2 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



large amounts of RNA as to exclude the possibility that all of this 

 RNA is localized in the cell nucleus. These observations were made 

 on sea urchin eggs, which had been studied by Needham and 

 Needham a little earlier (1930). They found that there is no increase 

 of "nucleic acid phosphorus" during development, although the 

 number of nuclei increases tremendously. They concluded that 

 DNA must be present in large amounts in the cytoplasm of the 

 unfertilized eggs and that it migrates from the cytoplasm to the 

 nucleus when the latter multiplies. But the combined use of the 

 Feulgen reaction and the specific method of Dische for DNA 

 estimation clearly showed that DNA is not present in large amounts 

 in the cytoplasm of unfertilized sea urchin eggs and that there is 

 considerable synthesis of this nucleic acid during development. It 

 was therefore postulated that, during cleavage, DNA is synthesized 

 at the expense of a reserve of cytoplasmic RNA ("conversion" 

 hypothesis) and it could, in fact, be demonstrated that unfertilized 

 sea urchin eggs contain large amounts of RNA (Brachet, 1933). 

 More than 25 years have elapsed since these experiments were 

 made and one may wonder how much truth there was in the ideas 

 presented at that time by the Needhams and Brachet. It is beyond 

 the scope of the present lectures, which deal with the biological 

 role of RNA, to go into the still obscure question of DNA synthesis 

 in developing eggs. The interested reader will find a full account of 

 the problem in the author's two recent books (Brachet 1957, 1960). 

 It can be said, however, that there is no doubt that DNA is syn- 

 thesized during development and that unfertilized eggs contain a 

 RNA store ; but it is now unlikely that RNA is directly converted 

 into DNA and the nature of the precursors used for DNA synthesis 

 in developing eggs remains controversial. Part of the DNA present 

 in the nuclei might come from a small reserve of DNA already 

 present in the unfertilized egg; but it is likely that the main part of 

 the DNA is synthesized at the expense of soluble deoxyribo- and 

 ribonucleosides or nucleotides present in the acid soluble fraction. 

 Many unsuccessful attempts were made by the author, around 

 1935, to detect the localization of RNA in unfertilized eggs with the 

 basic dye methyl green. This dye was said to stain nucleic acids in 



