50 - The Cell 



(with rare exception) a constant number of 

 chromosomes in the many cells of each differ- 

 ent organism. This constant is usually an 

 even number, and this number is the same 

 for all individuals belonging to a given spe- 

 cies. The chromosome number varies, in dif- 

 ferent organisms, from 2 to 200 or more, but 

 usually it is below 50 (man has 46). Not only 

 the number of chromosomes but also the size 

 and shape of each chromosome remain con- 

 stant. Usually the chromosomes of a given 

 species are individually distinguishable, and 

 exactly the same set appears at mitosis in all 

 cells of the individual and of the species. 

 The chromosomes that appear in any cell 

 during the prophase are exactly the same as 

 those that apparently disappeared during 

 the preceding telophase. In some cases the 

 separateness of the chromosomes can be seen 

 at all times, even throughout the intermitotic 

 period. But usually this cannot be seen di- 

 rectly, although other evidence indicates that 

 it is true. In other words, each chromosome 

 has a persistent individuality. Each grows 

 and replicates its own unique genie mate- 

 rials during the intermitotic period and each 

 divides at every mitosis. Thus every chromo- 

 some maintains its own peculiar composition 

 and structure throughout innumerable cell 

 generations. The essential function of the 

 intricate mechanism of mitosis is to achieve 

 an exactly equal division of the parent chro- 

 mosomes into two equivalent daughter sets 

 that then are segregated into the two new 

 daughter cells. By assembling the many kinds 

 of genie materials, each essential to the con- 

 tinued life ol the cell, into a small, definite 

 number of precisely packaged groups, and 

 by having every genie unit in each group 

 replicate itself before mitosis takes place, the 

 cell has achieved a relatively simple, highly 

 effective means by which it assures an exactly 

 equal, quantitative as well as qualitative, 

 division of these materials between the 

 daughter cells. It is estimated (p. 496) that 

 at least 10,000 genes are present in the aver- 

 age cell. II each gene had to be sorted and 

 separated on an individual basis, the mech- 



anisms of mitosis would doubtless have to be 

 far more complicated than they are. 



The dividing of the other parts of the cell 

 is not so exactly equal, because there is no 

 mechanism to accomplish a precise partition- 

 men t of the other parts. If a plant cell, for 

 example, contains a certain number of chlo- 

 roplasts, about half of these usually go to a 

 particular daughter cell. But sometimes the 

 numbers received by the two daughter cells 

 may be quite unequal. The cytoplasm as a 

 whole is divided quite equally in most cells, 

 but in some cases, such as dividing yeasts, one 

 of the daughter cells regularly receives by 

 far the larger share (Fig. 10-2). The signifi- 

 cance of mitosis becomes clear when it is 

 realized that each chromosome carries a spe- 

 cific group of hereditary determinants and 

 that these carry coded instructions (p. 134) by 

 means of which each cell is enabled to fulfill 

 the destinies of its own particular lineage. 

 When a cell multiples by mitosis, its own 

 complex of chromosomes and genes remains 

 unchanged from generation to generation, 

 and consequently there is no change in its 

 intrinsic hereditary potentialities. 



Amitosis. In rare instances cells have been 

 observed to divide without forming chromo- 

 somes. The nucleus merely pinches into two 

 parts and then the cleavage furrow or cell 

 plate cuts through the cytoplasm. Such cases 

 of amitotic division are considered to be 

 more or less abnormal and relatively unim- 

 portant, because daughter cells produced by 

 amitosis have lost the potential of perpetuat- 

 ing their lineage indefinitely. 



HAPLOIDY AND DIPLOIDY IN RELATION TO 

 FERTILIZATION 



Still another important fact about the 

 chromosomes has not been mentioned. When 

 counted and matched against one another, it 

 is usually found that the chromosomes make 

 up a duplicate set. The duplicate, or diploid, 

 set of chromosomes possessed by one species 

 of animal is shown in Figure 3-8. This rela- 

 tively simple case shows that the eight chro- 



