Chapter *19 



STRUCTURAL CHANGES 

 WITHIN CHROMOSOMES 



t: 



|he two classes of mutation dis- 

 cussed in Chapter 18 dealt with 

 changes in chromosomal con- 

 tent involving whole chromosomes, either 

 individually or in sets. Sometimes, geneti- 

 cally detected mutations are found to be 

 based upon the gain, loss, or shift of a part 

 of one or more chromosomes. It is these 

 structural changes in chromosomes, compris- 

 ing our third category of mutations, that we 

 shall now consider. 



All such structural changes in chromosomes 

 are preceded by chromosome breakage. 

 When a chromosome is broken, the two ends 

 produced are "sticky" and are capable of 

 joining to each other. When the ends pro- 

 duced by several breaks join together the 

 chromosomes formed are never branched, 

 demonstrating that each broken end can join 

 only to one other broken end. However, any 

 broken end can join to any other broken end. 

 Moreover, an end produced by breakage 

 cannot join to the normal (unbroken) end of a 

 chromosome. Thus, originally free ends of 

 chromosomes are not sticky, having genes 

 called telomeres that serve to seal them off 

 and make it impossible for normal ends to 

 join to each other or to ends produced by 

 breakage. 



The ends produced by one break usually 

 join together, in what is called restitiitional 

 union, even when ends produced by other 

 breaks coexist in the same nucleus, indicating 

 that proximity of broken ends favors their 

 joining together. Although restitutional 

 150 



union usually occurs, so that the original 

 linear order of the chromosome is reconsti- 

 tuted, under certain conditions, to be de- 

 scribed more fully, the ends uniting may be 

 such that a gene arrangement other than the 

 original one is produced. The latter union 

 is, therefore, of a nonrestitutional, or ex- 

 change, or cross-union type. Let us see how 

 nonrestitutional unions produce various struc- 

 tural changes in chromosomes. 



Consider first the consequences of a single 

 chromosome break (Figure 19-1). Diagram 

 1 represents a normal chromosome whose 

 centromere is indicated by a black dot. In 

 diagram 2 this chromosome is broken. If 

 these broken ends join together, i.e., restitute, 

 no chromosomal rearrangement is produced. 

 Although, as was said, restitution is the rule, 

 on some occasions it may fail to occur. Per- 

 haps restitution sometimes fails because fol- 

 lowing breakage the new ends spring apart or 

 are moved apart by Brownian movement or 

 protoplasmic currents. In that case, if the 

 chromosome replicates preparatory to a 

 subsequent nuclear division, a daughter 

 strand will be produced just like the parent 

 strand, i.e., with a break in the same position. 

 This is shown in diagram 3, in which the two 

 broken sister strands are indicated. The union 

 of piece a, containing no centromere, to piece 

 b', the centromere-bearing piece of the 

 other sister chromosome, would be, in effect, 

 restitution, as would be the joining of a' to b. 

 (Sometimes, only one of these restitutional 

 unions may occur.) 



But, if no restitution occurs either before 

 or after the chromosome replicates, the ends 

 closest together will usually join together, 

 these being the corresponding ends of the 

 sister strands (a with a' and b with b'). The 

 results of such nonrestitutional unions are 

 shown in diagram 4. Shown there is one 

 chromosome without a centromere (an acen- 

 tric chromosome) and one that has two centro- 

 meres (a dicentric chromosome). Note that 

 both the acentric and dicentric chromosomes 



