312 CELL HEREDITY 



The best primer described to date for this system is a single-stranded 

 DN A obtained from the bacterial virus 0X174. When the more usual 

 double-stranded DNA is used as primer, it must be opened up in some 

 manner, for example, by heating to break hydrogen bonds. 



The localization of DNA synthesis on chromosomes has been demon- 

 strated most directly by autoradiographs of cells after treatment with 

 tritiated thymidine. Because of its very weak /3-emission upon disin- 

 tegration, tritium (H'^) provides the best material for localization of the 

 radioactive atoms within cells at the magnification level of the light 

 microscope. With the use of tritiated thymidine, in a wide variety of 

 cell types, the chromosomes have been described as the unique site of 

 DNA synthesis. 



Evidence has been obtained in E. coli for the existence of a regulatory 

 mechanism which controls the time of DNA synthesis. Using a double 

 mutant which requires thymine — a constituent of DNA — and histidine 

 — a constituent of protein — it has been shown that each cycle of DNA 

 replication must be preceded or accompanied by protein synthesis. 



These observations are particularly interesting in relation to a phenom- 

 enon which has been called "thymineless death," and which consists of 

 the exponential death of thymineless cells when incubated in a medium 

 supplying everything necessary for growth except thymine. The cells be- 

 have as if the attempt to make DNA in the absence of thymine were 

 lethal. However, cells can be rescued from thymineless death by inhibit- 

 ing protein synthesis. The regulatory mechanism which blocks DNA 

 synthesis in the absence of protein synthesis also blocks the occurrence 

 of thymineless death. 



Studies of RNA synthesis have not yet provided much definitive in- 

 formation. This class of macromolecules, which are of key importance 

 in protein synthesis and, perhaps, as carriers of primary genetic informa- 

 tion, have thus far eluded the systematic analysis of their synthesis. The 

 fragmentary evidence available must be considered preliminary. 



An enzyme, polynucleotide phosphorylase, which catalyzes the poly- 

 merization of ribonucleotides, was discovered by Grunberg-Manago and 

 Ochoa. It has been isolated from bacteria and other cell types. Sub- 

 strates of the reaction are the diphosphates of the four ribonucleotides 

 found in RNA. If all four nucleotide diphosphates are present, the 

 product will contain them all, but the presence of all four is not obliga- 

 tory for polyribonucleotide synthesis. No primer need be added to this 

 system, but since the enzyme has not yet been fully purified, it is possible 

 that some unidentified constituent of the enzyme preparation is required 

 for the reaction. 



By providing only one nucleotide diphosphate as substrate, it has been 



