is 



CHAPTER 2 



formed, adjacent ones at right angles to each 

 other. 



As diplonema continues, the chromosomes 

 become shorter and thicker, more compacted 

 than they ever become in mitosis. 



5. In some animals, during the formation 

 oi female gametes especially, a diffuse or 

 growth stage follows diplonema, in which 

 the chromosomes ami nucleus revert to the 

 appearance kmnd in a nondividing cell. 

 During this stage a great amount of cyto- 

 plasmic growth takes place. In human be- 

 ings this stage may last for decades, after 

 which the rest of meiosis occurs and mature 

 eggs ready for ovulation are produced. 



6. Diakinesis (Figure 2-2D) is charac- 

 terized by the maximal contraction of diplo- 

 nema chromosomes, or by maximal recon- 

 traction 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. 



Metapha.se I (Figure 2-2E) is attained 

 by the movement of chromosomes to the 

 midspindle, as in mitosis, except that they 

 move as bivalents, made up of a tetrad of 

 chromatids still held together by chiasmata. 

 Between diplonema and metaphase I the 

 chiasmata move toward the end of the chro- 

 mosome arms, that is. away from the centro- 

 mere, especially if the bivalent is short. As 

 a consequence of this chiasma terminaliza- 

 tion the number of chiasmata present at 

 metaphase I may be less than it was at diplo- 

 nema. 



During anaphase I (Figure 2-2F) the uni- 

 valents in each bivalent separate from each 

 other at the region of the centromere and 

 proceed to opposite poles of the spindle. 

 This movement completely terminalizes all 

 remaining chiasmata. The dyad nature of 

 each univalent is readily seen in the figure. 

 In telophase I the two daughter nuclei are 

 formed, and interphase I (Figure 2-2G) 



follows. The length of interphase I varies 

 in different organisms. 



Each daughter nucleus undergoes the sec- 

 ond meiotic division, which proceeds as ex- 

 pected from mitosis. In prophase II, each 

 univalent (equivalent to a chromosome with 

 its two visible chromatids) contracts; at 

 metaphase II (Figure 2-2H) each lines up 

 at the equator of the spindle independently; 

 at anaphase II the members of a dyad sepa- 

 rate and go to opposite poles as monads 

 (each equivalent to a single chromosome, 

 since now each contains two visible chro- 

 matids). Because two nuclei undergo this 

 second division, four nuclei are formed at 

 telophase II (Figure 2-21). Photographs 

 of the meiotic process in corn can be seen 

 in Figure 2-4 (pp. 20-21 ). 



Chromosomal Segregation 



Consider next the consequences of meiosis. 

 The organism undergoing meiosis starts its 

 existence as a zygote produced by fertiliza- 

 tion involving the union of two haploid sets 

 of chromosomes, one maternal and one 

 paternal. When meiosis is completed the 

 diploid, paired, chromosome number is re- 

 duced to the haploid, unpaired, chromosome 

 number. Since any postmeiotic nucleus nor- 

 mally contains only one representative of any 

 given pair of chromosomes present in a pre- 

 meiotic nucleus, chromosome segregation 

 has occurred. Two questions come to mind 

 at this point. First, is the haploid set of 

 chromosomes, or genome, in a gamete com- 

 posed of replicas of all the chromosomes 

 contributed by the female parent or of all 

 those contributed by the male parent? 



For typical meiosis, the answer depends 

 upon two events. The first of these is the 

 manner in which the centromeres of the 

 bivalents arrange themselves at the equator 

 of the spindle at metaphase I. Relative to 

 the poles of the spindle, each bivalent ar- 

 ranges itself at the equator independently of 



