A variety of neural and muscular disorders are associated with expansions of trinucleotide repeat sequences. The table below lists several disorders, the repeat motifs, their locations, the normal number of repeats, and the number of repeats in the full mutations.
(a) Most disorders attributable to trinucleotide repeats result from expansion of the repeats. Two mechanisms are often proposed to explain repeat expansion: (1) unequal synapsis and crossing over and (2) errors in DNA replication where single-stranded, base-paired loops are formed that conflict with linear replication. Present a simple sketch of each mechanism.
(b) Notice that some of the repeats occur in areas of the gene that are not translated. How can a mutation occur if the alteration is not reflected in an altered amino acid sequence?
(c) In the two cases where the repeat expansions occur in exons, the extent of expansion is considerably less than when the expansion occurs outside exons. Present an explanation for this observation.
Disorder | Repeat | Location | Normal Number | Full Mutation |
Fragile X | CCG | 5′ untranslated region | 6–230 | >230 |
Huntington | CAG | Exon | 6–35 | 36–120 |
Myotonic dystrophy | CTG | 3′ untranslated region | 5–37 | 37–1500 |
Oculopharyngeal muscular dystrophy | GCG | Exon | 6 | 8–13 |
Friedreich ataxia | AAG | Intron | 20 | 200–900 |
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