388 CELL HEREDITY 



TRANSMISSION MECHANISMS 



The linear arrangement of genetic material has been estabUshed as a 

 characteristic feature of organisms from bacteriophage to man. Pre- 

 sumably this hnearity is a consecjuence of the geometry of the nucleic 

 acid molecule, which is a linear polymer. In classical genetic theory, 

 genes were viewed as indivisible units strung along a one-dimensional 

 chain. With the development of methods for the analysis of recombina- 

 tion within a gene, the evidence of linearity has been extended virtually 

 to the nucleotide level. In all organisms from phage to man, the chromo- 

 somal gene consists of a functional unit responsible for the appearance 

 of a particular gene product, and composed of a linear array of hundreds 

 or even of thousands of nucleotides. Presumably genetic information 

 resides in the linear sequence of these nucleotides which is rigorously 

 maintained in replication. 



The transmission of these giant polymers to progeny cells without loss 

 of their linear order has been achieved by means of chromosomes. In 

 the bacteria and viruses, mapping has thus far indicated an invariant 

 order which does not differ from strain to strain. In E. coli, for example, 

 despite the evidence of chromosome breakage by insertion of the F 

 factor, no evidence has been obtained of any inversions. Furthermore, 

 the gene order in the related genus. Salmonella, seems to be the same as 

 in £. coli. In organisms with true chromosomes in the sense of complex 

 nucleohistone bodies, considerable rearrangement of the gene order has 

 occurred. Translocations and inversions are common in wild popula- 

 tions. The significance of these differences will probably remain obscure 

 until more is known of the organization of the genetic material. 



The most perplexing question in the field of transmission genetics con- 

 cerns the mechanism of recombination between linked genes. Whether 

 exchanges occur during replication (Belling s hypothesis) or during the 

 prophase pairing of preformed chromosomes (partial chiasmatype hypo- 

 thesis), or whether both processes occur (or something quite different 

 from either one) remains one of the immediate and challenging questions 

 in genetic analysis today. It is related of cour.se to other unknowns: 

 Is DNA present as a continuous double helix or are there non-DNA con- 

 necting links? Are there functional differences of significance between 

 the bacterial chromosome and that of higher organisms? What controls 

 chromosome breakage and rejoining, and the healing of broken ends? 

 What is the structural basis of the phenomenon of position effect? 



The study of chromosome rearrangements by conventional recombina- 

 tion analysis led to the recognition of position effect in the 1930's. Two 



