The effect of mutations - silent, missense, nonsense
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of the genetic code, codons, and the mechanisms by which DNA sequence changes can affect protein synthesis. You can test your knowledge on these below.
What is a codon?
A sequence of three bases on mRNA that codes for a specific amino acid.
What is the role of tRNA during translation?
It carries amino acids to the ribosome and binds to codons via its anticodon.
What is meant by the term ‘degenerate genetic code’?
Most amino acids are coded for by more than one codon.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains the effect of mutations - silent, missense, nonsense or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What Happens When a Mutation Occurs in a Gene?
A mutation in the base sequence of DNA can change the codon on the mRNA.
This may lead to a change in the amino acid sequence of the polypeptide.
The impact of this change depends on the type of mutation.
Types of Mutational Effects on Protein Structure
Effect | Definition | Impact on Amino Acid Sequence |
Silent Mutation | A base is changed, but the codon still codes for the same amino acid | No change to the polypeptide |
Missense Mutation | A base change results in a different amino acid being coded for | Alters one amino acid in the polypeptide |
Nonsense Mutation | A base change results in a stop codon being formed | Premature end to translation – truncated / shorter polypeptide |
Mutations may also appear to be silent if:
The mutation happens in a non-coding region (eg.introns)
The change in the amino acid has no effect on the tertiary structure of the protein or the activity of the protein.
The mutation occurs in a recessive allele in a heterozygous individual
Mutations can also have a positive effect by producing new beneficial alleles that produce new phenotypes.
Example:
Original mRNA sequence
→ Met – Arg – Glu – Cys – Stop
Silent Mutation (e.g. )
→ Met – Arg – Glu – Cys – Stop
No change to the protein.
Missense Mutation (e.g. )
→ Met – Arg – Asp – Cys – Stop
One amino acid has changed.
Nonsense Mutation (e.g. )
→ Met – Arg – Stop
Protein is incomplete due to early termination.
No answer provided.
What is a Frameshift Mutation?
A frameshift mutation occurs when bases are inserted or deleted from the DNA sequence not in multiples of three.
This changes the reading frame of the mRNA codons from the point of mutation onward.
As the genetic code is read in triplets, this shift causes every codon after the mutation to be altered.
Types of Frameshift Mutations
Addition (Insertion) – extra base(s) inserted.
Deletion – base(s) removed.
Example:
Original mRNA:
→ Met – Arg – Glu – Cys – Stop
Frameshift Example (Insertion of one base):
→ Completely different amino acids from the point of mutation.
This has a much greater effect on the polypeptide and therefore the mutation has more of an effect on the individual.
No answer provided.
Key Terms
Codon – a set of three bases on mRNA that codes for an amino acid.
Silent mutation – changes a base but not the amino acid.
Missense mutation – changes a base and causes a different amino acid to be added.
Nonsense mutation – changes a base to create a stop codon.
Frameshift mutation - changes the sequence of codons and could change every amino acid after the mutation.
No answer provided.
Exam Tips
Always link the mutation to its effect on the amino acid sequence, not just the DNA.
Remember all the different reasons why the effect of a mutation may not be seen.
No answer provided.
Explain the difference between a silent mutation and a nonsense mutation. (2 marks)
A silent mutation changes a base in the DNA but still codes for the same amino acid due to the degenerate genetic code.
A nonsense mutation changes a codon to a stop codon, causing translation to end early.
As a result, the protein from a nonsense mutation is shorter and incomplete.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!