V. MOLECULAR MECHANISM OF MUTATIONS 



209 



nucleic acids has been proved for several microorganisms. We shall name 

 this structure a chromosome in all those cases in which it contains DNA, 

 even though the DNA may not be visible by Feulgen staining. One can 



Extranuclear (or extrachromosomal) 

 = cytoplasmic 



Chromosome 



number 



( polyploids 

 aneuploids ) 



Information 

 content 



Recombination 



( includes 

 transformation, 

 transduction, 

 etc.) 



Mutation 



Larger 

 alteration 



Fig. 1. Classification of heritable variations. 



then distinguish chromosomal and extrachromosomal variations; for 

 bacteria the latter are often called cytoplasmic variations. Since bipar- 

 tition of chromosomal variations into mutation and recombination re- 

 mains the same as above, the subsequent discussion about mutations 

 refers to both macro- and microorganisms. 



B. DEVELOPMENT OF GENETIC SYSTEMS AND DISCOVERY OF 

 THE FIRST MUT.AGENS 



The early genetic experiments were performed on higher organisms 

 such as garden peas (Mendel, 1865) and Oenothera (De Vries, 1909). 



