Chapter *4 

 MEIOSIS 



B; 



Y WHAT kind of process do 

 both male and female gametes 

 'come to contain only one set of 

 chromosomes, composed of one member of 

 each pair of chromosomes found in the 

 nucleus of an ordinary body, or somatic, cell? 

 Gametes cannot be produced by regular 

 mitotic division or they would be diploid. 

 The reduction from two sets to one has been 

 found to be brought about by another type 

 of indirect nuclear process, called meiosis, 

 which actually requires two successive nuclear 

 divisions to accomplish this result. 



Since the diploid number of chromosomes 

 is maintained generation after generation in 

 sexually reproducing forms, it is not surpris- 

 ing that meiosis always occurs at some time 

 in the life cycle of each sexually reproducing 

 individual. In most animals meiosis com- 

 prises the last two nuclear divisions before 

 the mature sperm or egg is produced. Meiosis 

 occurs at different times in the life history of 

 different plants, but almost never immedi- 

 ately before the formation of gametes. There 

 are minor variations in different species in the 

 details with which meiosis is carried out; 

 what is presented here is a generalized account 

 of its main features. 



In order that the cytological description of 

 the meiotic process may be more meaningful 

 to you, several assumptions will be made. 

 Suppose that the processes which direct the 

 nucleus to divide act especially early in the 

 case of meiosis, before the chromosomes have 

 attained the degree of coiling first seen in 

 mitotic prophase. Suppose further that a 

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relatively more uncoiled state of the chromo- 

 some is, under these conditions, associated 

 with an especially strong attraction of like 

 chromosome parts for Uke parts and that this 

 attractive force extends over considerable, 

 though still microscopic, distances. Then, 

 with one additional novelty which will be 

 described later, the meiotic process will occur 

 in a predictable way when the chromosomes 

 undergo in sucession two divisions of a mi- 

 totic nature. 



In prophase of the first meiotic division, 

 just as in the case of mitotic prophase, each 

 chromosome would contain two chromatids 

 plus an equal amount of chromosomal ma- 

 terial not yet visible as chromatids (Chapter 

 3). But, now, because of the early onset of 

 nuclear division, homologous chromosomes 

 would pair point for corresponding point 

 (making a bundle of four chromatids plus an 

 equal amount of future chromatid material). 

 Once paired, the chromosomes would pro- 

 ceed as pairs to the equator of the spindle for 

 the metaphase. (Recall that in mitosis, on the 

 other hand, each chromosome of the two sets 

 present behaves independently of its homolo- 

 gous chromosome in going to the spindle's 

 equator.) At anaphase the members of a 

 pair would separate and go to opposite poles 

 (note each anaphase chromosome would still 

 contain two chromatids plus an equivalent 

 amount of future chromatid material). In 

 the interphase which follows the first telo- 

 phase, no synthesis of future chromatid ma- 

 terial would take place since what was made 

 in the previous interphase had not been used 

 to make visible chromatids in the first meiotic 

 division. So, the second meiotic division 

 could now take place at any time and proceed 

 as a typical mitosis. In the second meiotic 

 prophase each chromosome would contain 

 two chromatids and two future chromatids. 

 Each chromosome would proceed to meta- 

 phase independently; at anaphase the two 

 chromatids would separate and go to oppo- 

 site poles of the spindle (once separated the 



