170 CELLS, TISSUES, AND ORGANISMS 



there may be some reason why it takes more than one gene of a given 

 kind to produce a gene product. But it may also be a reflection of the 

 reproductive habits of chromosomes, as we think it is in the case in 

 centrioles. 



In the last few years the study of the distribution of newly synthe- 

 sized DNA at mitosis by Plaut and Mazia ( 1956), Taylor et al. ( 1957), 

 LaCour and Pelc ( 1959 ) , and others has led to rather conflicting con- 

 clusions. The disagreements perhaps have been given undue impor- 

 tance because of the seeming implications for the theory of the replica- 

 tion of DNA molecules, even though all of the experimenters have in- 

 sisted that their work was concerned with the chromosomal level, not 

 with DNA as such. In terms of a generative model of chromosome re- 

 production, the controversy whether "old" and "new" chromosome ele- 

 ments can separate at the division following an interphase during 

 which DNA is labeled might not be related to DNA synthesis at all. 

 Under conditions where the "new" unit was not completed and 

 separable from its parent before division, the division would separate 

 pairs of strands consisting of one "old" and one "new." If the conditions 

 happened to be such that development and parturition took place be- 

 fore division, the four units could split at random. These conditions are 

 not hard to imagine, in view of the simplicity of the experiments with 

 mercaptoethanol. Any condition that delayed division without delaying 

 the development of the daughter units would suffice. 



Preparations for division 



So far as we know, the reproduction of the chromosomes and of the 

 mitotic centers are the only truly reproductive events within the cycle 

 of cell reproduction, and even the latter is doubtful in plant cells. If 

 only these two events take place, we will make cells with many nuclei 

 ( or polyploid or polytene nuclei ) and many centrioles, but we will not 

 get many cells. What are the further conditions of cell reproduction? 



In the long run, one of these is growth in its narrow sense— the pro- 

 duction of additional cytoplasmic structure and of additional enzymatic 

 machinery. This is not strictly a requirement for a given division. Divi- 

 sion without growth, producing cells of abnormally small size, is a com- 

 mon enough phenomenon, as in egg cells or in cells under certain condi- 

 tions of nutritional limitation. But it cannot go on indefinitely; cells 

 cannot divide without growth until they vanish. On the other hand, 

 growth is not a sufficient condition of division. There are many cases 

 where nuclear endoreproduction is accompanied by cytoplasmic 

 growth, as in the production of "giants" following irradiation of animal 

 cells in culture (Puck and Marcus, 1956). 



In order that cell devision follow the reproductive events we have 



