116 FIRST STEPS TOWARDS A CHEMISTRY OF HEREDITY 



space? We are all familiar with the four coordinates that define 

 the space-time continuum. It is curious that in biochemistry the 

 time coordinate is the one most easily defined, since through the 

 use of isotopic tracers the dating of a polymer often can be 

 achieved. But we know very little of the spatial conditions under 

 which the cell manufactures its constituents. It is even pos- 

 sible — and this should not be forgotten in all the talk about 

 ''templates" — that we may be dealing with templates in time 

 rather than space and that as yet undefinable time sequences 

 regulate the replication of cellular polymers. 



What I mean in speaking of the movement of precursors can, 

 perhaps, best be illustrated by an example relating to the for- 

 mation of protein and ribonucleic acid in rat liver^^- ^^ which is 

 shown in Table 25. It will be seen that in experiments of short 



TABLE 25 



UPTAKE in vivo OF RADIOACTIVE LEUCINE BY THE PROTEIN AND OF 



RADIOACTIVE SODIUM PHOSPHATE BY THE RIBONUCLEIC ACID IN CYTOPLASMIC 



FRACTIONS FROM RAT LIVER» 



Relative specific activity^ 



Protein 



Ribonucleic acid^ 



Fraction 



Minutes after injection 



o The values are adapted from recent datai^, i4, 



^ In each column the fraction having the lowest specific activity was taken 



as 1. 



^ The specific activities of the nucleic acid samples were computed through 



the summation of the weighted specific activities of the 2' -f 3' nucleotides 



produced by alkaUne hydrolysis. 



^ This represents the microsome portion insoluble in deoxycholate; the 



protein fraction I is insoluble at pH 13, fraction II insoluble at pH 6 



(Ref. 15). 



