Explain why a base deletion mutation is often more disruptive than a base substitution.

Explain why a base deletion mutation is often more disruptive than a base substitution.

• A. A deletion shifts the reading frame, altering amino acids from the mutation onward
• B. A substitution shifts the frame as well
• C. Deletion never affects protein
• D. Insertion always has no effect

Answer: (B)

Frameshifts are the key idea here. When a nucleotide is deleted, the grouping of nucleotides into codons is altered from the point of deletion onward. This shifts every downstream codon, so the amino acids that get built after the mutation are almost all wrong, and a premature stop codon often appears. The protein produced is usually much more severely defective or truncated because an entire stretch of the sequence is misread. In contrast, a base substitution changes only one codon. The rest of the reading frame stays the same, so downstream amino acids are read correctly. This means fewer downstream changes in the protein, though the substituted codon can still have a big effect if it alters a critical amino acid or creates a stop. There’s also the nuance that if a deletion (or insertion) involves a multiple of three nucleotides, the reading frame isn’t disrupted; you’d remove or add whole amino acids but keep the rest of the sequence read in the same frame, which can be less disruptive. So, the reason deletions are often more disruptive is the frameshift they cause, which scrambles the entire downstream protein sequence.

Frameshifts are the key idea here. When a nucleotide is deleted, the grouping of nucleotides into codons is altered from the point of deletion onward. This shifts every downstream codon, so the amino acids that get built after the mutation are almost all wrong, and a premature stop codon often appears. The protein produced is usually much more severely defective or truncated because an entire stretch of the sequence is misread.

In contrast, a base substitution changes only one codon. The rest of the reading frame stays the same, so downstream amino acids are read correctly. This means fewer downstream changes in the protein, though the substituted codon can still have a big effect if it alters a critical amino acid or creates a stop.

There’s also the nuance that if a deletion (or insertion) involves a multiple of three nucleotides, the reading frame isn’t disrupted; you’d remove or add whole amino acids but keep the rest of the sequence read in the same frame, which can be less disruptive.

So, the reason deletions are often more disruptive is the frameshift they cause, which scrambles the entire downstream protein sequence.