CHAPTER 48 



capable of some degree of self-replication, 

 but might not have been able to replicate any 

 of its mutant forms. The search for infor- 

 mation regarding nongenic systems having 

 some but not all of the properties of genetic 

 material is clearly highly desirable. Such 

 information may be sought in studies of 

 various polymers under test tube conditions. 

 Subgenic chemicals may occur in present- 

 day cells. Several constituents of the cyto- 

 plasm other than plastids are able to self- 

 replicate. These include the centriole and 

 the kinetosome (which have already been 

 discussed on pages 369-370 in connection 

 with episomes). If these structures prove to 

 be mutable and are still able to self-replicate, 

 they can be classed as cytoplasmic or extra- 

 nuclear genes. Experimental study of these 

 organelles will be expected to reveal the details 

 of their chemistry, and whether they possess 

 only the self-replicating capacity of genes, 

 never having had, or having lost, the ability 

 to reproduce themselves after mutation. In 

 any event, our knowledge of genes will be 

 increased by such studies. We would also 

 wish to know a great deal more about the 

 synthesis of RNA genes, and whether 

 ribosomes, which have the chemical compo- 

 sition of RNA genes,' are self-replicating and 

 mutable, in order to speculate fruitfully upon 

 the nature of the pregenic and the first genie 

 material. 



While we must conclude that we do not 

 yet have sufficient information to decide 

 which pathway led to the chemical evolution 

 of the first gene, we do have some evidence 

 on the subsequent history of genes. The 

 only genetic material found exclusively in free- 

 living organisms today is DNA, and this is 

 found in all such organisms, whether they 

 be unicellular or multicellular, plant, ani- 

 mal, or microorganismal. Whether or not 

 other types of genes exist or have existed, 

 DNA genes must have a selective advantage 

 for survival, having persisted as the main 

 1 See A. Van Kammen (1961). 



genetic material for about a billion years, 

 which is approximately the period that plants 

 and animals have been separate in their evolu- 

 tion. Moreover, it is hkely that the forma- 

 tion of chromosomes with telomeres and with 

 centromeres, as well as the establishment of 

 special methods of separating daughter and 

 homologous chromosomes (by mitosis and 

 meiosis), and of recombining them (fertiliza- 

 tion), were innovations involving DNA which 

 were established some time prior to the diver- 

 gence of the plant and animal kingdoms. 



Not only must there have been a chemical 

 evolution in DNA and the accessory ma- 

 terial which packages and recombines it, 

 but there probably was also an evolution in 

 gene activity. It is likely that the primitive 

 earth accumulated large amounts of different, 

 more or less complex, organic materials 

 which remained undegraded before the ad- 

 vent of the first genetic material. As the first 

 gene-containing organisms used up these 

 resources in their metabolism, however, there 

 would have been a selection in favor of mu- 

 tants capable of synthesizing such organic 

 materials from simpler organic, or from in- 

 organic, components. This means that 

 natural selection acted against those genes 

 which functioned auxotrophically (that is, 

 which were incapable of directing synthesis 

 of nongenetic material or directed the forma- 

 tion of no meaningful synthetic product) in 

 favor of those genes which functioned proto- 

 trophically (that is, which specified the syn- 

 thesis of a component no longer available 

 in the environment). Prototrophism would 

 also be advanced by the physical association 

 of genes involved in different portions of a 

 given biochemical sequence. This would 

 lead eventually to the selection of mutant 

 genes whose function, other than self-replica- 

 tion, was to regulate the functioning of other 

 genes. Thus, in addition to genes for struc- 

 ture, there would be expected to evolve genes 

 for function — operator genes located in 

 operons. 



