4.4 S. S. COHEN 



evidence to indicate that this organism contains at most trace amounts of 

 calcium and that the ion is not concentrated in chromosomes to any degree 

 higher than that prevaihng in the cytoplasm of body fluids. In recent studies 

 of the deformational changes effected in chromosomes by ethylenediamine- 

 tetraacetic acid (Kaufmann and McDonald, 1957), it was concluded that 

 alterations in the structural nucleoprotems arose from modifications of 

 the general ionic environment of the cell and not merely from the removal 

 of specific divalent cations. 



d. The Protein and Amino Acid Composition of Nuclei. A number of 

 reviews have appeared on the proteins of the nucleus (Hamer, 1951; Allfrey et 

 al, 1955b; Butler and Davison, 1957). Some of these proteins will be men- 

 tioned below in connection with the chemical dissection of chromatin. 

 Soluble proteins possessing the properties of globulins may be extracted from 

 the nuclei of calf thymus and liver with 0.14 M NaCl. In addition to these 

 poorly defined and relatively unfractionated materials, basic proteins are 

 prominent constituents of nuclei, associated with DNA in chromosome 

 structure. Relatively low molecular protamines, very rich m arginine and 

 devoid of aromatic amuio acids and sulfur-containing ammo acids, are found 

 in sperm. More complex basic proteins or histones are present in the somatic 

 cells of the same animal. The histones contain tyrosine and phenylalanine, 

 but lack tryptophan. Although it is commonly stated that histones contain 

 sulfur, the analyses of Hamer (1951) and of Daly et al (1951) indicate the 

 absence of cystme and methionme. Intermediate types of basic protems have 

 also been described in the sperm of sea urchins, starfish, andsquid (Hamer, 1955). 

 The most recent studies of histones derived from calf thymus nucleohistone 

 suggest that these substances consist of a very complex mixture, whose 

 fractionation is made difficult by their tendency to associate and to aggregate 

 (Butler and Davison, 1957). It has at least been shown that a lysine-rich 

 histone may be separated from an arginine-rich histone in such a mixture. 



Sperm nuclei are presumed to contain a genome which is also found in 

 somatic nuclei, although the latter nuclei contain two chromosome sets in 

 contrast to the haploid sperm. The fact that their nuclear proteuis are 

 qualitatively so different has been considered to point away from a key 

 genetic role of the proteins. In inamature salmon testis, the transformation of 

 somatic nucleohistone to sperm nucleoprotamme was fiirst studied by 

 Miescher (1897) in the 1870's. Nucleoprotamme m the salmon accounts for 

 at least 91 % of the nuclear mass (PoUister and Mirsky, 1946); a replacement 

 of histone by protamine must occur in this change. Alfert (1956) has sliown 

 that this replacement does indeed occur durmg the maturation process, more 

 particularly after completion of meiosis during an advanced stage of spermio- 

 genesis. A comparable result has been obtained for the replacement in bull 

 testis (Vendjely et al., 1957). 



