s 



CHAPTER 1 



of the spindle, which is represented by the 

 plane formed where the fingertips touch. 

 The rest of eaeli chromosome, being pas- 

 six e. can be in an\ position in the spindle. 

 When all the centromeres have arrived at 

 the equatorial plane of the spindle, mitosis 

 has reached the middle phase, or metaphase 

 (Figure 1-4E). 



Until this point the chromatids of a chro- 

 mosome are still attached to each other at or 

 near the centromere, although elsewhere they 

 are largely free. Next they also separate 

 at the centromere and the two daughter 

 centromeres suddenly move apart, one going 

 toward one pole of the spindle, the other 

 toward the other pole, with the rest of each 

 chromatid, which is now recognized as 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 1-4F). 



When the chromosomes have reached the 

 poles, the last stage, or telophase, occurs 

 (Figure 1-4G), in which the events appear 

 to be the reverse of those that happened in 

 prophase. Specifically, the spindle disinte- 

 grates, a new nuclear membrane is formed 

 around the chromosomes, and nucleoli re- 

 appear. The chromosomes become thinner 

 and longer and then can be seen to consist 

 of two delicate threads (chromatids) wound 

 one about the other. Finally, as the chromo- 

 somes lose their visible identity, the nucleus 

 enters the interphase, inter mitotic , or meta- 

 bolic stage (see again Figure 1-4A). 



The impression may have been gained 

 that, in one respect, the preceding general- 

 ized account of the mitotic phases was either 

 incomplete or misleading. It was stated that 

 the prophase chromosome is composed of 

 two chromatids or threads, that metaphase 

 puts these into position for separation at 

 anaphase, and that after separation their 

 newly attained individuality is recognized by 

 calling them chromosomes. But chromo- 



somes were defined as containing two visible 

 threads! The question rightly asked is: does 

 the anaphase chromosome contain the two 

 threads that are later seen at telophase? 

 This would be true if each chromatid some- 

 how visibly reproduced itself between the 

 time it was seen relatively uncoiled at pro- 

 phase and the next time it was seen rela- 

 tively uncoiled, at telophase. Remember 

 that we have been discussing the replication 

 of chromatids as detected by microscopic 

 observation. Chromosome and chromatid 

 replication can also be studied by other 

 means. Let us consider some evidence re- 

 garding chromosome replication at the chem- 

 ical level, which may help us understand its 

 replication at the visible level. 



Chromosomes ("colored bodies") are 

 unique since they are the only objects in 

 the cell that are made purple by the Feulgen 

 staining technique. It is possible to measure 

 the amount of chromosomal material by the 

 amount of purple stain held by the chromo- 

 somes. The amount of chromatin — Feul- 

 gen-stainable chromosomal material — does 

 not change between prophase and telo- 

 phase, but doubles over a period of hours 

 during the intermitotic stage. By the be- 

 ginning of prophase, therefore, each chro- 

 mosome, as revealed by its stainability, 

 has already replicated chemically. At the 

 visible level, however, this is not yet ap- 

 parent, so that each of the two visible 

 chromatids in a chromosome also contains 

 the chemical materials for an identical chro- 

 matid which is still not resolved as a sepa- 

 rate thread under the microscope. This new 

 material is unresolved 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 ap- 

 pear as one strand. Before the next occa- 

 sion when unwound threads can be seen — 

 that is, at the telophase of the same mitosis 

 — this replication at the visible level has al- 



