Trace A Mutation to A Phenotype
Follow a single DNA base change through the mRNA codon and amino acid to the trait it causes.
- DNA codes for protein: A DNA change can change a codon, which can change an amino acid in a protein.
- Proteins build traits: A changed protein can change how a cell or body works: that change is the phenotype (trait).
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Trace the sickle-cell mutation one step at a time: the DNA codon GAG becomes GTG, the mRNA becomes GUG, the amino acid changes from Glu to Val, and the trait becomes sickle-shaped red blood cells.
In sickle-cell, the DNA codon GAG is changed to GTG. Using the table, which amino acid now takes the place of glutamic acid (Glu)?
Approved| Step | Normal hemoglobin | Sickle-cell change |
|---|---|---|
| DNA codon | GAG | GTG (the middle A is replaced by T) |
| mRNA codon | GAG | GUG |
| Amino acid | Glutamic acid (Glu) | Valine (Val) |
| Protein | Normal hemoglobin | Sticky hemoglobin that clumps |
| Trait (phenotype) | Round, flexible red blood cells | Stiff, sickle (crescent) shaped red blood cells |
- A.A stop signal
- B.Hemoglobin
- C.Glutamic acid (Glu)
- D.Valine (Val)
Show the worked solution ▾
Answer: D. Valine (Val)
- Step 1: Find the changed codon: The table's sickle column shows DNA GTG, which gives the mRNA codon GUG.
- Step 2: Read the amino acid row: Across from GUG, the table lists valine (Val) as the amino acid.
Why it's right: The GAG-to-GTG change makes the mRNA codon GUG, and the table shows GUG codes for valine, which replaces glutamic acid.
- A: GUG codes for valine; it is not a stop signal.
- B: Hemoglobin is the whole protein, not a single amino acid.
- C: Glutamic acid is the normal amino acid that the mutation replaces, not the new one.
Aligned to Biotechnology Research and Experiments: trace a point mutation · reading level ~grade 9
A base change makes an mRNA codon read GAA where it once read GAG. Both codons call for glutamic acid. What happens to the protein?
Reviewed- A.The protein ends early, because any base change halts the build
- B.The protein gains one extra amino acid at that position
- C.The amino acid switches, because the codon letters are no longer alike
- D.Nothing changes, because both codons call for the identical amino acid
Show the worked solution ▾
Answer: D. Nothing changes, because both codons call for the identical amino acid
- Step 1: Follow the chain one link at a time: The DNA changed, so the codon changed.
- Step 2: Check the next link: The codon changed, but the amino acid it calls for did not, so the chain stops there.
Why it's right: More than one codon can call for the same amino acid. GAA and GAG both call for glutamic acid, so the protein is built exactly as before and the trait is unchanged.
- A: Only a stop codon halts the build, and GAA is not a stop codon.
- B: A base swap replaces a letter; it does not insert an extra amino acid.
- C: Different letters can still call for the same amino acid, which is what happens here.
Aligned to Biotechnology Research and Experiments · reading level ~grade 10
- In Unit 2.2 Decoding a Diagnosis, this skill turns class evidence into a result another person can check.
Fill these in as you work through the lesson.
- Mutation (a change in the DNA sequence):
- Codon (three bases that code for one amino acid):
- Amino acid (a building block of a protein):
- Protein (the molecule that does the cell's work):
- Phenotype (the visible trait that results):
- Hemoglobin (the protein in red blood cells that carries oxygen):
Trace the chain in order: a change in changes a , which changes an , which changes the , which changes the (trait).
- In sickle-cell, which single change in the DNA codon starts the whole chain?
- What amino acid replaces glutamic acid (Glu), and what protein does it change?
- Why does the sickle shape of the red blood cells count as the phenotype?
Starting from DNA codon GAG changing to GTG, walk through the table to name the new amino acid and the final trait. (Answer: GTG -> mRNA GUG -> Valine -> sticky hemoglobin -> sickle-shaped red blood cells.)