24 



CHAPTER 4 



chromatids may be called chromosomes, as 

 mentioned in the previous Chapter). By 

 telophase the future chromatid would become 

 visible so that each telophase chromosome 

 contains two chromatids. 



While mitosis always involves chromosome 

 duplication and separation alternately, in 

 meiosis one duplication is followed by two 

 separations. The result is the maintenance 

 of the diploid chromosome condition in 

 mitosis, but a reduction from the diploid 

 to the haploid (monoploid) condition when 

 meiosis is completed. 



Let us now examine the actual meiotic 

 process in some detail (Figure 4-1), after 

 which a more complete discussion will be 

 made of specific cytological phenomena and 

 their genetic implications. Prophase of the 

 first meiotic division {prophase I) is of long 

 duration, as compared to mitotic prophase, 

 and is divided into several substages each of 

 which has its own distinguishing character- 

 istics. 



1. As they emerge from the intermitotic 

 phase the chromosomes are long and thin, 

 more so than in the earliest prophase of 

 mitosis. This is the leptonema (thin thread) 

 stage of prophase I. 



2. Next the thin threads pair with each 

 other in a process called synapsis. This pair- 

 ing is very exact, being not merely between 

 homologous chromosomes, but between ex- 

 actly corresponding individual points of the 

 homologs. Synapsis proceeds zipperwise 

 until the two homologs are completely 

 apposed. This is the zygonema (joining 

 thread) stage. 



3. The apposition of homologs becomes so 

 tight that it is difficult to identify two separate 

 chromosomes {pachynema, thick thread, stage) 

 (Figure 4-2 A). 



4. Following this, the tight pairing of the 

 pachynema is relaxed and then it can be 

 clearly seen that each pair of synapsed 

 chromosomes contains four threads, two vis- 

 ible chromatids for each chromosome (Figure 





FIGURE 4-1. Meiosis in t/ie lily — general view. 

 {Courtesy of R. E. Cleland.) 



4-2B, C). A pair of synapsed chromosomes 

 is called a bivalent when referring to chromo- 

 somes, but is called tetrad when referring to 

 chromatids. Although the chromatids sepa- 

 rate from each other in pairs here and there, 

 they are still all in close contact with each 

 other at other places along their length. Each 

 place where the four chromatids are still held 

 together is called a chiasma (cross; the plural 

 is chiasmata) (Figure 4-3 A). A chiasma is 

 characterized by the fact that the two chroma- 

 tids which synapse to make a pair on one side 

 of the point of contact separate at the point 

 of contact and synapse with other partners 



FIGURE 4-2. {Opposite). Meiosis in the lily. The 

 leptonema and zygonema stages of prophase I 

 have been omitted. {Courtesy of R. E. Cleland.) 

 {By permission of McGraw-Hill Book Co., Inc., 

 from Study Guide and Workbook for Genetics, 

 by I. H. Herskowitz, copyright 1960.) 



