'■^^.^ W ■.•>^ 'f '■■' '1 



(Platell, Gross, Malkln and Hubbard, /. Mol. Biol. 7.?, 463, 

 1965; reproduced with permission of Academic Press.) 



Fig. 8. 



(Plate III, Gross, Malkin and Hubbard, /. Mol. Biol. 13, 

 463, 1965; reproduced with permission of Academic 

 Press.) 



They appear to be assembling complete ribo- 

 somes up to the very end of oogenesis. Now it 

 is interesting that when one plots specific 

 activities, determined after careful registra- 

 tion of counts and optical density for each 

 fraction, one gets the sort of pattern shown 

 by the heavy line in Fig. 9. There are several 

 things that could give rise to deviations from 

 constancy of specific activity in the manner 

 shown. If the counts really represent what's 

 present in bulk, then, of course, there should 

 be no deviations from constancy. Now one 

 possibility is that some extra counts are present 

 throughout the gradient; i.e., that there is not 

 complete coincidence between the optical density 

 and the radioactivity. In that case, the pattern 

 obtained will be of the type with maxima at the 

 positions of the optical density minima. 



There are two other possibilities, both of 

 them representing technical errors: (a) that 

 some highly radioactive bacterial RNA is pres- 

 ent as a contaminant which would sediment 

 slightly out of coincidence with the sea urchin 

 RNA because the sea urchin species sediment 

 at 18 and 28S, whereas the bacterial RNA sedi- 

 ment at 16 and 23S. On the other hand (b), per- 

 haps for some unknown technical reason, we've 

 failed to register the counts and optical densi- 

 ties accurately. In neither case would the 

 pattern of deviation from constancy of specific 

 activity be what is observed. A simple periodic 

 function ratio shows that the pattern obtained 

 would be one of constantly varying deviations 



across the peak, but no minima under the peak 

 of optical density. These functional points are, 

 however, less important than the fact that con- 

 stancy of specific activity across the ribosomal 

 density peaks is in fact obtained when the RNA 

 is labeled late in development at a time when 

 ribosomal RNA synthesis predominates. This 

 was demonstrated on an earlier slide. Since 

 these materials are treated and analyzed in the 

 same way as those obtained from the labeled 

 unfertilized eggs, there seems to be no doubt 



SP. ACT. 

 X 10"^ 

 ■50 



40 



--30 



--20 



10 15 20 



FRACTION NUMBER 



Fig. 9. 



25 



