Chapter 48 



THE BIOCHEMICAL EVOLUTION 

 OF GENETIC MATERIAL 



i; 



"n the course of studying the 

 nature and effects of genes, you 



.may have wondered about the 

 origin of genetic material on earth. This 

 would involve the first occurrence of a chemi- 

 cal substance capable of replicating itself 

 and some of its modifications. At present 

 we know of only two substances having these 

 properties, DNA and RNA. Some under- 

 standing of the problems involved in ex- 

 plaining gene origin may be obtained from 

 a consideration of our knowledge of present- 

 day genes and of biological evolution. 



Let us assume that the first gene was either 

 DNA or RNA. Given the presence of the 

 first gene, how did it manage to replicate? 

 We already know something regarding the 

 mechanism of biological replication of DNA. 

 The simplest DNA system capable of repli- 

 cating in vitro requires the presence of 

 (1) energy to separate the double-stranded 

 helix, (2) four kinds of deoxyriboside tri- 

 phosphates, and (3) the enzyme DNA 

 polymerase, not to mention water, at the cor- 

 rect pH, which contains the ions necessary 

 to activate the enzyme. It does not seem 

 very likely that the first gene was a DNA 

 polymer which replicated in this manner. 

 One reason for this opinion is based upon 

 the essential role played by the enzyme. 

 According to our present understanding, 

 the amino acid sequence in such an enzyme 

 is specified by a gene. It seems improbable 

 that so complex an enzyme could be formed 

 independently of gene action. However, one 

 439 



might require that the first gene be capable 

 of specifying DNA polymerase which would 

 then be available for subsequent DNA syn- 

 thesis. Even so, one would have to be con- 

 vinced that the amino acids in the enzyme, 

 the nucleoside triphosphate building blocks, 

 and the energy for chain separation are com- 

 ponents likely to be present in the environ- 

 ment of that time. The difficulty of chain 

 separation would be avoided if the first gene 

 were RNA, since this nucleic acid is usually 

 single-stranded. Unfortunately, we do not 

 know nearly as much about the biological 

 replication of RNA genes as we do about 

 DNA genes. 



The complexity of present-day replication 

 of DNA should not automatically exclude it 

 or RNA as being the material basis of the 

 first gene. Before making any decision on 

 this matter, we would need to have more in- 

 formation relative to DNA synthesis, includ- 

 ing answers to such questions as: (1) Can 

 any other simpler substance substitute, no 

 matter how inefficiently, for the polymerase? 

 (2) What is the shortest nucleotide sequence 

 capable of self-replication? (3) What, if any, 

 simpler substances can replace the triphos- 

 phates? 



Although DNA and RNA are recognized 

 as being genetic material today, it is possible 

 that the first genes were of neither type, but 

 of a related chemical composition. We may 

 even wonder if, at present, there are genes 

 which are neither DNA nor RNA. Note 

 that we have thus far bypassed the problem 

 of the origin of the first gene, and have con- 

 sidered only the matter of its replication 

 were it DNA or RNA. Although we 

 initially inquired about the origin of genes, 

 clearly the answer we suggest regarding the 

 mechanism of its replication will depend 

 upon the chemical identity we hypothesize 

 for the first gene. 



In considering the origin of the first gene 

 we should keep in mind the possibility that 

 its nongenic predecessor might have been 



