26 



CHAPTER 4 



B 



FIGURE 4-3. Lily diplonema showing chromatids 

 {1-4) with dijferent synaptic partners on dijferent 

 sides of a chiasma. {Courtesy of R. E. Cleland.) 



on the other side of the contact point, i.e., 

 the partners making up two synapsed pairs 

 of chromatids are different on the two sides 

 of the place of contact (Figure 4-3 B). 



It is, therefore, the occurrence of chiasmata 

 at least at one place along each tetrad that 

 prevents the bivalents from falling apart. 

 Chiasmata comprise the last feature unique 

 to meiosis, which is not found in mitosis 

 where tetrads are absent. As diplonema 

 (double thread) continues, the chromosomes 

 become shorter and thicker, more so than 

 they ever become in mitosis. 



5. In some animals, during the formation 

 of female gametes especially, a diffuse or 

 growth stage follows diplonema, in which 

 the chromosomes and nucleus revert to the 

 appearance found in a nondividing ceil. 



During this stage there occurs a great amount 

 of cytoplasmic growth. In human beings 

 this stage may last for decades, after which 

 the rest of meiosis occurs and mature eggs 

 ready for ovulation are produced. 



6. Diakinesis (Figure 4-2D) is character- 

 ized by the maximal contraction of diplonema 

 chromosomes, or by maximal recontraction 

 of the chromosomes which had entered a 

 diffuse stage. By the end of this stage nucleoli 

 and nuclear membrane have disappeared, the 

 spindle has formed, and prophase I is 

 completed. 



Metaphase I (Figure 4-2E) is attained by 

 the movement of chromosomes to the mid- 

 spindle, as in mitosis, except that they move 

 as bivalents, made up of a tetrad of chroma- 

 tids still held together by chiasmata. Ana- 

 phase I (Figure 4-2F) shows the bivalents 

 separating at their centromeres, moving to 

 opposite poles as univalents, each composed 

 of two chromatids (making now a dyad). 

 In telophase /the daughter nuclei are formed, 

 and interphase I (Figure 4-2G) follows. The 

 length of interphase I varies in different 

 organisms. 



The second meiotic division proceeds in 

 both daughter nuclei as expected from mitosis. 

 In prophase If the univalents containing 

 dyads (each equal to a chromosome with its 

 two chromatids) contract; at metaphase II 

 (Figure 4-2H) each univalent lines up at the 

 equator of the spindle independently; at 

 anaphase II the members of a dyad separate 

 and go to opposite poles as monads (each 

 equivalent to a single chromosome containing 

 its two chromatids). Since two nuclei 

 undergo this second division, there are four 

 nuclei formed at telophase II (Figure 4-21). 

 Photographs of the meiotic process in corn 

 can be seen in Figure 4-4 (pp. 28-29). 



Consider now the consequences of meiosis. 

 At the completion of meiosis each nucleus 

 contains one representative of each pair of 

 chromosomes that was present in the nucleus 

 at the start of meiosis. Meiosis, then, could 



