A single cytosine base is deleted from the middle of a coding sequence. What is the most likely consequence for the protein product?
AOne amino acid in the protein is changed at the deletion site
BOnly the codon containing the deletion is disrupted; downstream codons are unaffected
CAll codons downstream of the deletion are read in a shifted frame, typically producing a nonfunctional protein
DThe mutation is silent because the genetic code is degenerate
A single-base deletion shifts the reading frame for every codon after the deletion site. Because codons are non-overlapping triplets, removing one base changes which bases are grouped together from that point forward, typically producing a garbled amino acid sequence and often a premature stop codon. Degeneracy only helps when the same codon is produced — it cannot rescue a frameshift.
Question 2 True / False
A transition mutation that changes a codon from GCU to GCC will typically result in a change in the amino acid sequence of the protein.
TTrue
FFalse
Answer: False
Both GCU and GCC encode alanine. This is a silent (synonymous) mutation — a consequence of the degeneracy of the genetic code, where multiple codons specify the same amino acid. The DNA sequence changes, but the protein sequence does not.
Question 3 Short Answer
Why are frameshift mutations generally more damaging to protein function than missense point mutations?
Think about your answer, then reveal below.
Model answer: A frameshift shifts the reading frame for all codons downstream of the mutation, typically producing a completely different amino acid sequence and often a premature stop codon. A missense point mutation changes only a single amino acid, which may or may not affect protein function depending on its position and chemistry.
The reading frame defines how the nucleotide sequence is partitioned into codons. Disrupting the frame corrupts every downstream codon — essentially destroying the protein's blueprint from that point on. By contrast, a single amino acid substitution is a local change; if the new amino acid is chemically similar or is at a non-critical position, the protein may fold and function normally.