The human β-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

                   Intron 2                        Exon 3        _
wild type     5′-CCUCCCACAG      CUCCUG-3′
mutant.       5′-CCUCCCACUG      CUCCUG-3′

Speculate about the way in which this base substitution causes mutation of β-globin protein.

Key Concepts

Base Pair Substitution

A base pair substitution is a type of mutation where one nucleotide in the DNA sequence is replaced by another. This can lead to changes in the corresponding mRNA and potentially alter the amino acid sequence of the resulting protein. Depending on the location and nature of the substitution, it can be classified as silent, missense, or nonsense, each having different implications for protein function.

Intron and Exon Structure

Introns are non-coding regions of a gene that are transcribed into pre-mRNA but are removed during RNA splicing, while exons are the coding sequences that remain in the mature mRNA. The splicing process is crucial for producing a functional protein, and mutations in introns can affect splicing efficiency or accuracy, potentially leading to the inclusion of incorrect sequences in the final mRNA.

Protein Structure and Function

The structure of a protein is determined by its amino acid sequence, which is encoded by the mRNA. Changes in the mRNA due to mutations can lead to alterations in the protein's primary structure, which may affect its folding, stability, and overall function. In the case of β-globin, mutations can lead to disorders such as sickle cell disease, where the altered protein affects oxygen transport in the blood.