Chapter *49 



GENES — NATURE 

 AND CONSEQUENCE 



I 



"n the first Chapter, we postu- 

 lated the occurrence of genetic 

 .material responsible for pheno- 

 typic similarities and differences between 

 organisms. We later required that the 

 genetic material be self-replicating and capa- 

 ble of self-replicating some of its chemical 

 changes. Most of this book has been con- 

 cerned primarily with the further definition 

 or delimitation of the nature of the genetic 

 material. Each different way of studying 

 segments of the genetic material, or genes, 

 revealed additional properties of the genetic 

 material which were expressed in terms of 

 the operation used to detect them. Thus, in 

 the study of the chemical basis of the genetic 

 material, it was demonstrated that all known 

 genes are inseparable from nucleotides of 

 either RNA or DNA, the latter being the 

 typical genetic substance of chromosomes and 

 of free-hving organisms. RNA is usually 

 single-stranded and DNA double-stranded. 



The biological replication of DNA involves 

 strand separation and the action of a poly- 

 merase that takes directions from the single 

 DNA chain, in the utilization of deoxy- 

 riboside triphosphates to manufacture the 

 complementary DNA chain. Though DNA 

 is self-replicating, this does not mean that 

 the process is accomplished in one step. In 

 fact, it requires two replications before a 

 given strand can result in a copy of itself, 

 the first replication producing the comple- 

 mentary strand and the second replication 

 444 



producing a copy of the first strand. It is 

 probably justified to think of self-replication 

 as occurring in this indirect way for several 

 reasons. First, DNA replication is by single 

 strands, each of which apparently acts inde- 

 pendently of the other. Second, DNA oc- 

 curs in organisms like 0X174 in the single- 

 stranded condition, all strands being the same 

 complement. Here self-replication must be 

 considered a two-step process. Third, it is 

 possible that one of the two strands in a 

 double helix may be defective (mutant), so 

 that it is, at least in some places, incapable 

 of replicating a complement. Such a chain 

 would be incapable of both replication and 

 self-replication in its defective portion, while 

 its normal, complementary chain would be 

 capable of both. 



The preceding dealt with self-replication 

 at the chemical level. When does self- 

 replication occur at the informational level? 

 The transforming ability of hybrid DNA 

 double helices proves that a single DNA 

 strand is capable, subsequent to replication, 

 of providing all the information contained 

 in a double-stranded helix possessing correct 

 complementary pairs. It is likely that either 

 single strand of a normal double helix can 

 furnish this information after it has replicated 

 once. However, all or part of the informa- 

 tion used to make messenger RNA may be 

 carried by only one DNA strand. There- 

 fore, in order to carry the same functional 

 (messenger, or sRNA, or other) information, 

 DNA probably has to self-replicate. It 

 would seem desirable, therefore, to recog- 

 nize the difference between replication and 

 self-replication, at both the chemical and in- 

 formational levels. 



Does pure DNA have any of the proper- 

 ties of the genetic material? It is still an 

 assumption, however likely, that the DNA 

 primer placed in vitro has come from the 

 replication of DNA in vivo, rather than being 

 the product of the activity of some other sub- 

 stance, like protein or RNA. Accordingly, 



