Vol. 41, 1955 GENETICS: S. BENZER 345 



Pontecorvo.-) Such cases are of special interest for their bearing on the structure 

 and function of genetic determinants. 



The phenomenon of genetic recombination provides a powerful tool for separating 

 mutations and discerning their positions along a chromosome. When it comes to 

 very closely neighboring mutations, a difficulty arises, since the closer two mutations 

 lie to one another, the smaller is the probability that recombination between them 

 will occur. Therefore, failure to observe recombinant types among a finite number 

 of progeny ordinarily does not justify the conclusion that the two mutations are 

 inseparable but can only place an upper limit on the linkage distance between 

 them. A high degree of resolution requires the examination of very many prog- 

 eny. This can best be achieved if there is available a selective feature for the de- 

 tection of small proportions of recombinants. 



Such a feature is offered by the case of the rll mutants of T4 bacteriophage 

 described in this paper. The wild-type phage produces plaques on either of two 

 bacterial hosts, B or K, while a mutant of the rll group produces plaques only on 

 B. Therefore, if a cross is made between two different rll mutants, any wild-type 

 recombinants which arise, even in proportions as low as 10 ~*, can be detected by 

 plating on K. 



This great sensitivity prompts the question of how closely the attainable resolu- 

 tion approaches the molecular limits of the genetic material. From the experi- 

 ments of Hershey and Chase, ^ it appears practically certain that the genetic infor- 

 mation of phage is carried in its DNA. The amount of DNA in a particle of phage 

 T2 has been determined by Hershey, Dixon, and Chase^ to be 4 X 10^ nucleotides. 

 The amount for T4 is similar.^ If we accept the model of DNA structure proposed 

 by Watson and Crick,^ consisting of two paired nucleotide chains, this corresponds 

 to a total length of DNA per T4 particle of 2 X 10^ nucleotide pairs. We wish to 

 translate linkage distances, as derived from genetic recombination experiments, into 

 molecular units. This cannot be done very precisely at present. It is not known 

 whether all the DNA in a phage particle is indispensable genetic material. Nor is 

 it known whether a phage "chromosome" (i.e., the physical counterpart of a linkage 

 group identified by genetic means) is composed of a single (duplex) DNA fiber 

 or whether genetic recombination is equally probable in all chromosomal regions. 

 For the purpose of a rough calculation, however, these notions will be assumed to 

 be true. Thus we place the total linkage map of T4 in correspondence with 2 X 10* 

 nucleotide pairs of DNA. The total knoA\ai length of the three linkage groups^ in 

 phage T4 amounts to some 100 units (one unit = 1 per cent recombination in a 

 standard cross). In addition, there is evidence^ for roughly another 100 units of 

 length connecting two of the groups. Therefore, if we assume 200 recombination 

 units to correspond to 2 X 10^ nucleotide pairs, the recombination per nucleotide 

 pair is 10 ~^ per cent. That is to say, given two phage mutants whose mutations 

 are localized in their chromosomes at sites only one nucleotide pair apart, a cross 

 between these mutants should give rise to a progeny population in which one par- 

 ticle in 10^ results from recombination between the mutations (provided, of course, 

 that recombhiation is possible between adjacent nucleotide pairs). This compu- 

 tation is an exceedingly rough one and is only intended to indicate the order of mag- 

 nitude of the scale factor. Some preliminary results are here presented of a pro- 

 gram designed to extend genetic studies to the molecular (nucleotide) level. 



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