270 



NUCLEIC ACIDS AND GROWTH 



means of the Feulgen reaction have come to the conclusion that these discrepan- 

 cies are less a matter of technique than of materials : as pointed out by Fautrez and 

 his co-workers (1953, 1954), by Leuchtenberger (1954) and by Dalcq and Pasteels 

 (1955) ^^ recent papers, the synthesis of DNA does not necessarily always take 

 place at the same stage of the mitotic cycle, but the point may be different for 

 various tissues (Leuchtenberger, 1954). 



A different and interesting approach to the same problem has been made by 

 Felc and Howard (1952) who studied the incorporation of ^^P into DNA of bean 

 root cells with an autoradiography technique. These experiments have led to the 

 following main conclusions : DNA synthesis occurs during the first part of inter- 

 phase and there is neither synthesis nor loss of DNA in differentiating cells, which 

 can no longer divide. It is well worth adding that Felc and Howard (1952) also 

 studied the incorporation of inorganic ^-^S into the proteins of the same cells and 

 found that protein synthesis in the nuclei occurs at approximately the same time as 

 DNA synthesis. It would certainly be interesting to repeat these observations on 

 other material, in order to find out if they have general significance. 



Recent work by Stevens et al. (1953) and Daoust et al. (1954), related to the 

 same problem, has also produced much discussion: on the basis of extensive 

 mitotic counts, measurements of incorporation of -^^F into DNA and a number of 

 calculations, they concluded that twice as much DNA is synthesized as is needed : 

 therefore, in order to explain their results, they made the assumption that the 

 preexisting DNA is broken down at the time of mitosis : this means that the parental 

 DNA ^^F atoms are completely replaced during division. 



Such conclusions are in opposition to the hypothesis expressed by Watson and 

 Crick (1953a, 1953b) on chromosome reduplication: as we have already seen, 

 these authors suppose that DNA is formed of two complementary strands which 

 can unwind and separate, each serving as a template for the formation of its com- 

 plement (see Fig. i, p. 262) . In this case, DNA should be metabolically very stable : we 

 have seen that this has usually been found to be the case, except for the previously 

 mentioned claims of Daoust et al. (1954) and Stevens et al. (1953). Although it is 

 too early to draw final conclusions, it does not seem that the work of Daoust and 

 Stevens really carries conviction: it has been seriously critized on technical 

 grounds by Hotchkiss ( 1 955b) and contradictory results have recently been pub- 

 lished by Kihara and Sibatani (1955). According to these Japanese workers, 

 there is no turnover of DNA during growth, whether DNA is being synthesized 

 or not, and they obtained no evidence that preexisting DNA disappears during 

 mitosis. 



If, as it now seems, we can disregard Daoust and Stevens et al.'s conclusion, 

 there is no doubt that Watson and Crick's (1953a, 1953b) model of DNA structure 

 and their hypothesis of chromosome reduplication remains exceedingly useful. 

 It should be added, however, that the hypothesis presents a few weaknesses and 

 that attempts have already been made by Delbriick (1954) and by Dekker and 

 Schachman (1954) to improve Watson and Crick's (1953a, 1953b) scheme. 

 However it is too early to discuss further these hypotheses, since their value has 

 not yet been experimentally tested. 



