Genetic Material and Mitosis 



11 



true for the chromosomes — namely the in- 

 dependence with which each chromosome 

 arrives at metaphase. It might be thought, 

 when the chromosomes "disappear" during 

 interphase, that their individuality is lost 

 and even that their contents are dispersed. 

 That the nuclear material is not dispersed 

 into the cytoplasm between successive mi- 

 toses is indicated by the retention of the 

 full amount of chromosomal material within 

 the nucleus during interphase, insofar as re- 

 vealed by the Feulgen staining technique. 

 Even so, it is still possible that those com- 

 ponents of chromosomes which remain in 

 the nucleus become scrambled during inter- 

 phase and later resynthesize their proper 

 form during the next prophase. Four lines 

 of evidence bearing on this matter can be 

 mentioned. The first three come from study- 

 ing the appearance of successive mitoses. It 

 is possible to observe the relative positions 

 of the chromosomes at late anaphase or telo- 

 phase and also their relative positions as 

 they enter the next prophase. When this is 

 done, the chromosomes are found to have 

 held the same relative positions, as expected 

 had they retained their integrity during the 

 intervening interphase. Second, since the 

 nucleolus does not fragment during inter- 

 phase, those parts of the chromosomes, 

 called nucleolus organizers, with which the 

 nucleolus is associated probably remain as- 

 sociated during that interval. 3 Third, it 

 sometimes happens that two originally iden- 

 tical homologs are modified by mutation so 

 that each is changed in a different respect. 

 The finding, mitosis after mitosis, that both 

 homologs retain their separate differences is 

 evidence that each homolog has retained its 

 individuality cell generation after cell gen- 

 eration. Finally, more direct evidence on 

 the retention of chromosomal individuality 

 during interphase is available from cells of 



3 See also F. H. Ruddle (1962). 



larval salivary glands of certain flies. These 

 giant cells have interphase nuclei that con- 

 tain giant chromosomes which, though rela- 

 tively uncoiled, are clearly equivalent to the 

 more contracted chromosomes seen during 

 mitosis. 



The number of points of similarity be- 

 tween genetic material and chromosomes is 

 already impressive. However, if all nuclei 

 divide by mitosis, a gamete should contain 

 the same number of chromosomes as the 

 other cells derived from the original zygote; 

 and since the zygote of any generation com- 

 bines two gametes, the number of chromo- 

 somes should increase in the zygotes of suc- 

 cessive generations. One would therefore 

 expect an increase in the amount of genetic 

 material in successive sexual generations. 

 This expectation is not realized, however, 

 since one finds that all individuals of a 

 species have a characteristic, typically stable, 

 chromosomal content. In fact, as expected, 

 human gametes do not contain the paired, 

 diploid, chromosome number, that is, 23 

 pairs of nonhomologous chromosomes. In- 

 stead each usually contains 23 chromosomes, 

 one of each nonhomologous type, comprising 

 a complete, unpaired, haploid or monoploid, 

 set of chromosomes. The zygote, therefore, 

 has the diploid chromosome constitution re- 

 stored because each gamete furnishes a 

 haploid set of chromosomes, one set con- 

 tributed by the sperm from the father, and 

 another set by the egg from the mother. In 

 this way chromosomes remain as pairs, 

 sexual generation after sexual generation, 

 and the number of chromosomes in zygotes 

 remains unchanged. Clearly, then, the cell 

 divisions preceding gamete formation cannot 

 be invariably mitotic, but must involve at 

 some point a special mechanism for reducing 

 the number of chromosomes from diploid to 

 haploid. The nature of this special kind of 

 nuclear behavior is considered in the next 

 chapter. 



