Chapter 12 



STRUCTURAL CHANGES 

 IN CHROMOSOMES 



T 



|he two classes of mutation 

 dealt with in Chapter 11 in- 

 volved changes in chromo- 

 somal content of unbroken individual or sets 

 o( chromosomes. In some instances, mu- 

 tants are based upon the gain, loss, or shift 

 of a part of one or more chromosomes. 

 All such structural changes are preceded 

 by chromosome breakage, which — ignoring 

 chromatids for the present — results in two 

 new. "sticky" ends. When several breaks 

 are produced, the new ends can join together 

 but only in pairs, any new end capable of 

 joining any other new end. Moreover, an 

 end produced by breakage cannot join the 

 normal (unbroken) end of a chromosome. 

 Thus, originally free ends of chromosomes 

 are not sticky because they have genes, 

 called telomeres, which serve to seal them 

 off, making it impossible for a normal end 

 to join to any other. 



The two ends produced by one break 

 usually join together in what is called resti- 

 tutional union even when ends produced by 

 other breaks coexist in the same nucleus. 

 This indicates that proximity favors the 

 union of sticky ends. Although restitu- 

 tional union usually occurs and thereby 

 restores the original linear order of the chro- 

 mosome, the ends uniting may sometimes 

 come from different breaks, so that a new 

 chromosomal (gene) arrangement is pro- 

 duced. The latter union is, therefore, a non- 

 restitutioncd, or exchange, or cross-union 

 type. Let us see how nonrestitutional unions 

 164 



produce various structural changes in chro- 

 mosomes. 



Consequences of a Single Chromosome Break 



Consider first the consequences of a single 

 chromosome break; that is. a break through 

 both chromatids (Figure 12-1). Diagram 

 1 represents a norma] chromosome (its chro- 

 matids are not shown), whose centromere 

 is indicated by a black dot. In diagram 2 

 this chromosome is broken. If the new 

 chromosome ends join together, that is. 

 restitute, no chromosomal rearrangement is 

 produced. Although restitution usually oc- 

 curs, it may sometimes fail because the new 

 ends spring apart or are moved apart by 

 Brownian movement or protoplasmic cur- 

 rents. In nonrestitution. chromosome repli- 

 cation produces a daughter chromosome just 

 like the parent — with a break in the same 

 position — as shown in diagram 3 where the 

 two broken sister chromosomes are indi- 

 cated. The union of the piece containing 

 no centromere ( a ) to the centromere-bear- 

 ing piece of the other sister chromosome 

 ( b' ) would, in effect, be restitution as would 

 the joining of a' to b. (Sometimes, only 

 one of these restitutional unions occurs.) 



If restitution does not occur before or 

 after the chromosome replicates, the ends 

 closest together usually join together, these 

 being the corresponding ends of the sister 

 chromosomes (a with a' and b with b'). 

 As shown in diagram 4. the results of such 

 nonrestitutional unions are one chromosome 

 with no centromere (an acentric chromo- 

 some), and one with two (a dicentric chro- 

 mosome). Note that both the acentric and 

 the dicentric chromosomes are composed of 

 identical halves lengthwise, each, therefore, 

 being termed an isochromosome. (This dia- 

 gram shows the chromosomes contracting 

 preparatory to metaphase.) 



In diagram 5 we can see that in mitotic 

 anaphase the acentric isochromosome is not 

 pulled toward either pole, whereas the dicen- 



