 cytoplasm, the remainder being in the nucleus (Table VII, and Chapter 21). 



8' J. Brachet, (a) Enztjmologia. 10, 87 (1941); (b) Arch. biol. (Liege) 53, 207 (1941). 



*" T. Caspersson, (a) Chromosoma 1, 562 (1940); Nahirwissenschaften 29, 33 (1941). 



" M. M. Daly and A. E. Mirsky, J. Gen. Physiol. 36, 243 (1952). 



8* J. M. Denuc6, Biochim. et Biophys. Acta 8, 111 (1952). 



*^ P. Mandel, Exposes ann. biochim. med. 13, (1951). 



88 (a) P. Mandel and R. Bieth, Compt. rend. soc. biol. 142, 234 (1948) ; (b) P. Mandel, 



P. Metais, and R. Bieth, Compt. rend. soc. biol. 142, 1022 (1948) ; (c) P. Mandel and 



P. Metais, Compt. rend. soc. biol. 142, 241 (1948). 



