CHAPTER l6 



The Nature of the Gene 



A. CYTOLOGICAL CONSIDERATIONS 



I. Chromosome Movements^ 



There is very little experimental work which gives us any detailed 

 understanding of the nature of the forces which act on and between 

 chromosomes during the process of nuclear division; the nucleus is too 

 carefully guarded within the cell to be easily reached by our present 

 experimental methods. But the process of division, while remaining 

 fundamentally the same throughout the biological realm, suffers in the 

 different groups various modifications which can be regarded as natural 

 experiments and these are numerous enough to enable cytologists to 

 make deductions about the mechanisms involved. Particularly the 

 study of meiosis in polyploids, which has been very vigorously pursued 

 in recent years, has been found to provide crucial tests for many of the 

 possibilities suggested by the phenomena in simpler forms. There is no 

 space here to summarize the detailed evidence, which belongs to 

 cytology proper rather than to genetics. However, a short account of 

 the general conclusions arrived at must be given, since a knowledge of 

 the forces between chromosomes obviously has a bearing on our idea 

 of chromosome constitution. 



Two fundamental forces are involved throughout the whole of 

 mitosis and meiosis; firstly, an attraction between similar genes in 

 pairs, which leads to pairing in meiotic prophase, and secondly a re- 

 pulsion between pairs of chromonemata, and presumably between 

 individual pairs of genes, which leads to the diakinesis configuration 

 and aids in terminalization. To these we must add repulsions firstly 

 between centromeres and centrosomes, which cause the movement of 

 chromosomes on to the metaphase plate, and secondly between centro- 

 meres, which aid in terminalization and cause the first part of anaphase 

 separation. Finally, the later part of anaphase separation is due to the 

 elongation of the region of the spindle between the two separating sets 

 of chromosomes. These forces are operative partly within the fluid 

 nuclear sap and partly within the semi-solid spindle which is formed 

 after the nuclear membrane breaks down. 



^ Darlington 1937. 



