146 



CHARLKS A. THOMAS, JR. 



various points. This picture of the circuhir ehroinosonic was greatly 

 stroiij^theiu'd hy the (hscovery of certain Hfr strains which had a ?'rr?/ 

 high frequency of transfer (Vhf) of the donor chromosome (Tayloi' ami 

 Ad(>lherg. 1900). These strains are able to transfer tiie entire male 

 genome to the female cells in approximately 111 minutes. Using three 

 different \'hf strains, these authors were able to demonstrate more 

 conclusively the contimious linkage of all known chromosomal markers 

 and identify more precisely the separation (in minutes) between markers 

 distributed aix)un(l the map. In Fig. II their maj) determined l)y kinetic 

 experiments is shown. 



ser/gly 



ade 



his 



Fig. 11. Tho kinetic linkage map of E. coli. The map represents the circular 

 linkage group of the F"^ ancestor of three Vhf strains. Total injection requires about 

 111 minutes; the various markers enter sequentially and at intervals shown (in 

 minutes). (From Taylor and Adelbcrg, 1960.) 



The genetics of episomes is a subject of some beauty and detail. The 

 reader is referred to the excellent review by Campbell n962). An im- 

 portant kind of episome is the F+ unit which may be united with any of 

 a number of different segments. This episome may be transferred from 

 a male to a female bacteria by a process called sexduction which is 

 analogous to the transfer of the entire chromosome just mentioned 

 (Jacob et al., 1960). This transfer can be accomplished to cells other 

 than coli and in this wav Scrratia cells can be infected with a coli 



