Fig. 2.11. A, structure of a 

 chromosome; diagrammatic. 

 B, relation of chromosomes to 

 the mitotic spindle; diagram- 

 matic. (Redrawn from F. 

 Schrader, Mitosis, copyright 

 1944 by Columbia University 

 Press, printed by permission.) 



STRUCTURE AND ACTIVITIES OF CELLS 

 Center 



Chromosome 



Kinetochore 

 - Pellicle 



Genonema 



Matrix 



Interzonal 

 fiber 



Continuous 

 fiber 



Chromosomal 

 fiber 



of a rod-like chromosome depends on the position of the kinetochore, a faintly 

 stained region of the chromosome that becomes associated with a spindle fiber 

 (Fig. 2.11). The kinetochore is capable of duplication and resembles the 

 centriole in a number of respects; its structure in electron micrographs has 

 not been reported. If the kinetochore is near the end of a chromosome, it 

 appears straight; a subterminal or median position of the kinetochore pro- 

 duces a J- or V-shaped chromosome. No matter what the shapes and sizes of 

 the chromosomes are, we find that there are two of each kind as they become 

 fully condensed toward the end of the prophase. Two chromosomes that are 

 alike in shape and size are known as homologous chromosomes. The total 

 number of chromosomes visible at the end of the prophase is the diploid 

 number characteristic of any species. In any given kind of animal or plant, 

 the same number of pairs of chromosomes will be found in all the cells of the 

 body, with the exception of the mature germ cells (Fig. 2.12). In some 

 species the males have one fewer chromosomes than the females; that is, one 

 chromosome is unpaired. This same kind of chromosome occurs as a pair in 

 the female. Chromosomes that diff^er in number in the two sexes are known 

 as sex chromosomes (p. 209); the other chromosomes, of which there are two 

 of each kind in both sexes, are called autosomes. Coincident with the forma- 

 tion of the mitotic spindle and the condensation of the chromosomes, the 

 nuclear membrane begins to disappear, first in the region next to the spindle, 

 and the nucleolus is lost to view. The chromosomes take up a position on the 

 spindle midway between the centrioles to form the equatorial plate; at this 

 time a spindle fiber connects the kinetochore with the centriole (Fig. 2.11). 

 These changes mark the end of the prophase. Observations on certain kinds 

 of living cells growing in a nutrient medium (tissue-culture conditions) indi- 

 cate that the changes of the prophase take about 8 minutes to occur. 



41 



