The human beta-globin wild-type allele and a certain mutant allele are identical
ID: 61137 • Letter: T
Question
The human beta-globin wild-type allele and a certain mutant allele are identical in sequence except for a single base-pair substitution that changes one nucleotide at the end of intron 2. The wild-type and mutant sequences of the affected portion of pre-mRNA are
A) Speculate about the way in which this base substitution causes mutation of beta-globin protein.
B) This is one example of how DNA sequence change occurring somewhere other than in an exon can produce mutation. List other kinds of DNA sequence changes occurring outside exons that can produce mutation. In each case, characterize the kind of change you would expect to see in mutant mRNA or mutant protein.
Intron 2 Exon 3 wild type 5’-CCUCCCACAG CUCCUG-3’ mutant 5’-CCUCCCACUG CUCCUG-3’Explanation / Answer
(A) The presence of Adenine at the end of intron 2 is needed for the splicing of the intron sequence. The change of Adenine for Uracil causes the spliceosome to skip the splicing site and the following exon sequence, generating a mutated human beta-globin that doesn't include Exon3.
(B) Besides exons, a gene structure will include the following sequences: enhancers, silencers, a promoter, cap, 5' UTR, introns, 3' UTR, and polyA tail. All of those sequences have been shown to regulate gene expression, and if mutated, they can affect the expression of mRNA or generate a mutated protein. For example, mutation of a sequence inside within an intron that is relevant for correct splicing will produce a mutated protein that in which an exon(s) could be missing. Mutation of the promoter sequence would inhibit transcription and therefore production of mRNA, while mutation of enhancer would only decrease mRNA production, and mutation of silencers will increase mRNA production, the same for mutations in 3' UTR, which would affect its translation. The only other sequence in which a mutation could cause a mutated protein, besides introns, are cap, 5' UTR, and poly A tail. A mutation in cap sequence can affect splicing. A mutation in some 5' UTR can cause errors in splicing as some 5' UTR contain introns, that if mutated can remain in the protein sequence. A mutation in poly A tail would cause the degradation of ending sequences, which would produce a shorter mutated protein.
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