THE CENTRAL PROBLEMS OF THE BIOCHEMISTRY OF CELL DIVISION 48 1 



On the other hand, cvtochemical research has yielded some sohd facts 

 on changes of chromosome composition during the mitotic cycle : (i) There 

 is an " RNA cycle". Chromosomes acquire RNA during prophase, carry 

 it through the period of their mitotic movements, and give it up at the 

 end of the mitotic period [17]. This is not to say that interphase chromo- 

 somes do not contain some RXA, but the fact that they acquire more of 

 it and distribute it bv the mitotic mechanism raises some interesting 

 speculative possibilities [i<S]. (2) There is a cycle of changes in staining 

 properties which can be interpreted as the incorporation of phospholipids 

 in chromosomes at prophase and its release from the chromosomes at the 

 end of the mitotic period [19]. (3) The chromosomes lose their nucleolar 

 substance, including an unknown component identified by its reactions 

 with silver, during prophase and reacquire it at telophase [11]. Various 

 inferences can be made about these changes, but none of them is very 

 compelling as yet. All we can say, and it is not trivial, is that the 

 mitotic cvcle does involve some important changes in chromosome 

 chemistry. 



! 4. The mitotic apparatus : general 



Once the chromosomes have reproduced, the problem of mitosis is to 

 separate the daughters and to collect them into two equivalent nuclei. The 

 astoundinglv precise events can be described in a formal way: (i) There 

 are two poles ; (2) Each of a pair of sister chromosomes may be engaged by 

 (or "attracted" to) one pole and the two may not be engaged by the same 

 pole. The realities are, fortunately, embodied in a definite structure, which 

 we call a mitotic apparatus [20]. 



The general structural features of the mitotic apparatus are these : 

 (i) It is a "solid" coherent body, having properties sufficiently 

 different from the rest of the cvtoplasm to permit its isolation, which will 

 be described below. 



(2) It is looselv describable as a rather unstable gel in which there are 

 oriented regions or structures which are observed as fibres, both at the 

 submicroscopic and microscopic levels [21]. In contrast with the rest of 

 the cytoplasm, it does not contain mitochondria or other larger particles, 

 but it does contain smaller particles similar to those seen elsewhere in the 

 cell [22]. 



(3) The " fibres ", observed either in fixed material or in living material 

 with the aid of polarization optics, appear to include specific connections 

 between chromosomes and poles. Visually, it would appear that chromo- 

 somes are engaged to the poles bv these chromosomal fibres and are 

 "pulled" to the poles by them. At least, the fibres do predict the paths the 

 chromosomes will follow. 



VOL. II. 2 I 



