Chapter III 1 51 -~-— * 



Variation and Its Chemical Correlates \ ^^tlSJ* 



C. A. Knight ^''"»-— ^ 



Virus Laboratory, University of California, Berkeley, California 



I. Introduction 127 



A. Historical 127 



B. Terminology 128 



C. Frequency of Mutation 129 



D. Significance of Mutation 130 



II. Plant Virus Strains 130 



A. Characteristics of Strains 130 



B. Methods for Seciu-ing Strains 131 



1. Naturally Segregated Strains 131 



2. Isolation from Distinctive Spots 131 



3. Passage in a Different Host 132 



4. Exposure to Elevated Temperatures 132 



5. Isolation from Local Lesions 133 



C. Criteria of Strain Relationship 133 



1. Serological Tests 134 



2. Cross-Protection Tests 135 



3. Similarity in Host Range 136 



4. Similarity in Method of Transmission 136 



5. Similarity of Response to Genetic Change in Host 136 



6. Similar Resistance to Destruction of Lifectivity 137 



7. Coincidence of Specific Chemical and Physical Properties 137 



D. Evaluation of Criteria of Strain Relationship 138 



III. Chemistry of Virus Strains 139 



A. Protein Components 139 



1. Compositions of Strain Proteins 139 



2. Structm-al Featm^es of Strain Proteins 143 



B. Nucleic Acids 146 



1 . Compositions of Strain Nucleic Acids 146 



2, Structural Featui-es of Strain Nucleic Acids 149 



IV. Production of Variants by Chemical and Physical Treatments 150 



A. Chemical Derivatives of TMV 151 



B. Relation of Radiations to Production of Variants 152 



V. Summary and Perspectives 152 



References 153 



I. Introduction 

 A. Historical 



Evidence that viruses are not stable biological entities was provided by 

 Pasteur's classic experiment with rabies at least a decade before it was 



127 



