Chapter 42 



THE ORIGIN AND EVOLUTION 

 OF GENETIC MATERIAL 



I 



n considering the nature and 

 effects of genes, the origin and 

 evolution of genetic material 

 has been neglected. Before taking this up 

 in detail, it would be desirable to reconsider 

 the nature of presently-known genetic ma- 

 terials, DNA and RNA. 



The replication of either type of nucleic 

 acid involves the use of a single or double 

 polynucleotide strand as a template for com- 

 plementary monomers subsequently joined 

 to form complementary polynucleotides. 

 The nucleic acid properties responsible for 

 its ability to serve as a template must in- 

 clude a specific physical configuration of 

 linearly-arranged monomers as well as a 

 specific pattern of net electrical charges. 

 Although the utilization of the nucleic acid 

 template for the formation of a complement 

 is a relatively passive process with regard 

 to the polynucleotide strand, it is an active 

 process if we consider the highly specific 

 action of nucleic acid polymerase or syn- 

 thetase. Since the nucleic acid fiber which 

 serves as a template is mostly passive, it is 

 not surprising that nucleic acid can be used 

 as a template by different enzymes, pro- 

 vided that the raw materials collected on 

 the template have suitable physical and elec- 

 trical properties. That nucleic acids are 

 used as templates for the formation of poly- 

 mers not their complements, is exemplified 

 by DNA used as template by DNA-depend- 

 ent RNA polymerase to make RNA, and 

 by RNA used in vitro and in vivo as a tem- 

 509 



plate to make DNA. Whether nucleotides 

 other than those in DNA and RNA, or still 

 other substances, make use of the nucleic 

 acid template in a similar manner is yet to 

 be determined. (Note that the basic pro- 

 teins in nucleoproteins may be associated 

 with nucleic acids through a template 

 mechanism, at least in part.) The simplest 

 and the broadest working hypothesis, there- 

 fore, is that all the functional characteristics 

 of genes depend upon the linear sequence 

 of nucleotides and upon the ways this poly- 

 mer is used as a template by various sub- 

 stances and enzymes; in other words, the 

 only junction of genetic material is to serve 

 as a template. 



Although nucleic acid is self-replicating, 

 the process is apparently not accomplished 

 in one step. In fact, two replications seem 

 to be required before a given strand can be 

 duplicated. The first replication produces 

 a complementary strand; the second repli- 

 cation produces a copy of the first strand. 

 We are probably justified in thinking of 

 self-replication in this way for the follow- 

 ing reasons: 



1 . The self-replication of single-stranded 

 DNA and RNA viruses must be con- 

 sidered a two-step process. 



2. Double-stranded nucleic acid may 

 preferentially or exclusively replicate 

 one of the complements. 



3. One of the two strands in a double 

 helix may be defective (mutant) and 

 incapable, at least in some places, of 

 both replication and self-replication; 

 however, its normal, complementary 

 chain would be capable of both. 



Consequently, we can now define genetic 

 material as any template whose use eventu- 

 ally results in its self-replication and which 

 either retains this ability after mutation or 

 is a mutant of a template which has this 

 ability. We can also consider genetic ma- 

 terial to be any substance which produces 



