IT) 



CHAPTER 2 



in mitosis, but a reduction from the diploid 

 to the haploid condition upon the completion 

 of meiosis. 



I et us examine in some detail the actual 

 meiotic process as seen under the micro- 

 scope (Figure 2-1 ). Prophase of the first 

 meiotic division (prophase I ) is of long dura- 

 tion, as compared to mitotic prophase, and 

 is divided into several substages, each with 

 its own distinguishing characteristics. 



1 . As they emerge from the interdivision 

 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 

 pairing is very exact, being not merely 

 between homologous chromosomes, but be- 

 tween exactly corresponding individual 

 points of the homologs. 1 Synapsis proceeds 

 zipperwise until the two homologs are com- 

 pletely apposed. This is the zygonema (join- 

 ing thread) stage. 



3. The apposition of homologs becomes 

 so tight that it is difficult to identify two sep- 

 arate chromosomes in the pachynema (thick 

 thread) stage (Figure 2-2A). 



4. Next, the tight pairing of the pachy- 

 nema is relaxed, whereupon it can be clearly 

 seen in the diplonema (double thread) stage 

 that each pair of synapsed chromosomes 

 contains four threads, two visible chromatids 

 for each chromosome (Figure 2-2B, C). A 

 pair of synapsed chromosomes is called a 

 bivalent (composed of two univalents) when 

 referring to chromosomes, but is called a 

 tetrad (composed of two dyads or four 

 monads) when referring to cytologically de- 

 tectable chromatids. 



Although the chromatids in a tetrad sepa- 

 rate from each other in pairs here and there, 

 they are all still in close contact with each 

 other elsewhere. Each place where the four 



1 See H. Jehle (1963) for a discussion of the 

 physical basis for the attraction of like for like. 



figure 2-1 . Meiosis in the lily 

 (Courtesy of R. E. Cleland.) 



-general view. 



chromatids are still held together is called 

 a chiasma (Greek, cross; plural, chiasmata) 

 (Figure 2-3 A). In a chiasma the two chro- 

 matids that synapse to make a pair on one 

 side of the point of contact, separate at that 

 point and synapse with other partners 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 2-3B). A 

 tetrad typically has at least one chiasma. 

 The occurrence of a chiasma assures that 

 the univalents are held together. When sev- 

 eral chiasmata occur per bivalent, loops are 



figure 2-2 (opposite) . Meiosis in the lily. 

 The leptonema and zygonema stages of pro- 

 phase I have been omitted. (Courtesy of R. E. 

 Cleland.) (By permission of McGraw-Hill 

 Book Co., Inc., from Study Guide and Work- 

 book for Genetics, by I. H. Herskowit: :. copy- 

 right I960.) 



