74 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



It was shown by Witkin (195 1) that if bacteria which produce easily 

 recognisable variant colonies are irradiated at a stage of growth when the 

 majority of cells have a single chromosome, then complete variant colonies 

 are produced. If they have two or four chromosomes then sectored colonies, 

 with half or quarter variant sectors respectively, are produced. The segregation 

 of the variant character, produced by alteration of an irradiated chromosome, 

 takes place after a delay of two vegetative divisions, representing the process 



Fig. 33 

 FATE OF CHROMOSOMES IN CELL DIVISION 



A. Equational division. This does not occur in bacteria. Even if the occurrence of a 

 classical mitosis (for which there is no reliable evidence) were accepted, it would entail migration 

 of chromosomes across their own partners and across the newly-formed cell division. 



B. Reductional division. This appears to be the normal condition in bacteria. It was 

 originally postulated bv cytologists (e.g. in the first edition of this book), but is now supported 

 by genetical evidence also (e.g., Ryan, F. J. and Wainwright, J. K., J. Gen. Microbiol., 11.364). 

 Thus the vegetative cells are haploid but multinucleate, and mutant characters are rapidly 

 segregated. 



of nucleur segregation described above. Filamentous cells, derived from the 

 vegetative fusion process, behave as if they possessed a single chromosome ; 

 an observation which confirms the occurrence of a fusion process. 



The nucleus of Azotohacter resembles that of yeasts in the possession of a 

 number of chromatinic granules which appear to be attached to the nuclear 

 membrane. In the case of yeasts these granules are believed to be the chromo- 

 somes. In mitosis these chromosomes take part in the formation of two 

 ribbon-like complexes. In Azotohacter, the nuclear material at cell division 

 takes the form of two or four large rods, which divide with the cell and then 

 fragment to reform the vesicular nucleus with its surrounding granules. 



