16 THE BIOSYNTHESIS OF PROTEINS 



(chromosome or linear set of genes). Each site can exist in two alternative 

 forms: normal (+) or mutated (— ). 



■//////. ■/ ■//.■'/// ,, 7/////////^ 



Recombination 

 A- B-' 



A- B- 



FiG. 12. 



If the two mutants A+ B~ and A~ B+ are crossed and if recombination 

 occurs in between the two sites, a pair of recombinants will be obtained : 

 A+ B+ and A- B~. Obviously A+ B+, which possesses the two normal sites, 

 is normal ('wild type'). A" B- to the contrary is a double mutant. 



Instead of looking for recombinants between the two strains A+ B~ and 

 A~ B+, let us introduce these genomes within the same cell under con- 

 ditions where there is no recombination (as in heterozygotes, heterocaryons, 

 etc.). Each cell contains a sample of the normal site A+ and a sample of the 

 normal site B+, but these are located on two separate pieces of genetic 

 material. If A and B belong to regions of the genome which can each per- 

 form their function independently of the other, the function of both A+ and B+ 

 will be accomplished and the heterozygote or heterocaryon will behave as 

 the wild type, or in a very closely similar way. Complementation will be 

 observed. 



For instance, if a heterocaryon is made between two mutants of Neuro- 

 spora, one tryptophan-less and one adenine-less, the heterocaryon will have 

 no requirement, because the nuclei which cannot control the synthesis of 

 tryptophan synthetase cause the formation of the enzyme of adenine meta- 

 bolism that the other type of nuclei cannot control, and vice versa. 



But if the two mutation sites A and B both belong to the same functional 

 unit of genetic material, this unit is bad in both sets of genes: in one set it 

 does not work properly because site A is bad, in the other set, because site 

 B is bad. There is no complementation if the two mutation sites belong to 

 the same functional unit of genetic material. 



Two mutations which do not show complementation when they are on 

 two separate pieces of genome (trans), although they can restore the wild 

 type when they are recombined within the same piece of genetic material 

 (cis) are said to be comprised within the same 'cistron' (Benzer, 1957). 



If we are studying the genetic control of synthesis of a particular protein, 

 a cistron is thus a unit of nuclear genetic material which accomplishes one 

 indivisible function required for the synthesis of that specific protein. 



