Chapter 45 



EXTRANUCLEAR GENES 

 AND THEIR INTERRELATIONS 

 WITH NUCLEAR GENES 



WHEN a DNA phage enters the 

 stage of vegetative replica- 

 tion, the synthesis of host 

 DNA and protein ceases and the cell pro- 

 ceeds to make virus DNA and protein. While 

 we do not know the detailed mechanism 

 whereby this metabolic shift is produced, it 

 could involve the utilization of the host's 

 cistronic products as well as the direct sup- 

 pression of host gene replication and/or 

 cistronic functioning. It would seem of 

 interest, in this connection, to discuss whether 

 or not there is evidence of a direct inter- 

 relationship between chromosomal and non- 

 chromosomal genes. We have already pre- 

 sented some evidence bearing on this during 

 our study of episomes of F and temperate 

 virus types. It should be recalled that when 

 such episomes are associated with their 

 chromosomal locus, replication, of the same 

 episome located nonchromosomally, is re- 

 pressed. 



Is there any evidence for the occurrence of 

 genes which are always physically unassoci- 

 ated with nuclear chromosomes, that is, 

 genes which are not chromosomal or epi- 

 somal but always extrachromosomal? It is 

 possible that some virulent phages are com- 

 posed of genes of this type. It is even 

 possible that some of the genes in temperate 

 phages are always extrachromosomal, in view 

 of the fact that only the genes of the prophage 

 are intimately associated with a chromosomal 

 locus. We may now ask two questions: By 

 409 



what means can we search for and prove the 

 occurrence of extrachromosomal genes? If 

 they exist, can we find any direct interactions 

 between them and particular chromosomal 

 genes? We can seek the answers to these 

 two questions in investigations of organisms 

 whose cells possess a definite nuclear mem- 

 brane (and hence a definite nucleus). Ac- 

 cordingly, we shall be concerned with the 

 identification of extranuclear genes and their 

 interrelations with nuclear genes. 



How are we going to recognize an extranu- 

 clear component as being genie? We can do 

 so by testing whether that component is oper- 

 ationally genie — that is, by testing its chemi- 

 cal, recombinational, mutational, phenotypi- 

 cal. and replicative properties. If such studies 

 reveal properties which satisfy our operational 

 definitions of a gene, the component is genie. 



Let recombination be the first operational 

 method used in the search for extranuclear 

 genes. To detect recombination we shall 

 require that the extranuclear gene produce a 

 recognizable phenotypic effect. We shall also 

 require that such a gene be capable either of 

 mutation or recombination, or both. In 

 other words, we need to have changes, in- 

 volving either the kind or quantity of such a 

 gene, or both, so that we are provided with 

 phenotypic alternatives having an inborn 

 basis. 



How would we actually proceed to look 

 for an extranuclear gene in Drosophila'} We 

 would initially observe the occurrence of 

 (preferably, clearly) different phenotypic al- 

 ternatives which occur generation after gener- 

 ation under the same environmental condi- 

 tions. By a series of crosses we would 

 proceed to test whether the occurrence of the 

 alternatives was associated with the presence 

 of a particular chromosome (X, Y, II, III, 

 IV) or a group of chromosomes. If it is, it is 

 hkely that the phenotypic alternatives are due 

 to some genie factor linked to (hence located 

 on) a chromosome. (Then by additional 

 appropriate crosses and/or cytological stud- 



