1 6 LURIA 



Addendum 



THE DISTRIBUTION OF SPONTANEOUS PHAGE MUTANTS 

 FROM INDIVIDUAL BACTERIA 



Information on the kinetics of production of phage material has 

 been obtained by analysis of the distribution of mutants (r and w 

 types) in single bursts of phage T2. 



On the one hand, if each new replica of a genetic determinant were 

 produced independently of others produced in the same cell each mu- 

 tation would give rise to one mutant particle per burst, which would 

 lead to a nonclonal distribution of mutants in individual bursts. On 

 the other hand, if each determinant, after being produced, acts itself 

 as a model for new replicas, the mutants will be present in clones, 

 each clone representing the offspring of one mutation; smaller clones 

 will be more frequent than larger ones, since they stem from later 

 mutations, more frequent because of the presence of a larger population. 



Analysis of 3600 bursts gave 67 bursts containing at least one r 

 or w mutant. The distribution is clearly clonal, with clones of mutants 

 ranging in size from 1 to over 40 individuals. The observed distribu- 

 tion agrees wdth the one calculated, assuming constant probability of 

 mutation per reduplication. The quantitative analysis is not yet com- 

 plete. The uni-mutational origin of each clone was confirmed by 

 crosses of four pairs of r mutants, each pair from one clone, between 

 members of each pair and among different pairs. All intrapair crosses 

 gave no r+ recombinants, all interpair crosses gave several r+ recom- 

 binants, proving the allelism (and probable identity) of members of a 

 clone and the nonallelism of members of different clones. 



The clonal distribution of spontaneous mutants is the more remark- 

 able since in recombination experiments the recombinants of any one 

 type are distributed nonclonally (Poisson distribution, Hershey and 

 Rotman, 1949, and Doermann unpublished). The combined data in- 

 dicate the existence of a phase of autocatalytic replication of genetic 

 material during which genetic recombinations have not yet taken place. 

 It confirms the conclusion that little or no reproduction can occur after 

 recombination. In this respect recombination is a terminal phenom- 

 enon. However, in order to explain the lack of correlation between 

 opposite recombinant types one has to assume that recombination pre- 

 cedes, or is concurrent with, the formation of fully active particles 

 (cf. p. 11). 



