18 



CHAPTER 3 



This movement of centromeres occurs in a 

 particular direction relative to a structure 

 called the spindle which has been in the 

 process of formation throughout prophase. 

 When completed, the spindle has an appear- 

 ance similar to what you will see upon spread- 

 ing your fingers and touching corresponding 

 fingertips together. Your wrists serve as the 

 poles of the spindle and your fingers as spindle 

 fibers. The chromosomes migrate from what- 

 ever position in the spindle region they may 

 have, so that their centromeres come to lie 

 in a single plane perpendicular to the axis 

 between the poles, that is, at the equator of 

 the spindle, which is represented by the 

 plane formed where your fingertips touch. 

 Now the rest of the chromosome, being pas- 

 sive, can be in any plane in the spindle. Once 

 all the centromeres have arrived at the equa- 

 torial plane of the spindle, mitosis has reached 

 the middle phase, or metaphase (Figure 3-1 E). 



Until now the chromatids of a chromosome 

 are still attached to each other at or near the 

 centromere, although elsewhere they are 

 largely free. They next also separate at the 

 centromeres, the two centromeres suddenly 

 moving apart, one going toward one pole of 

 the spindle, the other toward the other pole, 

 with the rest of each chromatid, now called 

 a chromosome, being passively dragged along. 

 This stage, in which the chromatids separate, 

 move toward and arrive at the poles as chro- 

 mosomes, is called anaphase (Figure 3-1 F). 



Once the chromosomes are at the poles, 

 the last stage, or telophase (Figure 3-lG), 

 occurs, in which the events that follow ap- 

 pear to be the reverse of those that happened 

 in prophase. Specifically, the spindle disin- 

 tegrates, a new nuclear membrane is formed 

 around the chromosomes, and nucleoli re- 

 appear containing material probably drained 

 away from the chromosomes. The chromo- 

 somes become thinner and longer and then 

 can be seen to consist of two delicate threads 

 (chromatids) wound one about the other. 

 Finally, as the chromosomes lose their visible 



identity the nucleus enters the intenniiotic, 

 interphase, or metabolic stage. A general 

 view of mitosis in the onion root tip is shown 

 in Figure 3-2. 



FIGURE 3-2. Mitosis in t/ie onion root tip — 

 general view. {Courtesy of R. E. Cleland.) 



In reading this generalized account of the 

 mitotic phases, you may have gained the 

 impression that, in one respect, it was either 

 incomplete or misleading. For it was stated 

 that the prophase chromosome is composed 

 of two chromatids, that metaphase puts these 

 into position for separation at anaphase, and 

 that once separated their newly attained in- 

 dividuality is recognized by calling them 

 chromosomes. But chromosomes were de- 

 fined as containing two visible threads! The 

 question rightly asked is, does the anaphase 

 chromosome contain the two threads or 

 chromatids that are seen at telophase? This 

 would be true if each chromatid somehow 

 visibly reproduced itself between the time 

 it was seen relatively uncoiled at prophase 



