1962 M. II. F. WI LKI N S 



from cancerous tissues, or in calf thymus DNA separated into fractions of 

 different base composition by my colleague Geoffrey Brown. 



We also made a study, in collaboration with Harriet Ephrussi-Taylor, of 

 active transforming principle from pneumococci, and observed the same 

 DNA structure. The only exception to double-helical DNA so far found 

 is in some very small bacteriophages where the DNA is single-stranded. We 

 have found, however, that DNA, with an unusually high content of adenine, 

 or with glucose attached to hydroxymethylcytosine, crystallized differently. 



DNA Structure is Not an Artefact 



It did not seem enough to study X-ray diffraction from DNA alone. Ob- 

 viously one should try to look at genetic material in intact cells. It was pos- 

 sible that the structure of the isolated DNA might be different from that /'// 

 vivo, where DNA was in most cases combined with protein. The optical 

 studies indicated that there was marked molecular order in sperm heads and 

 that they might therefore be good objects for X-ray study, whereas chromo- 

 somes in most types of cells were complicated objects with little sign of 

 ordered structure. Randall had been interested in this matter for some years 

 and had started Gosling studying ram sperm. It seemed that the rod-shaped 

 cephalopod sperm, found by Schmidt to be highly anisotropic optically, 

 would be excellent for X-ray investigation. Rhine 17 , while making a study 

 of liquid crystals from many branches of Nature, had already taken diffrac- 

 tion photographs of such sperm; but presumably his technique was inad- 

 equate, for he came to the mistaken conclusion that the nucleoprotein was 

 liquid-crystalline. Our X-ray photographs (Wilkins and Randall 18 ) showed 

 clearly that the material in the sperm heads had 3 -dimensional order, i.e. it 

 was crystalline and not liquid-crystalline. The diffraction pattern (Fig. 4) 

 bore a close resemblance to that of DNA (Fig. 5), thus showing that the 

 structure in fibres of purified DNA was basically not an artefact. Working 

 at the Stazione Zoologica in Naples, I found it possible to orient the sperm 

 heads in fibres. Intact wet spermatophore, being bundles of naturally ori- 

 ented sperm, gave good diffraction patterns. DNA-like patterns were also 

 obtained from T2 bacteriophage given me by Watson. 



s-43 



