Predict effects of nucleotide insertions and deletions
An insertion or deletion changes the DNA sequence; if the number is not a multiple of three, downstream codon grouping shifts.
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 an insertion mutation?
What you will learn
- Describe insertion and deletion mutations
- Explain frameshift effects when one or two bases are inserted or deleted
- Distinguish frameshifts from changes in whole codons
Codons are read in groups of three
An insertion adds one or more nucleotides to DNA; a deletion removes one or more. If the number inserted or deleted is not a multiple of three, the reading frame can shift from the mutation onward.
A frameshift changes how downstream bases are grouped into codons. This can alter many amino acids and may create a premature stop codon. The effect depends on the position and the resulting sequence.
Multiples of three have a different pattern
Insertion or deletion of three bases can add or remove one amino acid without shifting the grouping of all later codons. A one- or two-base change typically shifts the reading frame; adding or removing a multiple of three does not create that same frame shift.
Do not state that every insertion or deletion has an identical outcome. Use the number of bases and whether the reading frame changes to justify the prediction.
Pause and explain
Why can a one-base insertion alter many downstream codons?
Worked example
One nucleotide is deleted from the middle of a coding sequence. Explain why several downstream amino acids may change.
Show the worked solution
- Codons are triplets.
- Deleting one base changes the grouping of later bases.
Answer The deletion of one nucleotide shifts the reading frame because codons are read in triplets. Downstream bases are regrouped into different codons, so several amino acids and possibly a stop signal may change.
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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.
Explain the likely consequence of deleting one base from a coding sequence.
Give me a hint
Track the effect on triplet grouping after the deleted base.
Compare my reasoning
- Codons are read in groups of three.
- A one-base deletion shifts the reading frame and changes downstream codons.
A one-base deletion shifts the reading frame because the sequence is read in triplets. Downstream codons may change, altering several amino acids and possibly creating a stop codon.
Look for these in your work
- Frameshift explained
- Downstream effects stated
Compare insertion of three bases with insertion of one base.
Give me a hint
Check whether the codon grouping shifts.
Compare my reasoning
- One base typically shifts the reading frame.
- Three bases can add one amino acid without shifting downstream triplets.
Insertion of one base typically shifts the reading frame and changes downstream codons. Insertion of three bases can add one amino acid while preserving the grouping of later codons.
Look for these in your work
- One-base effect correct
- Three-base distinction correct
A student says “every insertion makes the entire protein sequence different.” Evaluate the claim.
Give me a hint
Consider insertion position, number of bases and downstream reading frame.
Compare my reasoning
- A one- or two-base insertion can shift the reading frame.
- An insertion of a multiple of three can add amino acid(s) without shifting later codons; effects depend on position.
The claim is too broad. A one- or two-base insertion can shift the reading frame and change many downstream codons, but insertion of a multiple of three can add amino acid(s) without shifting later codon grouping. The effect also depends on its position.
Look for these in your work
- Claim evaluated
- Frame and multiple-of-three cases distinguished
Common mistakes
- Calling a one-base insertion a substitution
- Claiming any deletion changes every amino acid in the protein
- Calling a three-base deletion a frameshift without checking the reading frame
What can you explain now?
What is an insertion mutation?
Compare with the explanation
One or more nucleotides are added to DNA
Insertion adds nucleotide(s).
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Make it stick
When do insertions and deletions shift the reading frame, and how does that differ from a multiple-of-three change?
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.