236 CELL HEREDITY 



ni'oclassical nu'thods of transformation and transduction. Nonchromo- 

 sonial elements liaxc been identified l)v their al)errant segregation pat- 

 terns in nieiosis, w hieh make them unmapable in anv chromosomal link- 

 age group, and often hv their segregation in mitosis under conditions 

 w here chromosome segregation is not occurring. 



Tile study of replication is potentially a powerful tool in the analysis 

 of heredity. The term replication is employed in modern genetics to 

 mean the origin of a new unit by copying from a pre-existing unit of the 

 same kind, either directly in one step or indirectly with the intervention 

 of complementary intermediates. The only cell elements which need be 

 considered as possible carriers of heredity are those which do replicate, 

 and do so, furthermore, with conservation of specificity. As yet, the only 

 biological system smaller than the whole cell in which the process of 

 replication has been shown to occur is the DNA. 



Other species of molecules to be considered as possible carriers of 

 heredity include RNA and proteins. With neither class of compounds 

 has the mode of synthesis been fully established (see Chapter 11). 

 Protein specificity is probably determined by a coding device utilizing 

 RNA, and does not seem to require a pre-existing protein molecule of 

 the same kind for its synthesis. On the other hand, studies of protein 

 synthesis have been limited largely to a few soluble proteins, and much 

 less is known about the synthesis of the structural proteins, some of 

 which like the cytochromes are enzymatically very active. 



Very little is known as yet about RNA synthesis (see Chapter 11), but 

 again there are different types of RNA and some of them may be repli- 

 cated. The existence of viruses, in which the hereditary information is 

 carried by the RNA, requires that that RNA at least be replicated with 

 conservation of its specificity. Since a replication mechanism must exist 

 for viral RNA, perhaps the same mechanism is employed for some cellu- 

 lar RNA's as well. 



If it were possible to detect molecular replication directly, a method 

 would thereby be available to screen for hereditary determinants on that 

 criterion alone. Since no such methods are yet available, we must turn 

 to the classical methodology of genetics for evidence of nonchromosomal 

 heredity. 



Nonchromosomal determinants have been identified primarily by their 

 nonsegregation at meiosis. In such instances, crosses between contrast- 

 ing parents give rise to progeny exclusively of one parental type. An 

 example is the inheritance of streptomycin resistance in Chlamydomonas 

 (see Chapter 3 for its life cycle). 



The wild-type cells are streptomycin-sensitive (ss), and by treating a 

 large population of cells with the drug, it is possible to isolate resistant 



