2(»S 



CHAPTER 20 



the emergent beam shows qo regularity when 



it is refracted. But it' the material is com- 

 posed oi macromolecular units and or mo- 

 lecular subunits spatially arranged in a reg- 

 ular manner, then the emergent beam will 

 form an X-ray diffraction pattern. This 

 particular X-ray pattern can be used to 

 identify units and subunits that arc repeated 

 at regular intervals. It is known that each 

 nucleotide in a DNA chain occupies a length 

 of 3.4 A along the chain; this repetition is 

 detectable by the characteristic X-ray dif- 

 fraction pattern it produces — the black spots 

 located symmetrically near the upper and 

 lower edges of both photographs in Figure 

 20-3. 



X-ray diffraction patterns have been ob- 

 tained from the DNA of numerous species. 

 In some cases the DNA was not removed 

 from the nucleus; in other cases it was re- 

 moved and also separated from the nucleo- 

 protein protein. The spacings between DNA 

 parts, and hence the X-ray diffraction pat- 

 terns, depend upon the degree to which the 

 DNA is hydrated. In all cases, provided 

 that the DNA is similarly hydrated, essen- 

 tially the same patterns attributable to DNA 

 are found (Figure 20-3). In addition to 

 the 3.4 A repetition, a study of these com- 

 mon patterns shows other repeated units 

 which can be explained only if DNA does 

 not usually occur as a single strand. (On 

 the other hand, X-ray diffraction studies 

 show that chromosomal RNA usually is 

 single-stranded. ) Here then is clear evi- 

 dence for the existence of a secondary 

 structure to DNA normally found in all 

 chromosomes. (We might infer some sort 

 of secondary organization for DNA from the 

 independent observation of the equivalences 

 A = T and G = C.) The simplest explana- 

 tion consistent with the diffraction results 

 of M. H. F. Wilkins and co-workers was 

 proposed by J. D. Watson and F. H. C. 

 Crick (1953a). They hypothesized that 

 DNA is normally two-stranded (Figure 



20-4 ) ; each strand being a polynucleotide, 

 and the two strands coiled around each other 

 in such a manner that they cannot be sepa- 

 rated unless the ends arc permitted to re- 

 volve. This kind of coiling is called plecto- 

 nemic (coiled like the strands of a rope) in 

 contrast with paranemic coiling, which per- 

 mits the separation of two coils without 

 revolving their ends (just as two bedsprings 

 pushed together can be separated). 



The Watson-Crick model for the second- 

 ary organization of DNA macromolccules 

 involves a double helix in which each strand 

 is coiled right-handedly (clockwise). This 

 coil direction is the same as that found in 

 the secondary structure of amino acid chains. 

 polypeptides. The model shows the pentose- 

 phosphate backbone of each strand on the 

 outside of the spiral (comprising the rib- 

 bon), whereas the relatively flat organic 

 bases projecting into the center (as bars) lie 

 perpendicular to the long axis of the fiber 

 (indicated by a vertical interrupted line). 

 The backbone completes a turn each 34 A. 

 Since each nucleotide occupies 3.4 A along 

 the length of a strand, 10 nucleotides occur 

 per complete turn, and each successive nu- 

 cleotide turns 36° in the horizontal plane 

 (so that 10 nucleotides complete the 360° 

 required for a complete turn). 



The two helices are held together by 

 chemical bonds between bases on different 

 strands. The two strands can form a reg- 

 ular double helix with diameter uniformly 

 about 20 A only if the bases on different 

 strands join in pairs, each pair composed 

 of one pyrimidine and one purine. A pair 

 of pyrimidines (being single rings) would 

 be too short to bridge the gap between back- 

 bones, whereas two purines (being double 

 rings) would take up too much space. 

 Moreover, it is found that the pyrimidine- 

 purine pairing must be either between C and 

 G or between T and A, for only in this way 

 is the maximum number of stabilizing bond- 

 ages between them produced. The type of 



