Predict effects of base substitutions
A substitution replaces one DNA base pair; it may be silent, change an amino acid or create a stop codon.
Section 6.2 · Protein synthesis
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain DNA, RNA and protein synthesis. Written practice is self-checked; it does not replace teacher review or practical assessment.
Before you begin
- Recognise that DNA stores genetic information and that proteins are made from amino acids.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a substitution mutation?
What you will learn
- Describe a base substitution
- Compare synonymous, missense and nonsense outcomes using codons
- Explain why a substitution may have different effects
One base pair is replaced
A substitution changes one base pair in DNA to another. After transcription, the corresponding mRNA codon may be different.
If the altered codon specifies the same amino acid, the substitution is synonymous. If it specifies a different amino acid, it is missense. If it becomes a stop codon, it is nonsense and translation can stop early.
Use the code table, not intuition
The effect depends on the specific codon and the substitution position. The genetic code’s degeneracy means some substitutions do not change the amino acid. A changed amino acid or early stop may alter the polypeptide.
When a question supplies a genetic-code table, translate both the original and altered mRNA codons using that table before deciding the effect.
Pause and explain
What happens in a synonymous substitution?
Worked example
An mRNA codon changes from UUU to UUC. The supplied table shows both code for phenylalanine. Classify the result.
Show the worked solution
- The codon changed.
- The amino acid remained phenylalanine.
Answer This is a synonymous substitution: the codon changed from UUU to UUC, but both specify phenylalanine, so the amino-acid sequence is unchanged at this position.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
An mRNA codon changes but the supplied code table shows that both codons specify the same amino acid. Describe the likely polypeptide effect.
Give me a hint
Compare the amino acids rather than only the codon letters.
Compare my reasoning
- The codon is different.
- The amino acid is unchanged, so the polypeptide sequence at that position is unchanged.
This is a synonymous substitution. The codon changes, but both specify the same amino acid, so the polypeptide sequence is unchanged at that position.
Look for these in your work
- Substitution recognised
- Amino-acid effect described
A substitution changes a codon for an amino acid into a codon for a different amino acid. Name the outcome.
Give me a hint
Use the term for an amino-acid replacement.
Compare my reasoning
- The codon now specifies a different amino acid.
- This is a missense substitution.
This is a missense substitution because the changed codon specifies a different amino acid.
Look for these in your work
- Different amino acid identified
- Term correct
A substitution creates a stop codon in the middle of a coding sequence. Predict the likely translation effect.
Give me a hint
Consider where translation terminates.
Compare my reasoning
- A stop codon signals termination.
- The polypeptide is likely to be shorter than the original.
The substitution creates a premature stop codon, so translation is likely to end early and produce a shorter polypeptide. The exact effect on function depends on the protein and mutation position.
Look for these in your work
- Premature termination stated
- Effect appropriately qualified
Common mistakes
- Calling every substitution a frameshift
- Assuming a changed codon must change the amino acid
- Using the DNA triplet directly in an mRNA code table without transcribing it
What can you explain now?
What is a substitution mutation?
Compare with the explanation
One DNA base pair is replaced by another
A substitution replaces a base pair.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How can one base substitution be synonymous, missense or nonsense?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.