Chapter 2 



MEIOSIS AND 

 CHROMOSOMAL SEGREGATION 



H 



"ow do both male and female 

 gametes come to contain 

 only one set of chromo- 

 somes, composed of one member of each 

 pair of chromosomes found in the nucleus 

 of an ordinary body, or somatic, cell? If 

 gametes were produced by regular mitotic 

 division, they would be diploid. The reduc- 

 tion from two sets to one is brought about 

 by another type of indirect nuclear process, 

 called meiosis, which actually requires two 

 successive nuclear divisions to accomplish 

 its result. 



Meiosis 



To render the cytological description of the 

 meiotic process more meaningful, several as- 

 sumptions will be made. Suppose that the 

 processes directing the division of the nucleus 

 act especially early in the case of meiosis, 

 before the chromosomes have attained the 

 degree of coiling first seen in mitotic pro- 

 phase. Suppose further that a relatively 

 more uncoiled state of the chromosome is, 

 under these conditions, associated with an 

 especially strong attraction between homo- 

 logs of like chromosome parts for like parts 

 and that this attractive force extends over 

 considerable, though still microscopic, dis- 

 tances. Then, with one additional novelty 

 yet to be described, the meiotic process will 

 occur in the following predictable way when 

 the chromosomes, without further replica- 

 15 



tion, undergo two successive mitotic divi- 

 sions. 



In prophase of the first meiotic division, 

 just as in mitotic prophase, each chromo- 

 some contains two chromatids plus an equal 

 amount of chromosomal material not yet 

 visible as chromatids (see p. 8). But 

 now, because of the early onset of nuclear 

 division, homologous chromosomes pair 

 point for corresponding point (making a 

 bundle of four chromatids plus an equal 

 amount of future chromatid material). Ac- 

 cordingly, the chromosomes proceed as pairs 

 to the equator of the spindle for the meta- 

 phase. (Recall that in mitosis, on the other 

 hand, each chromosome of the two sets pres- 

 ent goes to the equator of the spindle in- 

 dependently of its homologous chromo- 

 some.) At anaphase the members of a pair 

 separate and go to opposite poles, each ana- 

 phase chromosome still containing two chro- 

 matids plus an equivalent amount of future 

 chromatid material. In the interphase after 

 the first telophase, no synthesis of future 

 chromatid material takes place since what 

 was made in the previous interphase had 

 not been used to make visible chromatids 

 in the first meiotic division. The second 

 meiotic division may start at any time and 

 proceed as a typical mitosis. In the second 

 meiotic prophase each chromosome contains 

 two chromatids and the material for two 

 future chromatids. Each chromosome pro- 

 ceeds to metaphase independently; at ana- 

 phase the two chromatids separate and go 

 to opposite poles of the spindle (after sep- 

 aration the chromatids may be called chro- 

 mosomes). By telophase the future chro- 

 matid becomes visible; thus each telophase 

 chromosome contains two chromatids. 



Although mitosis always involves chromo- 

 some duplication and separation alternately, 

 one duplication is followed by two separa- 

 tions in meiosis. The result is the mainte- 

 nance of the diploid chromosome condition 



