148 BIG MOLECULES 



The celebrated French work on the transplanting of DNA in ducks seems to 

 open the doorway to studies on higher animals. The long extrapolation to 

 humans may turn out to be correct, although it is certainly not yet justified, 

 for this will take generations to prove. 



Bacterial transformation: If pure DNA, extracted from a suspension of bac- 

 teria of one type (A) is added to a suspension of another type (B), the 

 progeny of the thus-infected B type have characteristics of A. 



Virus reproduction: Bacteriophage T 2 , a virus, which can reproduce only 

 after it has entered into a living bacterial cell, can be split — the protein part 

 from the nucleic acid part (DNA). The DNA, shorn of its protein, can enter 

 the bacterial cell and rapidly reproduce the intact T 2 phage particles again. 



Virus "synthesis'''': Tobacco mosaic virus can be split chemically into pro- 

 tein + RNA. One can then reconstitute the virus, using protein of strain A 

 and RNA of strain B. The progeny are of strain B only, having resnythesized 

 their original protein. 



Genetic recombination of bacteria: In fertile strains of bacteria, in which DNA 

 can be passed from the donor to recipient cells, the extent of the appearance 

 of the characteristics of the donor in the progeny is proportional to the 

 amount of DNA transferred. 



Some Properties of DNA and RNA 



These "nucleic" acids (found in the cell nucleus and in the cytoplasm) 

 are substituted sugar molecules which are polymerized through phosphate 

 linkages. In DNA the sugar is desoxyribose; in RNA it is ribose. Both have 

 5-carbon rings. The substituent groups on the sugar molecules are organic 

 nitrogen bases. These are ringed compounds with two nitrogen atoms in the 

 ring, and are four in number: adenine, guanine, cytosine, and thymine (in 

 DNA) or uracil (in RNA). Linkages, etc., are shown in Table 6-3. 



From X-ray diffraction studies it is known that DNA is a helical molecule 

 with 10 sugar residues per turn of the helix. In the "dry" (70 per cent RH) 

 crystalline state two helices are found interlocked (Figure 6-11), each with 

 its phosphate-sugar chain facing to the outside, and the purine and pyrimi- 

 dine bases, hydrogen-bonded together, facing to the inside. f 



At cell division, the two interlocking helices separate, and each repro- 

 duces, probably by a process analogous to crystal growth, as though each 

 helix, separated, acts as a template or a die for the "casting" operation 

 which forms the new molecule. That this occurs at mitosis, suggests that 

 the helices are pulled apart by a force which exists only at mitosis. For 

 instance if two ends, one from each helix, are attached to the membrane 

 which encloses the nucleus, in the expansion before division (25 per cent by 

 one measurement) the DNA helices could be pulled apart; then if each 

 template reproduces its opposite by "condensation," two DNA molecules 



| A single-stranded DNA is known, in phage <pX 174. 



