THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



27 



Metaphase, the phase of separation. The metaphase begins as the 

 chromosomes reach the center of the spindle. It can be seen at this 

 point that a spindle fiber is attached to each centromere of the chromo- 

 somes. Next, the centromeres divide so that there is one for each 

 of the coiled gene strings or chromatids. The spindle fibers now seem 

 to exert a pull on these centromeres, and they are pulled along to the 

 poles at each end of the spindle dragging their chromatids behind them. 



A 







t 



Omit this one 



Fig. 2.5. The life cycle of a single chromosome. Starting with the chromosome in 

 interphase (A), we see how it becomes duplicated and then shortens by coiling (C and 

 D). After separation at metaphase (G), the chromosome uncoils as it goes into an- 

 other interphase (J). 



Thus, the actual splitting of the chromosomes is accomplished ; al- 

 though, as we have seen, it was preceded by a duplication of the genes 

 and the gene strings some time earlier. As a chromosome is thus 

 separated, each chromatid now becomes a separate chromosome. It 

 is no longer a half-chromosome, but a complete chromosome in itself. 

 Anaphase, the phase of migration. After the chromosomes have 

 been pulled apart to form two distinct groups, the cell is said to be in 

 the anaphase. In this stage each centromere leads the way with the 

 balance of the chromosome being dragged along behind. This causes 



