Structural Changes Within Chromosomes 



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are composed lengthwise of identical halves, 

 each being termed, therefore, an isochromo- 

 some. This diagram shows these chromo- 

 somes contracting preparatory to metaphase. 



Diagram 5 shows that in mitotic anaphase 

 the acentric is not pulled to either pole while 

 the dicentric is pulled toward both poles at 

 once. The acentric chromosome is, therefore, 

 not included in either daughter nucleus and 

 so is lost to both. (The acentric pieces in 

 diagram 3 will be lost in any subsequent 

 division, whether they join to each other or 

 do not join at all.) The dicentric isochromo- 

 some, being pulled to both poles at once, 

 forms a bridge that may prevent any of the 

 chromosome from entering either daughter 

 nucleus, so that the dicentric is lost. Alter- 

 natively, the centric regions of the dicentric 

 piece may enter the daughter nuclei, and 

 either the bridge may snap at any one of a 

 number of places between the centromeres, 

 so that the daughter nuclei are free of each 

 other, or the bridge may persist so that the 

 daughter nuclei are joined together. 



The amount of phenotypic detriment that 

 a single nonrestituting break will produce in 

 the daughter cells and their progeny cells will 

 depend upon the particular chromosome in- 

 volved, the place of breakage, and the fate of 

 the dicentric piece. Suppose first, for ex- 

 ample, that chromosome IV of Drosophila 



151 



FIGURE 19-1. Conse- 

 quences of a single non- 

 restituting ciiromosome 

 brea/<. 



(which you recall is often viable as a haplo-IV 

 individual) is the chromosome involved. The 

 break may occur at any locus in IV and the 

 loss of the genes in the acentric piece, though 

 detrimental, will not usually cause invia- 

 bility, nor will the loss of the entire dicentric 

 fragment if excluded from both daughter 

 nuclei. The phenotypic consequences would 

 probably be the same should the bridge be- 

 tween daughter nuclei snap (in which case, 

 you recall, each daughter nucleus certainly 

 will be deficient at least for the genes in the 

 acentric piece). However, note what hap- 

 pens when a bridge, involving a dicentric 

 isochromosome linearly differentiated as 

 a.bcddcb.a, snaps other than between the d 

 segments. If it snaps between b and c, one 

 fragment would be still more deficient, yet 

 viable in the present example, while the other 

 would contain an extra dose of the genes in 

 the cd segment (most probably viable). 

 Regardless of where the bridge snaps, how- 

 ever, both daughter nuclei would carry an 

 unjoined centric fragment which, after repli- 

 cating, would usually form a new dicentric 

 isochromosome which, at the next mitotic 

 division, would again form a bridge. It is 

 possible, then, to have a cycle of bridge- 

 breakage-fusion-bridge events in the course of 

 successive mitotic divisions. 

 If, however, the bridge does not break, and 



