Extranuclear Genes 



377 



figure 29-6. Electron micrographs of {A) 

 mouse heart mitochondria (X60,000), (B) 

 Neurospora mitochondria prepared to show 

 cristae with elementary particles attached 

 (X67,200), and (C) the outline of Neuro- 

 spora mitochondrial DNA (X ,29,300). (Cour- 

 tesy of Dr. Walther Stoeckenius, The Rocke- 

 feller Institute, New York.) 



sensitive strain segregate two sensitive to 

 two resistant — just as the alleles for mating 

 type segregate — a chromosomal basis for the 

 mutation is indicated. These chromosomal 

 mutants are of the sr-100 class. Strepto- 

 mycin also acts not as a selective agent but 

 as a mutagen to produce individuals — called 

 sr-500 mutants — resistant to 500 jxg strepto- 

 mycin per vial. Sr-500 mutants do not show 

 meiotic segregation in the haploid F, or the 

 backcross progeny. Moreover, in crosses 

 with ss (streptomycin-sensitive) individuals, 

 the sr-500 factor is usually received by all 

 progeny when the sr-500 parent is of mating 

 type + (m/+), and by none of the progeny 

 when the sr-500 parent is — (mt~). These 

 and other characteristics demonstrate that 

 sr-500 is caused by a "nonchromosomal" 



,. • as ;.■ ■^<:k-.--..- 



gene; that is, a gene which does not comply 

 with the usual transmission rules for chro- 

 mosomal genes in sexually reproducing in- 

 dividuals. Streptomycin also induces a large 

 number of auxotrophic mutants whose basis 

 also proves to be in nonchromosomal genes. 

 Exceptional zygotes occur which contain 

 the nonchromosomal genes from both par- 

 ents. Using such zygotes, the results of 

 single factor crosses and of reciprocal two- 

 factor crosses show that the nonchromo- 



