46 - The Cell 



around the center of the cell, near the now 

 clearly defined spindle. Soon, however, they 

 are moved into a precisely regimented ar- 

 rangement at the exact center of the spindle 

 — that is, halfway between the poles and 

 regularly disposed around the spindle axis 

 (Fig. 3-2). While the chromosomes maintain 

 this mid-point position, the cell is said to be 

 in the metaphase of mitosis. The final stages 

 of prophase, while the chromosomes are 

 being moved to the metaphase position, are 

 usually referred to as metakinesis. 



Anaphase. Anaphase is the period that 

 embraces all stages during which the sister 

 chromosomes are separated from one an- 

 other, as they are moved toward, and finally 

 to, opposite poles of the spindle (Figs. 3-2 

 and 3-4). Each of the chromosomes originally 

 present in the parent cell has undergone 

 replication into identical pairs of sister chro- 

 mosomes, and now the sister members of each 

 pair regularly pass to opposite poles of the 

 mitotic apparatus. 



The exact mechanism that brings about 

 the anaphase movements of the chromosomes 

 is still somewhat problematical. Considerable 

 evidence exists, however, to indicate that the 

 chromosomes are pulled along the length of 

 the spindle by contractile processes inherent 

 in the traction fibers. The kinetochore of 

 each chromosome, to which a traction fiber 

 is attached, regularly leads the way, and the 

 limbs, on either side of the kinetochore, ap- 

 pear as if they were being dragged through a 

 viscous medium. Thus during anaphase, the 

 chromosomes display • and V shapes, depend- 

 ing upon whether the kinetochore is exactly 

 centered, and the limbs of the bent chromo- 

 somes regularly trail away from the poles of 

 the spindle. The traction fibers can be ob- 

 served to become shorter and shorter as the 

 chromosomes move from the equator to the 

 poles. The fibers do not thicken as they 

 shorten, however, as might be expected if the 

 contraction were of an elastic nature. Indeed, 

 a few biologists have postulated the exist- 

 ence of unknown forces — of attraction be- 

 tween the kinetochores and the mitotic cen- 



ters, or of repulsion between sister kineto- 

 chores. According to such views the fibrous 

 structure of the spindle would be interpreted 

 as a molecular orientation resulting from 

 lines of force generated by the attraction or 

 repulsion. A less ephemeral mitotic structure 

 appears more likely, however, especially in 

 view of the important recent work of Mazia 

 and Dan. These investigators have shown 

 that the mitotic apparatus can be isolated as 

 a stable, integral structure, to which the chro- 

 mosomes remain attached, when certain cells, 

 particularly dividing egg cells, are fixed in 

 cold alcohol and then treated with detergents 

 that dissolve or disperse all other parts of the 

 cytoplasm (Fig. 3-3). 



Not all of the force that operates to sepa- 

 rate the daughter groups of chromosomes 

 can originate in the traction fibers, however. 

 While the traction fibers are shortening, the 

 fibers of the central spindle are lengthening. 

 But here again no change in the caliber of 

 the libers has been observed. The precise 

 mechanisms by which these processes of 

 shortening and lengthening are brought 

 about remain obscure. Nevertheless, it must 

 be recognized that the lengthening of the 

 central spindle participates in the work of 

 moving the daughter groups of chromosomes 

 to opposite sides of the dividing cell. 



Telophase. During telophase, the final 

 phase of mitosis, the new nuclei of the two 

 prospective daughter cells become organized 

 and the cytoplasm of the parent cell becomes 

 divided, usually in roughly equal fashion, be- 

 tween the two resulting daughter cells (Fig. 

 3-2). 



Soon after reaching the poles of the spin- 

 dle, the chromosomes begin to elongate and 

 to lose their compact, densely staining form, 

 thus making it difficult to distinguish them 

 individually (Fig. 3-2). Meanwhile, a new 

 nuclear membrane is formed, or perhaps as- 

 sembled, around the chromosomes, and the 

 nucleus of each daughter cell takes on the 

 appearance of the parent nucleus before 

 mitosis began. This telophase reorganization 

 of the daughter nuclei probably represents a 



