528 - Heredity and Evolution 



lethal (p. 512) — because each segment of a 

 DNA molecule is able to self-template its 

 own replication. A mutation, accordingly, 

 may involve a structural change at one or 

 more points along the length of a gene, and 

 it is perfectly possible that more than one 

 mutational change may occur within the lim- 

 its of a single gene. 



Crick's addition-subtraction analysis of the 

 B gene of the T 4 virus indicates that the code 

 within a single gene is written serially in 

 triplets. Also it indicates that the code starts 

 at one end of a genie segment. Thus, if a 

 single addition or subtraction occurs near 

 the start of a genie series, the whole code is 

 disrupted; that is, the whole gene becomes 

 inoperative. Consider, for example, one end 

 of a gene that carries (via RNA) the follow- 

 ing code: 



alanine --cysteine-- glycine-- alanine -- 

 CCG. . . UUG. . . UGG. . .CCG. . .U. 



.etc. 



If, however, a subtraction occurs at the first 

 C. code would now read: 



CGU . . . UGC . . . GGC . . . CGU 



etc. 



Thus the code would be completely changed 

 and the gene could not fulfill its proper func- 

 tion. If, however, a subtraction near the 

 beginning of a genie series is balanced by a 

 nearby addition, most of the gene triplets 

 would remain unchanged. Then the gene 

 might continue to function, although there 

 might be some slight change in its effects. 

 Perhaps an altered triplet, if it still remains 

 a triplet, might summon a single different 

 amino acid into a protein chain. This, in- 

 deed, appears to be the case in certain herita- 

 ble changes, such as sickle cell anemia (p. 

 505). 



An interesting phenomenon is observed in 

 the replication of T 4 phage virus, when two 

 differently mutated viral strands are under- 

 going replication in a single bacterial cell. 

 An exchange of DNA material between the 

 strands may take place; and the points at 

 which such exchanges occur do not neces- 

 sarily lie between two genie segments. Some- 

 times, indeed, a break may be localized 

 -with in a genie segment. 



Simplest to analyze are cases where dele- 

 tions have been induced within the same 



PARTS OF A GENIC SEGMENT OF DNA 

 DEFICIENT; NONFUNCTIONAL PERFECT; FUNCTIONAL 



+ + + + - 



+ + + + + + + 



NONOVERLAPPING 

 DEFECTS 



^T^ RECOMBINATION 



ju ^ 



+ 4- + + + 



DEFICIENT; NONFUNCTIONAL 



+ + ~ 



DEFICIENT; NONFUNCTIONAL 



OVERLAPPING 

 DEFECTS 



+ + + + + 



NO PERFECT 

 -^- RECOMBINATION 

 POSSIBLE 



Fig. 27-4. Recombination within a genie segment. Each minus sign ( — ) indicates a deficiency— 

 presumably a missing base pair. Plus sign (-)-) indicates normal base pairs. (Modified from 

 Benzer.) 



