!(>(• 



( ii \i- ni< I 2 



or absence of all of the genes located in the 

 bridge. 



Suppose, however, that the broken chro- 

 mosome is one of the largo autosomes of 

 Drosophila. Detriment or death to one or 

 both daughter cells may occur because of 

 the genes lost when either the acentric or 

 the dicentric fragment is left out of one 

 or both daughter nuclei. In addition, suc- 

 cessive bridge-breakage-fusion-bridge cycles 

 ma) harm future cell generations via the 

 abnormal quantities o\' chromosomal regions; 

 that is. the ancuploidy. resulting from the 

 off-center breakage of dicentric isochromo- 

 somes. Other things being equal, shorter 

 dicentrics are expected to break more often 

 than longer ones. Of course, any inter- 

 nuclear bridge that does not break may frus- 

 trate future nuclear division. 



Single chromosome breaks can occur in 

 either the somatic or the germ line. In the 

 latter case, aneuploid gametes may be pro- 

 duced. Since the genes are found to be 

 physiologically inactive in the gametes of ani- 

 mals, aneuploid genomes can enter the egg 

 and sperm without impairing their function- 

 ing (as implied on p. 104). Accordingly, 

 in animals, aneuploid genomes can be carried 

 by unaffected gametes into the zygote, which 

 may subsequently suffer dominant harmful 

 or lethal effects. In many plants, however, 

 the products of meiosis form a gametophyte 

 generation which performs physiological 

 functions requiring gene action, in which 

 case, ancuploidy is usually more lethal or 

 detrimental before fertilization than after. 



Chromatid Breaks 



A break can be produced in one and not the 

 other chromatid of a chromosome. Such 

 chromatid breaks are more likely to restitute 

 than chromosome breaks, since the unbroken 

 strand serves as a splint to hold the newly- 

 produced ends close to each other. What 

 appears under the microscope as a break 

 involving only one chromatid may initially 



have been a chromosome (or isochromatid) 



break that was followed by restitution of one 

 but not (yet) the other chromatid. 



Nonrestituted chromatid fragments be- 

 come nonrestituted chromosome fragments 

 if they persist long enough to replicate. To 

 be seen cytologically, a conjoined chromatid 

 or chromosome break produced during inter- 

 phase usually has to persist until nuclear 

 division occurs. Some chromatid and. per- 

 haps, chromosome breaks induced in con- 

 tracted ( metaphasc ) chromosomes may not 

 be visible, the pieces being held together 

 without joining by the nongenetic auxiliary 

 material in a chromosome. To detect such 

 unjoined breaks one would have to wait until 

 the next division. Essentially all ends pro- 

 duced by breaks are not sticky when the 

 chromosome is contracted as during nuclear 

 division; joinings are restricted largely, if not 

 completely, to the period between late telo- 

 phase and early prophase. Accordingly, the 

 later in this period a break is produced, the 

 less likely it is that the ends will join; broken 

 ends produced between early prophase and 

 late telophase have the maximum time for 

 joining but probably also the maximum op- 

 portunity to cross-unite. 



For simplicity, the discussion which fol- 

 lows is restricted to isochromatid breaks that 

 fail to restitute. The reader is given the 

 task of working out the consequences of 

 aneuploidy resulting from single nonresti- 

 tuted broken chromatids. The lack of fur- 

 ther discussion on this type of mutation does 

 not reflect on the relative frequency or im- 

 portance of chromosome versus chromatid 

 breaks. Agents capable of producing chro- 

 mosome breaks can also produce chromatid 

 breaks; moreover, certain agents may prefer- 

 entially produce chromatid breaks. 



Consequences of Two Breaks in One 

 Chromosome 



When a chromosome is broken twice, the 

 two breaking points may be paracentric, that 



