254 



HEINZ SCHUSTER 



virus (3000 A.), is 33,000 A. A nucleic acid chain of 2 X 10 6 molecular 

 weight conforms with this length, assuming a normal spacing of about 5 A. 

 between P atoms along the helix. TMV-RNA occurs, therefore, as a single 

 chain with an "open" arrangement of its bases. This is in contrast to DNA 

 in which every purine base is coupled via H bonds to a pyrimidine base in 

 the complementary helix. 44 It is unlikely that H bonds may link bases from 



Fig. 2. Schematic representation of a single-strand ribonucleic acid helix of 40 A. 

 radius in position within the TMV rod. The spacings of the purine and pyrimidine 

 bases, ribose rings, and phosphorus atoms with respect to the helix are made approxi- 

 mately to scale; # = phosphorus, O = sugar, | = bases. [W. Ginoza, Nature 181, 

 958 (1958).] 



44 J. D. Watson and F. H. C. Crick, Nature 171, 737 (1953). 



