The Origin and Evolution of Genetic Material 



513 



In considering the origin of the first gene, 

 we should keep in mind the possibility that 

 its nongenic predecessor might have been 

 capable of self-replication to some degree, 

 but might not have been able to replicate 

 any of its mutant forms. The search for 

 information about nongenetic systems with 

 some but not all the properties of genetic 

 material is clearly highly necessary and de- 

 sirable. 



Subgenic chemicals may occur in present- 

 day cells. Constituents of the cytoplasm 

 which contain DNA and are able to self- 

 replicate include chloroplasts, mitochondria, 

 the centriole, and the kinetosome. If the 

 DNA in these structures is mutable and still 

 able to self-replicate, it can be classed as 

 cytoplasmic or extranuclear genetic mate- 

 rial. Experimental study of these and other 

 organelles is expected to reveal details of 

 their chemistry. We would also like to 

 know a great deal more about the synthesis 

 of RNA genes; how the metagonic RNA 

 of Paramecium replicates in Didinium; and 

 whether it is still self-replicating after mu- 

 tation. Answers to such questions would 

 be valuable in enabling us to speculate more 

 fruitfully upon the nature of pregenetic and 

 primitive genetic materials. 



Eras IV and V. The early environment 

 in these eras was probably much the same 

 as in Era III, but after an increase followed 

 by a decrease in carbon dioxide, a large in- 

 crease in the amount of free oxygen took 

 place. Though we do not yet have suf- 

 ficient information to decide precisely which 

 pathways led to the chemical evolution of 

 the first gene, we do have some evidence 

 concerning the subsequent history of genes 

 in organisms. The only genetic material 

 found exclusively in free-living organisms 

 today is DNA; this substance is found in 

 all such organisms, be they unicellular or 

 multicellular, plant, animal, or microorgan- 

 ismal. Whether or not types of genes other 



than DNA and RNA have ever existed, 

 DNA genes must have a definite advantage 

 for survival — after all, they have persisted 

 as the main genetic material for about a 

 billion years, approximately the period the 

 evolution of plants and animals have been 

 separate. It is likely that the formation of 

 chromosomes with telomeres, centromeres, 

 and polynemy, as well as the establishment 

 of special methods of separating daughter 

 and homologous chromosomes (by mitosis 

 and meiosis) and of recombining them 

 (fertilization) were innovations involving 

 DNA which occurred some time prior to 

 the divergence of the plant and animal 

 kingdoms. 



Evolution must have led to the transcrip- 

 tion of only one of the two complementary 

 polynucleotide strands in producing com- 

 plementary RNA and viruses with single- 

 stranded nucleic acids, and must have oc- 

 curred in the genetic code and the appara- 

 tus for translation. The quantity of DNA 

 per organism and the basic proteins which 

 regulate gene activity as well as the mate- 

 rials which serve to regulate mutability must 

 also have evolved. 



We may reasonably suppose that an evo- 

 lution in gene activity also took place. On 

 primitive earth it is likely that large amounts 

 of different, more or less complex, organic 

 materials accumulated and remained unde- 

 graded before the advent of the first gene- 

 containing organisms. As the organisms 

 used up these resources in their metabo- 

 lism, however, there would have been a se- 

 lection in favor of those mutants capable of 

 synthesizing such organic materials from 

 simpler organic, or from inorganic, compo- 

 nents. 7 This hypothesis means that natural 

 selection acted in favor of those mutant 

 genes which specified the synthesis of a com- 

 ponent no longer available in the environ- 



"See N. H. Horowitz (1945). 



