186 R. Jeener 



any ^^p appearing in the culture medium. The fact which 

 we beheve important to emphasize is that this decrease takes 

 place at a rate (defined as 1/r. dr/dt) which is almost constant 

 whether the quantity of RNA decreases, remains constant or 

 rapidly increases. We would have expected that the specific 

 radioactivity of the RNA would depend at any instant on the 

 relation existing between the number of newly formed mole- 

 cules and the number of old molecules, i.e. that it would 

 diminish as rapidly as the quantity of RNA would become 

 smaller or as the rate of increase of RNA goes up. 



Two hypotheses seem to be able to account for the fact, at 

 first difficult to interpret, that the decrease of the specific 

 radioactivity of the RNA is independent of quantitative 

 variations of the RNA. We could consider that the rate of 

 renewal of the RNA molecules or of the phosphate groups of 

 their constituent nucleotides is sufficiently high so that the 

 specific radioactivity of the RNA remains at any instant 

 independent of the quantitative variations that it undergoes. 

 But, in this case, the specific radioactivity of the RNA 

 phosphorus should be very close to that of the intracellular 

 inorganic phosphate. This is by no means the case in our 

 experiments. Moreover, the turnover of the nucleic P should 

 be very high, whereas the maximum value ascribable to it is 

 3 to 6 per cent per hour. This fiirst hypothesis being discarded, 

 we do not see any possibility of avoiding the other one, i.e. 

 to assume that the quantity of RNA synthesized at any 

 instant is proportional to the quantity of RNA present, which 

 involves necessarily the consequence that the evolution of the 

 specific radioactivity of RNA is independent of its concen- 

 tration. 



The conclusion to which we are thus led only becomes 

 significant if we remember that the RNA is part of ribonucleo- 

 protein particles. We may consider their autocatalytic multi- 

 plication as possible, if we remember the analogy of the 

 smallest particles and the viruses, and the arguments in favour 

 of the "genetic continuity" of many types of larger and more 

 complex particles, such as mitochondria, plasts, kinetosomes, 



