480 DANIEL MAZIA 



may or may not be important for our understanding of its genetic functions, 

 but they may be the heart of many problems of cell division : the way in 

 which the DNA synthesized in one generation is distributed among 

 descendants, the relation between the timing of genetic reproduction and 

 the timing of cell division, the realization of mutational events, and all of 

 the problems of the realization of two fully operational chromosome sets 

 from one. Some of the experimental problems are out of reach at present, 

 but others are simple enough. For example, there is preliminary evidence 

 [12] that some of the chromosomal proteins are in fact made at a different 

 time than the time of DNA synthesis. On the other hand, the histones 

 seem to be incorporated into chromosomes in parallel with DNA 

 synthesis [13]. 



The reproduction of the nucleolar equipment is rather unusual in that 

 the original material is given up by the chromosome in prophase and two 

 nucleoli, containing at least some new RNA [14], appear at late anaphase. 

 There is no substantial chemical evidence concerning the reproduction of 

 the kinetochores, although there are many interesting cytological inferences. 



3. Chromosomes in the mitotic cycle 



The structural behaviour of the chromosomes in the mitotic cycle is 

 one of the dramatic events of the life-history of the cell. As everyone 

 knows, the chromosome substance, which is so highly extended and 

 attenuated between divisions that individual chromosomes cannot be 

 discerned, undergoes during prophase a "condensation" which most 

 cytologists attribute to superimposition of several orders of helix formation. 

 The sense of this event is clear enough in terms of the mechanics of 

 mitosis; the genetic material can be transported in compact packages. 



A little can be said about the physiological import of this "condensa- 

 tion". As has already been mentioned, the condensed state of the chromo- 

 some seems to be incompatible with its ability to synthesize DNA ; the 

 evidence for this is quite good. A case can be made for the proposition that 

 the genetic function of the chromosomes, the control of synthesis, is 

 interrupted during the period when they are condensed [i] but we need 

 not discuss this now. Since the prophase coiling of the chromosomes is 

 the most convenient signal that a given cell is committed to division, its 

 initiation and mechanism represent a major problem, and one that has 

 excited considerable and stimulating speculation [15, 16]. Many of the 

 cytological hypotheses postulate various changes in structural composition, 

 such as the packing of the primary genetic threads into a " matrix", but I 

 am afraid that most of what has been said represents ingenious inference. 

 We simply do not possess any solid facts that bear directly on chromosome 

 coiling. 



