50 MAURICE J. BESSMAN 



is composctl of 100% alternating deoxyadenylate-deoxythymidylate 

 residues. 



Another interesting experiment of Trautner et al. (19()2) was the 

 preparation of a product from a reaction mixture primed with K. coll 

 DNA in which 100% of the dTMP residues were rcphiced by 5-bronio- 

 deoxyuridyhite (Baldwin et al., 1961). A nearest neighbor analysis of 

 this DNA revealed that its dinucleotide sequences were indistinguishable 

 (±2%) from those in normal E. coli DNA. These results are not in 

 keeping with a report of Shapiro and Chargaff (1960), based on the 

 analysis of an acid-hydrolyzed product that marked changes in sequences 

 in E. coli DNA result from substitution of 5-bromouracil. 



The general conclusion that may be drawn from the studies on the 

 incorporation of base analogs into DNA is that, in vitro, purine and 

 pyrimidine bases supplied as deoxyribonuclcoside triphosphates usually 

 are incorporated into DNA at positions predicted by the base-pairing 

 relationships of the Watson-Crick model. This does not mean that these 

 same analogs supplied as free bases, deoxyribonucleosides, or deoxyribo- 

 nucleotides will be incorporated into DNA in vivo since these compounds 

 may be excluded by being blocked at any one of the several steps leading 

 to the deoxyribonuclcoside triphosphates (Kalle and Gots, 1961 ; Fried- 

 kin and Kornl)crg, 1957; Bessman et ol., 1958b). Another interesting 

 method of exclusion is based on the specific hydrolysis of the deoxynu- 

 cleoside triphosphates by enzymes such as dCTPase (Zimmerman and 

 Kornbcrg, 1961; Koerner et al., 1960) and dUTPase (Bertani et al., 

 1961; Greenberg and Somerville, 1962a). These two enzymes are pre- 

 sumably involved in keeping cytosine out of the DNA of T-evcn 

 bacteriophages and uracil out of E. coli DNA, respectively. 



V. Synthesis of DNA by Calf Thymus Polymerase 



There have been numerous reports of the synthesis of DNA in vitro 

 catalyzed by extracts prepared from higher organisms. With the excep- 

 tion of veiy few, these have been done with imfractionated preparations 

 which contain several activities related and unrelated to the direct 

 reaction. These investigations, although important in establishing an 

 over-all picture of the fate of labeled nucleic acid precursors in cell 

 homogenates cannot provide the detailed information necessary to define 

 the pathways and mechanisms of DNA synthesis. Bollum (1959a,b, 

 1960a,b) has investigated the synthesis of DNA using a partially puri- 

 fied enzyme from calf thymus glands. His experiments will be discussed 

 as representative of the properties of DNA synthesis in animals although 

 the generalization may not be applicable to all systems. No obvious 



