162 CELLS, TISSUES, AND ORGANISMS 



Reproductive events of the cell cycle 



We are sure of the existence of one set of events in the reproduc- 

 tive cycle of the cell that involves genuine self-replication— the repro- 

 duction of the genetic elements of the chromosomes. For the present, let 

 us assume that this is primarily the replication of DNA; if this is a mere 

 hypothesis, it is surely one of the most stimulating hypotheses in the 

 history of biology. In higher organisms, at least, the genetic operations 

 are carried out by chromosomes, which are complex and contain a 

 good deal more than DNA. We shall have to consider the character of 

 the reproduction of the chromosome as a whole. 



A second example of self-reproduction in the cell cycle is the 

 multiplication of centrioles. This clearly is essential for the reproduc- 

 tion of animal cells, many protozoan cells, and some lower plants. I 

 shall not discuss the controversy about the presence of centrioles or 

 their functional equivalents in higher plants, having dealt with it at 

 length in another place ( Mazia, 1961 ) . Sufiice it to mention that many 

 of the phenomena attributed to the centrioles in animal cells are also 

 observed in the division of plant cells, but the corresponding physical 

 particles have yet to be observed in plant cells. 



Present-day theory about cell reproduction in general does not 

 require any other self-reproducing elements. Additional elements meet- 

 ing some or all of the specifications of self-reproducing particles are 

 known— for example, plastids in plants and kinetosomes in flagellates 

 and ciliates— and others have been invoked to account for special cases. 



Reproduction of centrioles 



The name first given to the centrioles by Van Beneden— "polar 

 corpuscle"— describes their significance more vividly than the name that 

 has come into common use. They are indeed the physical embodiments 

 of the poles of mitosis, and mitosis would make no sense if it were not 

 polarized. The centrioles define the destinations of the chromosomes 

 when they move apart to form two separate daughter nuclei. The basic 

 principle of mitosis is that sister chromosomes move to diflFerent cen- 

 trioles, normally to sister centrioles, and never to the same one. If we 

 add that the plane of mitotic division is exactly midway between the 

 centrioles and exactly at right angles to the axis connecting the cen- 

 trioles— and both of these generalizations are valid for normal division 

 of animal cells— we can see that the existence of a "polar corpuscle" has 

 a profound meaning for cell division. The idea of a "pole" in this con- 

 text is richer than the physical abstraction to which we ordinarily 

 apply the term, being represented by distinct particles having the 



