10 : 3/ Cellular Events Produced by Ionizing Radiations 



191 



controls. During mitosis, each chromosome is split and, therefore, the 

 daughter cells have the same character as the original cell. 



During meiosis, however, the two homologous chromosomes line up 

 together, entwine about one another, and then separate along the 

 spindle to opposite poles. Thus, the egg and sperm cells end up with 

 half the number of chromosomes as the normal body cells. This 

 division is not completely random because each egg or sperm cell con- 

 tains one member of each pair of chromosomes. When the sperm 

 fertilizes the egg cell, the normal number is re-formed. Figure 3 illus- 

 trates diagrammatically the chromosome changes in meiosis. 



(a) Interphase. As in Fig. 2(a). 



(d) 



Late anaphase. As in Fig. 2(d), 

 except each daughter cell has half 

 the original number of chromosomes. 

 Crossing over can occur in early 

 anaphase. 



(e) Telophase. 



Two haploid cells 

 are formed. Note 

 each chromosome 

 is double stranded. 



(b) Late prophase. 

 As in Fig. 2(b). 

 Cell is called 

 diploid. 



Metaphase. Homologous 

 chromosomes pair up at 

 centromeres and line up 

 along equatorial plane. 

 Pairs twist around each other, 

 forming 4 -stranded groups. 

 Crossing over can occur. 



(f) Haploid cells grow and undergo mitosis, resulting in 

 four haploid cells. Chromosomes are shown within 

 these for diagrammatic purposes. Some of the last 4 

 grow, forming double-stranded chromosomes and 

 becoming the active cells of sexual reproduction. 



Figure 3. Note that if there are pairs of homologous chromo- 

 somes, the cell is called diploid, whereas if there are only 

 half this number of chromosomes, the cell is called haploid. For 

 discussion of crossovers, see Figure 4. Modified from Life : 

 An Introduction to Biology by G. G. Simpsom, C. S. P. Hendrigh, 

 and L. H. Tiffany, © 1957, by Harcourt, Brace & World, Inc. 



