510 



CHAPTER 42 



the same phenotypic effects as known ge- 

 netic material. 



Should pure DNA in vitro be considered 

 to be genetic material? Originally, the 

 identification of genetic material depended 

 upon its presence in organisms and its pro- 

 duction of a phenotypic effect; we can now 

 dispense with these requirements. Pure vi- 

 rus DNA in a test tube should be consid- 

 ered genetic material even though no longer 

 within an organism, recombining, mutating, 

 replicating, or performing any phenotypic 

 function. This statement is valid on the 

 basis that such DNA either is known to or 

 expected to possess genetic properties when 

 introduced into an organism. DNA syn- 

 thesized in vitro is physically and chemically 

 almost identical to chromosomal DNA. It 

 is capable of: 



1. Replicating itself and some of its vari- 

 ants 



2. Undergoing strand separation and re- 

 combination 



3. Producing a phenotypic effect by ge- 

 netic transformation. 



We may conclude, therefore, that DNA 

 synthesized in vitro also fulfills the require- 

 ments of our definition for genetic material. 



The simplest biological synthesis of DNA 

 or RNA requires the presence of nucleoside 

 triphosphates; an enzyme (DNA polymer- 

 ase or RNA synthetase); and water — at the 

 correct pH — containing the ions necessary 

 to activate the enzyme. It seems unlikely 

 that the first gene-like material had these 

 numerous and specific requirements for rep- 

 lication. We may hypothesize that in the 

 course of evolution, the first really success- 

 ful genetic material resembled RNA rather 

 than DNA; although DNA (by lacking an O 

 at the 2' position) is more stable than RNA 

 as a template, it is the RNA polyribotide 

 which serves as a carrier (as sRNA) for 

 amino acids. This amino-acid carrying 

 ability may have led to the synthesis of the 



polymerizing enzymes essential for rapid 

 gone synthesis, and lor the synthesis of other 

 proteins (including enzymes) which stab- 

 ilize and preserve the chemical integrity of 

 the genetic material. 



The preceding discussion leads to the 

 question of the origin of genetic material 

 on earth. Were RNA (and or DNA) and 

 proteins present during the early stages of 

 genetic evolution? Would their presence 

 in early evolution correlate with existing 

 knowledge about the course of chemical 

 evolution on earth? 



Eras of Chemical Evolution ' 



Era I. We now understand that at an early 

 prebiotic stage in its history — some 4 bil- 

 lion years ago — the earth had a reducing 

 atmosphere rich in water, hydrogen, meth- 

 ane, and ammonia, but poor in free oxygen 

 and carbon dioxide. Using mixtures of 

 these and similar compounds predicated to 

 have been present in such a reducing at- 

 mosphere plus a source of energy (such as 

 electrical discharges, sunlight and ultravio- 

 let light, microwaves, ultrasonic vibrations, 

 heat, high energy electrons, X rays, and 

 proton irradiation), it is possible to produce 

 in the laboratory a large number of simple 

 radicals and organic molecules. Moreover, 

 since a projectile propelled through a gas 

 and into a liquid can cause the formation of 

 a large number of complex chemical com- 

 pounds, it is very likely that in prebiotic 

 times chemosynthesis was also induced by 

 meteorites.- Some of the compounds syn- 

 thesized experimentally in a "primitive" 

 atmosphere include alanine, glycine, glutamic 

 acid, aspartic acid, acetic acid, formic acid, 

 proprionic acid, lactic acid, succinic acid, 

 some fatty acids, urea, some sugars, phos- 

 phoric acid, adenine, and uracil. Although 

 we are not yet able to determine which of 



1 See article by H. Gaffron in M. Kasha and B. 



Pullman (1962). 



-See A. R. Hochstim (1963). 



