106 



Cellular Structure and Activity 



pole and remains outside the metaphase 

 plate (Hvighes-Schrader, '43b). Congression 

 into the metaphase plate thus seems to be 

 accomplished by the chromosomal fibers. 

 Through them the chromosome or bivalent 

 becomes attached to opposite poles of the 

 spindle and the tension on the chromosomal 

 fibers moves the chromosomes until equi- 

 librium is reached in the metaphase plate 

 (cf. Schrader, '47). In living cells such a 

 movement of chromosomes back and forth 

 in the long axis of the spindle has often been 

 described and is easily seen in most films of 

 dividing cells. The pre-metaphase stretch ob- 

 served in spermatocytes of several insects 



SPINDLE LENGTH 



CHROMOSOME SEPARATION 



pulsive forces can produce similar patterns, 

 but tell us nothing about the forces involved 

 in the spindle. Most likely a complex inter- 

 action of chromosomal fibers, electrostatic 

 charges on chromosomes, and intermolecular 

 attraction between spindle micelles, tending 

 to crowd out foreign bodies (Ostergren, '51), 

 is responsible for the metaphase arrange- 

 ment. 



Anaphase Movement. Of all the various 

 aspects of mitosis hardly any has attracted 

 the attention of cytologists more than the 

 strikingly regular movement of chromo- 

 somes at anaphase. For years it has been the 

 subject of much speculation and some experi- 



SPINDLE LENGTH 



CHROMOSOME SEPARATION 



Fig. 22. Curves of chromosome separation and spindle elongation. A, In forms with diffuse kinetochore 

 (Hemiptera and Homoptera; after Ris, '43. B, In forms with localized kinetochore (grasshopper spermato- 

 cytes, Ris, '49; chicken fibroblasts, Hughes and Swann, '48; Hughes and Preston, '49). 



(Hughes-Schrader, '43a) and the metaphase 

 position of multivalents give further sup- 

 port to this hypothesis (Ostergren, '51). The 

 details and mechanisms of this process, how- 

 ever, are completely unknown. What causes 

 orientation of the kinetochores in the spindle 

 and assures that chromosomal fibers attach 

 to opposite poles? How can we explain the 

 different behavior of kinetochores in mitosis 

 and meiosis? (For a stimulating and interest- 

 ing discussion of these problems see Oster- 

 gren, '51.) Another interesting aspect of 

 metakinesis is the spacing of chromosomes 

 in the metaphase plate. The chromosomes 

 are either all on the periphery of the spindle, 

 the arms of long chromosomes directed radi- 

 ally away from the spindle, or they are 

 evenly spaced in the equatorial plane. Even 

 then the radial arrangement of chromosome 

 arms is often striking. Large chromosomes 

 are usually near the periphery, smaller ones 

 in the center. In different cells even of the 

 same organism this metaphase arrangement 

 shows often striking variations and may 

 show a constant pattern characteristic for 

 the type of cell (Wilson, '32). 



Model experiments with floating magnets 

 show that a balance of attractive and re- 



mental analysis. (For a critical discussion of 

 the various hypotheses, see Schrader, '53.) 



The initial separation of the chromatids 

 takes place also in colchicine-treated cells 

 and is therefore independent of the spindle 

 apparatus (Levan, '38) and may be due to 

 a swelling and dissolution of some material 

 that holds the chromatids together at meta- 

 phase (cf. Carlson, '52). For the movement 

 to the poles, however, spindle and chromo- 

 somal fibers are indispensable. Analysis of 

 chromosome movement in living cells has 

 shown that two factors are involved. The 

 pole-ward movement is correlated with a 

 shortening of the chromosomal fiber. Just 

 how this occurs is not known but probably 

 some kind of contraction of the organelles is 

 involved (see review by Cornman, '44). The 

 second factor is the lengthening of the 

 spindle. The spindle suddenly swells in the 

 equator and then stretches in its long axis. 

 Since the chromosomes are attached to the 

 spindle poles they are thus further moved 

 apart. These two components in the ana- 

 phase movement are sometimes separated in 

 time (Fig. 22/4) — so far this has been found 

 only in animals with diffuse kinetochore 

 (Ris, '43) — but they usually occur simul- 



