190 



Cellular Events Produced by Ionizing Radiations / 10 : 3 



results from certain types of irradiation), the chromosomes do not divide. 

 The forces causing the chromosomes to adhere to the spindle at the 

 centromere, to separate, and to migrate are not at all understood. The 

 observed phenomena known as anaphase are shown in Figure 2d. Note 

 that each half of the cell now has the same number and types of chromo- 

 somes as the original one in Figure 2b. 



Centriole 

 \ Nucleolus 



Cytoplasm^ 



Nucleus with 

 Chromatin 



(e) Telophase. 



New nuclear 



membranes appear. 



Chromosomes 



elongate. 

 Cells divide. 



(a) Interphase. Vegetative growth phase. 

 Chromosomes do not 

 exist as distinct entities. 



(b) Late prophase. 

 Nucleolus has 

 faded. Nuclear membrane 

 has disappeared. 

 Centriole has divided 

 and spindle is forming. 

 Double stranded chromosomes 

 have formed; dots represent 

 centromeres. 



(c ) Metaphase. Chromosomes line 

 up on equatorial plate 

 midway between centrioles. 

 Chromosomes attach to 

 spindle at centromeres. 



(d) Late anaphase. In anaphase 

 centromeres divide. Daughter 

 centromeres move as if pulled to 

 opposite centrioles. 

 Cell starts to divide. 



Figure 2. Diagrammatic outline of mitotic cycle. Each cell 

 starts with two homologous chromosomes, distinguished in the 

 diagram by showing their centromeres as dots and squares. 

 Modified from Life : An Introduction to Biology by G. G. Simpson, 

 C. S. P. Hendrigh, and L. H. Tiffany, © 1957, by Harcourt, 

 Brace & World, Inc. 



Finally, as illustrated in Figure 2e, new nuclear membranes form and 

 the cell pinches in two during the final stage called telophase. If no 

 spindle forms, each cell ends up with about half the original number of 

 chromosomes and eventually dies. In normal mitosis, by contrast, one 

 ends up with two duplicates of the original cell. These duplicates are 

 sometimes referred to as daughter cells. 



In the normal cells, the chromosomes occur as pairs. The two 

 members of the pair have similar shapes and are believed to control the 

 same characteristics. If the two members of the pair are not identical, 

 one will be dominant for each character and the other recessive; the cell 

 and the individual usually reflect only the dominant character. How- 

 ever, one chromosome will not be dominant for all the characteristics it 



