Chromosome reorientation

The user can define the duration of the electric current that will be applied in each direction (pulse). With each change in direction of the electric field, the DNA molecules reorientate themselves. The smaller chromosomes reorientate themselves more quickly than the larger ones (Figure 1.29). 

Ault JG, Rieder CL, Chromosome mal-orientation and reorientation during mitosis. Cell Motil. Cytoskeleton 1992 22 155-159. 57. 

This suggests that controlled distribution depends upon two processes, each of which requires explanation. First, for some reason the initial orientation is so ordered that the vast majority of all chromosomes achieve bipolar orientation directly and immediately in early prometaphase. Second, however, malorientations do occur and for some reason are almost invariably followed by reorientation and the indirect achievement of bipolar orientation. 

Wherever reorientation is an essential feature of mitosis, it probably has the same cause as in meiosis, although direct evidence is lacking (Nicklas and Koch, 1969). Thus, appropriate bipolar orientation subjects the mitotic chromosome to oppositely directed forces (cf. the deformations observed by Bajer, 1958a, Fig. 6). By analogy with meiosis, the resultant tension directly or indirectly stabilizes these orientations, and reorientation occurs until this state is reached. 

Only some unpaired half-bivalents amphiorient, and this variation may well seem puzzling. Briefly, a probable explanation is that on Ostergren s (1951) hypothesis, metiotic versus mitotic orientation depends upon the same factors involved in initial orientation the position of kinetochores relative to each other and preferential orientation. As has been seen, initial orientation is not errorless, because chromosomes may by chance lie in positions making appropriate orientation of their kinetochores more or less unlikely. Hence, amphiorientation of ha -bivalents may be viewed as an enor, especially likely when univalency induces frequent reorientation (see p. 272) and the half-bivalent is exposed repeatedly to the hazard of amphiorientation which may be possible whenever the half-bivalent s kinetochores happen to lie perpendicular to the spindle axis during reorientation. 

Exactly this is observed in studies on fixed cells (reviewed by John and Lewis, 1965, pp. 52ff.) and following experimental destruction of the linkage by micromanipulation of living cells (Nicklas, unpublished). A more exact prediction is possible from the postulated role of bipolar tension in reorientation unpaired meiotic chromosomes that retain the meiotic synorientation of both sister kinetochores to the same pole should be unstable and frequently reorient. If, however, they achieve amphiorientation, as do mitotic chromosomes, a stable bipolar orientation should result. Both conditions occur in nature and with the expected result (living cells with unpaired sex chromosomes, Dietz, 1956 Bauer et al., 1961 Nicklas, 1961 fixed cells, reviewed by John and Lewis, 1965). 

The fine structural and molecular events during reorientation are similarly unknown. Reorientation surely involves the loss and reformation of chromosomal spindle fibers (p. 261). But even whether loss and reformation are sequential or concurrent at one kinetochore is uncertain (brief review and speculation, Nicklas and Koch, 1969). Again, correlated in-vivo and electron microscopic observations on the same chromosome will be required. 

The distribution of chromosomes in meiosis is understandable in terms of initial orientation and reorientation. Direct evidence that initial orientation and reorientation are now explained at the cytological level comes from experiments on bivalents in which orientation and distribution are predictably altered the experimenter 1. can determine the distribution of any chosen chromosome 2. can induce unstable malorientation and 3. can stabilize malorientations and induce nondisjunction. The explanation offered is easily extended to chromosome distribution in mitosis and to most units other than bivalents in meiosis. Testable molecular hypotheses are readily suggested. These would relate features of the cytological explanations to the generally documented organizing center activity of kinetochores and to spindle fiber lability. 

Chromosome micromanipulation. II. Induced reorientation and the experimental control of segregation in meiosis. Chromosoma, 21 17-50. 

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