24 Nature of the Genetic Material 



those of Matsuura, Belling, and Darlington. But according to the well- 

 known experiments of Stern and McClintock and Creighton, de- 

 scribed in all textbooks, a segmental exchange, involving two breaks, 

 seems to be a necessity for crossing over, whatever the details, causes, 

 and forces may be. The same conclusion follows from unequal cross- 

 ing over of the Bar segment, also of Beadex (Green, 1953fo). The two 

 attempts at explaining crossing over without exchange of segments 

 are generally regarded as antiquated. This is true of my own theory 

 ( 19nb ) that individual genes are assembled to the chromosome after 

 the manner of antigen-antibody fixation, with a variable force pro- 

 viding for the numerical rules. Another large-scale attempt at explain- 

 ing crossing over without chiasmatype was made by Winkler (1930) 

 in his conversion theory (hardly noticed until recently) of direct 

 change of individual alleles into each other. 



It is rather remarkable that elements from these older theories 

 have been revived in a recent biochemical theory of Haldane ( 1954 ) , 

 which, however, is not worked out in detail to include the numerical 

 aspect of crossing over. He assumes that a chromosome is copied into 

 a different structure, related like antigen and antibody. This is a 

 template, or negative, which is recopied into two positives. It is further 

 assumed that original and negative are nucleic acid and protein, 

 respectively. "It is intelligible, that each [of the copies] should be a 

 mixed copy of maternal and paternal material. If this is correct, 

 crossing over, in the sense of chromosome breakage and subsequent 

 reunion, never occurs." Thinking in chemical terms, Haldane compares 

 the non-mechanical exchange with transpeptidization, and proposes 

 to consider the process of crossing over, as far as proteins are con- 

 cerned, as a series of simultaneous transpeptidizations. I quote these 

 views as a sign that the theory of crossing over is, after almost fifty 

 years, still or again in the stage of uneasy discussion, even of its 

 elementary aspects. But I may add, with due caution, that there are 

 indications that duplication of genetic material occurs — at least in 

 meiosis — independently of chromosome duplication. Thus it may be 

 possible to discern genie and non-genic parts on this level: genie parts 

 duplicated by a template mechanism, non-genic parts dividing simply 

 so as to assemble the new genie parts into a new chromatid. Belling's 

 old theory of crossing over led to such a view, which again may find 

 support in recent tests for Belling's theory. This hints at the possibility 

 that a real understanding of crossing over may also reflect upon our 

 views on chromosomal constitution and division, and thus on the 

 nature of the genie material (see Schwartz, 1954). 



