28 



COLLEGE ZOOLOGY 



^ Telophase 



m 



Y Anaph 



\ 



Centromere 



ase 



Metaphase 



Chromatids 



Prophase 



Figure 10. Structure of a chromosome during mitosis. Note spiralization (coiling) of chro- 

 monema throughout the cycle. The interphase chromosome consists of at least two chromo- 

 nemata. In the early prophase chromosome, note the two distinct chromonemata and how they 

 shorten by coiling; in the last stage of the prophase, observe that the chromonema of each half 

 chromosome (chromatid) has been duplicated. Metaphase shows that each half chromosome 

 is composed of two chromonemata. Anaphase shows that daughter halves of the duplicated 

 chromosome separate and move to opposite ends of the mitotic spindle. In the telophase the 

 chromosome forms from two daughter chromonemata. The chromosome is linearly differentiated 

 into a variety of genes, qualitatively different from one another insofar as they affect the develop- 

 ment of traits. The centromere has been indicated by a clear circle; this is the point of spindle- 

 fiber attachment. (After General Cytology by De Robertis, Nowinski, and Saez. Second edition. 

 Copyright 1954 by Saunders Company.) 



they appear and behave as a single structure. 

 As the prophase progresses, the chromosome 

 thickens and shortens, probably due to the 

 chromonemata becoming more tightly 

 coiled, like a spring (Fig. 10). 



The primary significance of mitosis is the 

 separation of the longitudinally duplicated 

 chromosomes into two identical groups, 

 constituting two daughter nuclei. The gen- 



eral result is that every cell in the body con- 

 tains the same number of chromosomes of 

 the same size, shape, and quality. 



Chromosomes have a persistent individ- 

 uality. Those that appear during the pro- 

 phases of mitosis are the same as those that 

 took part in the reconstruction of the nu- 

 cleus in the telophase of the preceding di- 

 vision. In some cases the chromosomes are 



