400 THE CHANGING GENERATIONS 



around the earth, would unite with oxygen and ammonia to produce 

 hydrocarbons and nitrogenous compounds. With time in abundance for 

 chance encounters between molecules, an ever-increasing variety of 

 chemical substances, both organic and inorganic, would have come into 

 existence, and among these we may suppose were the first simple proteins. 



In generalized terms a protein consists of a polypeptide chain folded 

 over on itself many times to form a three-dimensional particle, held 

 together by chemical linkages and with exposed peripheral valences that 

 can bind several or many such units into large colloidal particles. By 

 chance meetings in the molecular soup of the primordial waters a certain 

 proportion of the ancestral protein molecules may have made net gains 

 in size and complexity, and some of these augmented molecules presum- 

 ably had structures resembling those of nucleoproteins. 



Nucleoproteins appear to be the essential stuff of life. Direct analysis 

 and circumstantial evidence point to the conclusion that genes and 

 viruses are wholly or chiefly composed of these substances. Chromosomes 

 are largely nucleoprotein, which suggests but does not prove that this 

 is true of the genes. Several viruses have been isolated in pure (crystal- 

 line) form, and these are all nucleoproteins. Genes and viruses probably 

 approach one another in particle size, and both are able to duplicate 

 themselves by organizing protein molecules from their environments and 

 to mutate. These facts have led many biologists to the idea that the first 

 life units were nucleoprotein particles similar to the viruses we know 

 today. Beadle has proposed to call these hypothetical archetypic life 

 particles protogenes. 



Protogenes are compared with viruses rather than with genes, because the 

 former are less complex and more uniform, are not grouped into assemblages of 

 different but interacting units (chromosomes), can exist independently of cells 

 and cytoplasm, and can be chemically isolated. The viruses cannot be regarded as 

 surviving examples of protogenes, however, for all of them can multiply only 

 within living cells. The bacteriophages, for example, can exist in a free state as 

 submicroscopic, nonreproducing, physiologically inactive nucleoprotein particles. 

 These particles are able to adhere to and penetrate bacterial cells. Entry of the 

 particle disrupts the genetic, regulatory apparatus of the cell but leaves its 

 metabolic system intact. The genetic material of the virus takes over control and 

 guides the metabolic system of the cell in the production not of its normal prod- 

 ucts but of materials that become organized into new virus particles. These, 

 barring mutation or recombination, are exactly like the particle that originally 

 entered. Completion of the process is accompanied by breakdown of the cell 

 membrane and liberation of perhaps 100 times as many virus particles as entered 

 the cell. While the virus is within the cell, it is evidently dissociated into its 

 components, with later recombination. This is demonstrated by the fact that if a 

 bacterial colony is infected with two virus strains, one having the properties A b 

 and the other aB, the virus particles liberated on destruction of the cells include 

 not only these types but also the new combinations AB and ab. 



