Mitosis 



19 



and the next time it was seen relatively un- 

 coiled, at telophase. Note that we have been 

 discussing the replication of chromatids as 

 detected by microscopic observation. We 

 can also study chromosome and chromatid 

 replication using other operations. Accord- 

 ingly, let us consider some evidence regarding 

 chromosome replication at the chemical 

 level, in the hope that this will help us under- 

 stand its replication at the visible level. 



Chromosomes ("colored bodies") are 

 unique in the cell since they are the only 

 objects which stain purple by a procedure 

 called the Feulgen-Rossenbeck technique. 

 It is possible to measure the amount of 

 chromosomal material by the amount of 

 purple stain taken up by the chromosomes. 

 It is found that the amount of chromosomal 

 material does not change between prophase 

 and telophase, but doubles over a period of 

 hours during the intermitotic stage. At the 

 start of prophase, therefore, each chromo- 

 some has already replicated chemically. At 

 the visible level, however, this is not yet appar- 

 ent, so that each of the two visible chromatids 

 in a chromosome also contains the chemical 

 materials for an identical chromatid which 

 is still invisible under the microscope. This 

 new material is submicroscopic either because 

 it has not yet assumed a proper chromatid 

 form or has done so but is so tightly paired 

 with its sister chromatid that together they 

 appear as one strand. However, before the 

 next occasion when unwound threads can 

 be seen, that is, at the telophase of the same 

 mitosis, this replication at the visible level 

 has already been accomplished. So, the sub- 

 microscopic chemical reproduction which 

 takes place in a given interphase stage is not 

 visible in chromatid form until the next 

 telophase. 



What are the consequences of mitosis? 

 Speaking in terms of visible structures, the 

 chromosomal content of the parent nucleus 

 has become repeated in each daughter nu- 

 cleus, so that the subsequent division of the 



cytoplasm produces daughter cells whose 

 chromosomal composition is identical to 

 each other and to the parent cell from which 

 they were derived. The cells of different 

 species are different in that either they have 

 different numbers of chromosomes per nu- 

 cleus or the chromosomes differ in appear- 

 ance, or both. Different chromosomes may 

 differ from each other in their size, their 

 stainability with various dyes, and in the 

 location of their centromeres. Almost all 

 chromosomes have a subterminal centromere 

 which separates the chromosome into two 

 parts (arms); all chromosomes are linearly 

 arranged and unbranched. When very care- 

 ful analyses are made, any two species can 

 be shown to differ in their chromosome com- 

 plements. 



Examination at metaphase of the kinds of 

 chromosomes within the nucleus of sexually 

 reproducing organisms typically shows that 

 for each chromosome which arrives at the 

 equatorial plane there is another chromosome 

 very similar or identical to it in appearance 

 which also takes a position independently in 

 this plane. Chromosomes thus occur as pairs, 

 and the members of a pair are called Iwmolo- 

 gous c/iromosomes, or homologs, whereas 

 chromosomes of different pairs are non- 

 homologous, or non homologs. It should be 

 repeated that the members of a pair of homo- 

 logs assume their position at mitotic meta- 

 phase independently of each other. 



The number of chromosomes seen in 

 typical mitosis of the garden pea is 14 (2N, 

 or the diploid number of chromosomes) or 

 7 pairs; in Indian corn (maize) there are 10 

 pairs, in human beings 23. Whatever number 

 of chromosomes is present in the zygote, then, 

 other things being equal, the same number 

 of chromosomes will be found in every cell 

 of a multicellular organism descended from 

 the zygote by cell divisions in which mitosis 

 has occurred. 



From the information presented, we are now 

 in a position to hypothesize what may be the 



