Genes: Nature and Mode of Action - 525 



hundredfold. Finally, therefore, about 200 

 complete new virus particles can be liberated, 

 when the infected bacterium goes to pieces. 

 These studies on viral replication are im- 

 portant. Not only do they demonstrate that 

 a self-templated synthesis of viral DNA oc- 

 curs, but they also show that a close relation- 

 ship exists between DNA and protein syn- 

 thesis. Genie materials, apparently, are capa- 

 ble of templating the synthesis of other sub- 

 stances, as well as of themselves. In a sense, 

 therefore, genes are both autocatalytic and 

 heterocatalytic. 



DNA-RNA-PROTEIN INTERRELATIONSHIPS 



That ribonucleic acid (RNA) is essential 

 for the synthesis of proteins, including en- 

 zymes, was suggested some twenty years ago 

 by the work of T. O. Casperson of Stockholm 

 and of Jean Brachet of Brussels. But only 

 within the past five years, as the result of 

 many studies, has it become apparent that 

 the processes by which DNA, RNA, and pro- 

 teins are synthesized are related to one an- 

 other in a complex and fascinating manner. 



In the ordinary cycle of cell division, an 

 active new synthesis of DNA does not occur 

 during mitosis or meiosis — while the chro- 

 mosomes are compactly formed — but in be- 

 tween times — when the chromosomes are 

 greatly extended (Fig. 2-1 IB) and when (pre- 

 sumably) the helical structure of the DNA is 

 to a large extent uncoiled. 



It has recently been proved that DNA can 

 template the synthesis of RNA as well as of 

 itself. Each single strand of uncoiled DNA 

 assembles a complementary strand of RNA. 

 This assembling process follows the base-pair 

 rule — except that thymine (which is absent 

 from RNA) is replaced by uracil in the pair- 

 ing. Affiliation between the strands of DNA 

 and RNA is short-lived, however. After sepa- 

 ration, the RNA remains in single-stranded 

 state. But eventually, after self-templated 

 synthesis has occurred, DNA resumes its 

 double-stranded helical form. The sugar of 

 the sugar-phosphate chain, in the case of 



RNA is ribose, of course, rather than deoxy- 

 ribose. 



Such a heterocatalytic templating of RNA 

 synthesis has been demonstrated most di- 

 rectly in cell-free bacterial preparations by 

 Samuel Weiss at the University of Chicago 

 and by Jerard Hurwitz at New York Univer- 

 sity; and in phage virus replication by Elliot 

 Volkin, at the Oak Ridge National Labora- 

 tory. Moreover there has been considerable 

 confirmatory evidence derived from the cells 

 of higher organisms. 



By templating the synthesis of RNA, DNA 

 assumes indirect control over the synthesis 

 of the enzymes and the other protein com- 

 ponents of the organism, as was partly ex- 

 plained in Chapter 4. 



Kinds of RNA. There are at least three 

 kinds of RNA and each has a different mis- 

 sion to fulfill in the cytoplasm. Ribosomal 

 RNA, a very stable form of high molecular 

 weight, takes up a fixed position in the ribo- 

 somal membranes (p. 25). Here, presumably, 

 it determines the alignment of the second 

 kind, called template RNA. Template RNA 

 (also called messenger RNA) is likewise of 

 large molecular size, but it is relatively un- 

 stable. Template RNA keeps breaking down 

 as its mission in protein synthesis is consum- 

 mated. Consequently new template (messen- 

 ger) units must, from time to time, originate 

 from the nucleus. Template (messenger) 

 RNA, moreover, is the bearer of the genetic 

 code. The base sequence in this fraction de- 

 termines the amino acid sequence in the 

 protein being synthesized. In other words, 

 the template for protein synthesis is carried 

 by the messenger, or template, form of RNA. 



The third form of RNA is of low molecular 

 weight and it is freely dissolved in the cyto- 

 plasm. This form is called transfer RNA 

 (also sRNA, or soluble RNA) because it 

 serves to pick up amino acids from the cyto- 

 plasmic pool and to bring these units ot 

 protein structure into proper alignment upon 

 the synthesizing template. And here again 

 the base-pair rule operates to determine the 

 affiliations between the transfer and the tern- 



