Extranuclear Genes 



379 



1 



/ 



A 



B 



figure 29-7. The centromere and its gran- 

 ules in corn. (Courtesy of A. Lima de Faria, 

 "Compound Structure of the Kinetochore in 

 Maize," J. Hered., 49:299, 1958.) 



in the centromere and centrosome stain the 

 same (both seem to contain DNA); so does 

 the material surrounding these granules. In 

 the living cell centromere and centrosome 

 have a similar appearance. The granules 

 within the centromere are apparently thick- 

 enings of the DNA thread which passes from 

 one chromosome arm to the other. 



Centromeres are sometimes attracted to 

 each other and to the centrosomes, and at 

 anaphase the centromeres migrate toward 

 the centrosomes. The centrosome, too, has 

 the capacity to move, as demonstrated by its 

 preanaphase movement and its behavior 

 after a sperm has penetrated an egg. Thus 

 we see that the morphological similarity of 

 centromere and centrosome is paralleled by 

 their behavioral similarities. 



From these facts some sort of kinship is 

 suggested 1,; between centrosome and centro- 



mere. This view is strikingly supported by 

 the finding ,7 that during the meiotic divi- 

 sions leading to the formation of certain 

 mollusc sperms, certain chromosomes de- 

 generate and release "naked" centromeres. 

 These now-free centromeres group together 

 with the centrosome and thereafter duplicate 

 centrosomal behavior and appearance ex- 

 actly. In effect, then, the freed centromeres 

 become extra centrosomes. The preceding 

 circumstances suggest that the centromere 

 and centrosome may be two states of a 

 presently — or previously — existing episome. 

 The change from one episomal state to an- 

 other is probably influenced by the presence 

 of a nuclear membrane and by various other 

 genetic factors present in highly organized 

 cells, as well as by the occurrence of muta- 

 tions in the autonomous or integrated state 

 of the episome. 



At the base of each cilium or flagellum is 

 a granular body, the kinetosome, which is re- 

 sponsible for the motion of the organelle. 

 Considerable evidence has been brought 

 forth that kinetosomes contain DNA and are 

 homologous to centrioles (or centrosomes). 

 Perhaps kinetosomes, too, are episomes or 

 episomal derivatives. It has been suggested 

 that the episome F functions like a centro- 

 mere. Could it be that the movements at- 

 tributed to F and the centromere and centro- 

 some have the same basis as the flagellar and 

 ciliary movement produced by kinetosomes? 



In Trypanosoma, DNA and a histone-like 

 protein are found in the kinetoplast, a large 

 cytoplasmic organelle associated with mo- 

 tility as well as mitochondria. DNA repli- 

 cation occurs synchronously in nucleus and 

 kinetoplast. Kinetoplasts can be damaged 

 irreversibly if treated with acridine dyes. 

 Since the kinetoplast apparently has an ap- 

 preciable DNA content, additional molecular 

 information about it should prove quite valu- 

 able. 



16 Originally by C. D. Darlington. F. Schrader. 

 and others. 



17 See A. W. Pollister and P. F. Pollister (1943). 



