Ill NUCLEIC ACIDS AND GROWTH 265 



III. DNA AND growth: mitosis 



(a) DMA content per cell nucleus 



As already mentioned before, the Feulgen reaction gives unequivocal proof that 

 the chromatin and the chromosomes contain large amounts of DNA; in the 

 chromatin this DNA is associated with basic proteins (usually histones, which may 

 be replaced by the much simpler protamines in the sperm of various species). 

 Mirsky and Ris (1951) have shown conclusively the presence, in the chromosomes, 

 of more complex proteins, often called chromosomine or residual proteins, be- 

 cause they constitute a residue when interphase chromosomes, obtained from 

 broken nuclei, are thoroughly extracted with concentrated NaCl in order to remove 

 DNA and the histones. Apparently associated with these residual proteins also are 

 variable amounts of RNA; this represents 10% of the DNA content in liver cells, 

 but can reach similar concentrations to that of DNA in rapidly growing tissues 

 like wheat germs (Stern and Mirsky, 1952). 



Usually, the Feulgen reaction is positive in the chroinatin of resting nuclei or in 

 the chromosomes only, and there is no evidence, except for the case of unfertilized 

 eggs (which will be discussed in section v, p. 286) , that measurable amounts of DNA 

 exist in the cytoplasm. Since we know, from cytological evidence that chromoso- 

 mes are self-duplicating units, and from classical genetics that they contain the 

 genes, it may be supposed, as was first suggested by Boivin, Vendrely and Vendrely 

 (1948), that all the nuclei of resting cells from the same organism have the same 

 DNA content. In this hypothesis, it is assumed that all cells of a given organism 

 have the same genetic composition, an assumption which is not accepted by all 

 cytogeneticists (see for instance Sengtin, 1947; Pavan and Breuer). Nevertheless, if 

 the Boivin et al. hypothesis is correct, one could expect to find the DNA content 

 approximately constant for each set of chromosomes in the various cells of the 

 organism : for instance, the DNA content of a sperm cell should be one-half of 

 the amount found in a diploid somatic cell. 



This question of DNA constancy has been the subject of several recent reviews 

 (Swift, 1953; Alfert, 1954; Allfrey et al., 1955; Vendrely, 1955), which should be 

 read by all those further interested in the matter. In the writer's opinion, the 

 question is still controversial. Early work by Vendrely and Vendrely (1948) and 

 by Mirsky and Ris (1949), later confirmed by Leuchtenberger et al. (195 1), con- 

 sisted of an analysis of the DNA content of a given number of isolated cells {e.g. 

 sperm, liver cells, erythrocytes, leukocytes) : these experiments have clearly shown 

 that the DNA content of a haploid sperm cell is approximately one half the value 

 obtained for somatic cells. 



Difficulties arise, however, in the case of the latter, because not all somatic cells 

 are always diploid: in liver, for instance, a relatively large proportion of polyploid 

 cells is to be found. In order to study these more complex cell types, extensive use 

 has been made of quantitative microphotometric observations of the Feulgen- 

 staining material, as a measure of DNA content (Ris and Mirsky, 1949). Follow- 

 ing Ris and Mirsky's lead, many cytologists have performed a large number of 

 DNA estimations in several types of nuclei by this method (reviews by Swift, 1953; 

 Alfert, 1954; Allfrey et al., 1955). There is, unfortunately, wide disagreement in 



Literature p. 2gg 



