22 



( HAPTER 2 



m 

 m 

 P 



P 



T 



m 



rr 

 P 

 P 



^ 



m 



m 



HGURE 2-5. Chiasma showing paternal (p) and maternal (m) composition of strands. 

 Compare with Figure 2-3B. 



matids. then, arc uniparental replicas for this 

 region, one all-maternal and the other all- 

 paternal. The recombinational event that 

 produces the two biparental segments can 

 be called an exchange, although this should 

 not be taken to mean that what occurred 

 was a cross-union following breakage of a 

 paternal and a maternal chromatid at exactly 

 corresponding positions. It is likely that 

 synapsis at any level is strongest between 

 strands having the same parental derivation. 

 Accordingly, during diplonema of prophase 

 I, when the chromatids separate in pairs, 

 they should switch pairing partners at the 

 point where the two biparental chromatids 

 change their parental derivation. This 

 switching would produce the chiasma con- 

 figuration. Note in Figure 2-3B, where 

 chromatid material of one parental deriva- 

 tion is shown filled in whereas that of the 

 other is not, that it is necessary for chroma- 

 tids to change pairing partners in order to 

 maintain in synapsis all the corresponding 

 paternally-derived (and also all the corre- 

 sponding maternally-derived) portions of the 

 strands. Some of the exchanges may fail to 

 produce a visible chiasma later; this could 

 happen if the pairing were uniparental on one 

 side and biparental on the other side of a 

 point of exchange. On the other hand, every 

 chiasma can be taken to represent cytological 

 evidence of a prior intratetrad exchange. It 

 should be noted, however, that because of 



chiasma terminalization. the position of a 

 chiasma may be distal to the point where 

 the two biparental chromatids changed their 

 parental type. We will henceforth assume 

 that chiasma terminalization is absent dur- 

 ing diplonema, thereby making it possible 

 to equate the position of a chiasma with the 

 point of exchange. Accordingly, a tetrad 

 containing one chiasma would have the pa- 

 ternal (p) and maternal (m) linear consti- 

 tution shown in Figure 2-5, where the cen- 

 tromere is represented by C. The figure 

 shows that, after one exchange, one chro- 

 matid remains entirely maternal and one en- 

 tirely paternal, but the other two are bi- 

 parental in origin. Note again that only 

 two of the four chromatids are involved in 

 a single exchange. However, a tetrad nor- 

 mally contains several chiasmata; this means 

 that earlier each of the four chromatids had 

 probably exchanged at one place or another 

 with a chromatid derived from the other 

 parent and consequently has a biparental 

 composition. 



We can now return to the questions re- 

 garding the maternal-paternal chromosome 

 content of the haploid products of meiosis. 

 The centromeres in a bivalent separate at 

 anaphase I ; thus segregation of maternal from 

 paternal centromeres occurs at the first meio- 

 tic division. Then, as revealed by the loca- 

 tion and number of chiasmata at diplonema, 

 the monads having the maternal centromeres 



