Describe how a deletion mutation alters the structure of a gene.

• A. Removal of one or more bases occurs but does not change the reading frame.
• B. Removal of one or more bases causes a frameshift and base sequence change.
• C. Addition of bases produces a longer protein.
• D. Inversion of a gene segment has no effect.

Answer: (B)

The important idea is how deleting nucleotides affects how the gene is read. Deleting one or more bases can shift the grouping of codons that the ribosome reads. If the number of bases removed is not a multiple of three, the reading frame is disturbed downstream, so every codon after the deletion is read differently, producing a new sequence of amino acids and usually a nonfunctional protein. This is a frameshift mutation, and it also changes the base sequence that follows the deletion.

That’s why describing the effect as a frameshift with a changed downstream sequence best captures what a deletion mutation typically does to the gene’s structure. In-frame deletions (where a multiple of three bases is lost) preserve the reading frame but remove specific amino acids, which is a more limited scenario. Additions would involve inserted bases, not deletions, and inversions can disrupt gene function in different ways.