218 ROBERT L. SINSHEIMER 



Benzer selected groups of mutants in each cistron and using the property 

 of selective plating of recombinants on K12 (X), precisely mapped these 

 mutants. The smallest, nonzero, recombination frequency observed was 

 2 X 10™ 2 map units. Several mutants showed no recombinants when 10~ 6 

 map units would have been detectable and presumably are recurrent mu- 

 tations at the same site. Other mutants, which were presumably deletions 

 of a significant portion of the map, produced no recombinants at all with 

 all mutants known to be located within a certain map range, but recombined 

 with mutants outside this region. 



If it is assumed that the DNA complement of T4, 200,000 nucleotide 

 pairs, corresponds to 400 map units, then each rll B cistron, about 3 units, 

 would contain 1,500 nucleotide pairs (molecular weight around 2 X 10 6 ). 

 The smallest nonzero map distance, 2 X 10~ 2 map units, is then about 

 10 nucleotide pairs. 



The size of a mutation, then, can be estimated in various ways, such as 

 deviations from the additivity of map distances between very close mutants. 

 From such necessarily uncertain calculations, the maximum size of any 

 mutant which is observed to revert (thus excluding large deletions) is 

 estimated to be 0.05 map units (25 nucleotide pairs or less). The large dele- 

 tions can amount to several thousand nucleotide pairs. These values of 

 the size of a map unit or a mutation are probably to be regarded as upper 

 limits. 



A similar detailed analysis of the "h" region 139 has indicated that it is a 

 single cistron encompassing about 2 recombination units. 



(2) Ultraviolet Irradiation Studies. Much evidence indicates that ultra- 

 violet damage to phage is localized in the DNA. The cross section for 

 phage inactivation is nearly the same immediately after adsorption and 

 injection of DNA into the host cell as it is for free phage particles. 4089 

 The action spectrum for inactivation of T2 parallels the ultraviolet absorp- 

 tion curve of DNA. 140 Phage inactivation is subject to photoreactivation 

 which has been shown to be a property of free nucleic acid 46 (see Chapter 

 30). 



Therefore, it is pertinent that ultraviolet irradiation can destroy genetic 

 loci in the sense that after irradiation they will not appear in the progeny of 

 a mixed infection with unirradiated phage (i.e., no cross-reactivation). 50 

 Unlinked or weakly linked markers (separation greater than 12-20 recom- 

 bination units) are inactivated independently. Linked markers are inacti- 

 vated simultaneously more frequently than would be accountable by 

 chance. These results accord with the hypothesis that genetic loci consist of 



139 G. Streisinger and N. C. Franklin, Cold Spritig Harbor Symposia Quant. Biol. 21, 



103 (1956). 

 »° M. R. Zelle and A. Hollaender, J. Bacteriol. 68, 210 (1954). 



