Pyrimidine Moieties in Animals, Plants, and Bacteria 93 



The variation in composition of DNA molecules among dif- 

 ferent species of microorganisms is very great. C/ostricIium perfringens 

 has only 32 molar per cent (G + C) while at the other extreme 

 of the distribution, Alycobacterium phlei has 73 molar per cent 

 (G + C) (Table II). Fungi vary from 36 to 54 per cent (G + C) 

 in the four species investigated and the two protozoan strains so 

 far measured {Tetrahymena and Euglena) contain 26.5 and 47 molar 

 per cent (G+C), respectively (Table IV). The range of average 

 DNA values among algae is also rather great — 36.9 molar per 

 cent (G+C) for diatomic algae to 64 molar per cent (G+C) for 

 the green alga, Chlamydomonas reinhardi (Table III). The dis- 

 tribution of average values among different species of higher 

 plants (Table III) and invertebrates (Table V) is much narrower: 

 Plant DNA composition varies from 35 molar per cent (G+C) 

 for tobacco leaves to 48.4 molar per cent for Triticum vulgare, and 

 ainong invertebrate species the values vary from 34.9 molar per 

 cent for the echinoderm, Echinocardium cordatum to 44 molar per 

 cent (G + C) for the crab, Carcinus maenas. 



The range of values is very narrow indeed for the average 

 composition of the twenty-three vertebrate DNA species so far 

 examined. The values range from 40 to 44 molar per cent (G + C) 

 (Table VI). The range for DNA aniinal viruses appears to be 

 greater than that for the host animal species: 38 molar per cent 

 (G+C) for fowl pox virus and 50 molar per cent (G+C) for the 

 Shope papilloma virus of rabbits (Table VIII). Various insect 

 viruses manifest in their DNA average (G + C) contents of 31.5 to 

 58.7 per cent (Table IX), a range which is also somewhat broader 

 than the compositions of the few insect DNA's so far studied. 



The T-even bacteriophage DNA's have about 35 molar per 

 cent (G + C), a value outside the range of the bacterial host in 

 which the viruses grow {E. coli, 51 per cent (G+C) (Table VII). 

 The DNA from a number of other bacteriophages (T-1, T-3, T-7) 

 and the lysogenic phages (X, P22, salmonella Al) have average 

 molar (G+C) contents which are very similar to those of the 

 bacterial host cells {E. coli. Shigella, Salmonella). 



Base Composition of DNA Strands 



Since a DNA molecule consists of two complementary nucleotide 

 strands of opposite polarity, the question arises as to whether the 



