CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 179 



actual weight of DNA per cell, and by cytophotometric methods which 

 provide comparative data in arbitrary units for individual cells. The 

 methods complement one another to some extent. 



Table 7.1 summarizes values obtained by a number of investigators 

 for DNA content per cell in various organisms. There is a trend towards 

 a greater amount of DNA per cell as organisms become larger and more 

 complex, but no significant increase is found in organisms from fish to 

 man. Table 7.1 also lists values for haploid sperm as well as for diploid 

 cells of various tissues; in general, haploid cells have half the DNA con- 

 tent of diploids, and the diploid content is constant from tissue to tissue 

 in each species. Variations which have been reported are too small to 

 allow a definitive judgment of whether or not they are artifacts of the 

 method. 



In what physical state does the DNA exist in chromosomes? The best 

 answer to this question has come from X-ray diffraction studies by 

 Wilkins and co-workers of intact, isolated nuclei and of sperm heads, 

 comparing the results with those from isolated DNAs. The evidence 

 obtained supports the view that the majority, if not all, of the DNA 

 is in the same helical configuration in the chromosome as it is in solu- 

 tion. However, as shown both by X-ray diffraction and by chemical pro- 

 cedures, cellular DNA forms a complex with a particular class of basic 

 proteins — the histones — in all cells except bacteria and certain sperm; 

 in the sperm, a simpler class of proteins — the protamines — have been 

 substituted for histones. 



Recent studies of the DNA-histone complex, DNP, extracted from calf 

 thymus by exceedingly mild methods, indicate that the DNP is an 

 elongate fibrous particle of about 18 million molecular weight, of which 

 8 million is attributable to a single DNA molecule, the rest to many his- 

 tone molecules associated with the DNA in a stable configuration. It 

 appears probable that at least half of the histone is in a helical con- 

 figuration, and that both the DNA and protein extend the entire length 

 of the fiber. Electron micrographs of these fibers shown in Plate III in- 

 dicate a diameter of about 30 A for the nucleoprotein, and about 20 A 

 for the DNA itself extracted from the DNP. Several investigators have 

 proposed that the histones are wound around the DNA double helix, 

 lying in the "deep groove (Figure 1.2) where they can be held by the 

 combined effect of many hydrogen bonds and van der Waal electrostatic 

 forces. There is as yet no critical evidence that this interpretation 

 is correct, but a number of observations, particularly from X-ray dif- 

 fraction studies, do support it. 



Mirsky and Ris estimated that the chromosomes of calf thymus con- 

 tain about 39 per cent DNA, a similar amount of histone protein, and 



