THE PLAN OF CELLULAR REPRODUCTION 169 



Reproduction of chromosomes 



A number of the important questions regarding the reproduction 

 of the chromosomes have been solved by considering DNA to be a 

 tracer of the genome. I might add, lest we forget, that the pioneer 

 cytochemical studies on the relation between chromosomal DNA con- 

 tents and genetic constitution were an important factor in the growth 

 of confidence in DNA as the carrier of genetic information. Some of 

 the main generalizations are: (1) DNA doubles between divisions- 

 there is no DNA synthesis during the mitotic period when the chromo- 

 somes are fully condensed; (2) the time of DNA synthesis between 

 divisions varies— there may be a considerable delay between division 

 and the beginning of DNA doubhng or between the completion of 

 DNA doubling and the next division; (3) in cells not destined to di- 

 vide, e.g., many diflFerentiated cells, there is no DNA synthesis— the 

 DNA content is that received at the last division; ( 4 ) DNA tends to be 

 metabolically stable, although there are some regions of chromosomes 

 in which it is not. 



But these important discoveries have^ their limitations. The chromo- 

 some as it goes through the whole cell cycle is not just a test tube full 

 of DNA, or, if it is, we have to solve the problem of the reproduction of 

 the tube as well as of its contents. There have been some important ad- 

 ditional advances; for example, the discovery that the basic proteins— 

 e.g., histones— associated with DNA generally double exactly in parallel 

 to the DNA (summary by Alfert, 1958). But this is still not the whole 

 story. There are proteins other than histones; there is the nucleolus 

 viewed as a part of the chromosome complex, and there are the kineto- 

 chores, the elements by which the chromosomes move in mitosis. What 

 can we say about the plan of reproduction of the whole chromosome— 

 the processes whereby we go from one complete chromosome to two? 



Again, it is interesting to note that the chromosomes have been re- 

 garded as multiplex units by many cytologists. There have been many 

 reasons, chiefly direct observations and interpretations of radiation- 

 induced breakage, for thinking that a chromosome is at all times com- 

 posed of at least two equivalent strands, which we may call, noncom- 

 mittally, chromonemata. For example, this two-stranded structure has 

 often been resolved in chromosomes at anaphase, a stage where the 

 units produced during the previous interphase are being separated 

 without further synthesis of DNA. There is also some evidence that the 

 bipartite chromosome may be further subdivided into strands. The 

 hterature on the multiplicity of chromosomes has recently been sum- 

 marized by SteflFenson ( 1959 ) . The multiplicity of the chromosomes, if 

 true, may merely reflect advantages to the cell in carrying around a 

 supply of spare genetic parts. It may also be a functional necessity; 



