Insertions and deletions at Higher Level: The same statement as SL. At HL the same idea is used on purpose: gene knockout often deletes or disrupts a gene so that its product stops working.
Practise this as you read
- Deduce a codon after a base is deleted.
- Explain why insertions and deletions usually stop a polypeptide working.
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The big idea: Inserting or deleting a base shifts the reading frame. Every codon after it changes, so the polypeptide is likely to stop working.
The points to remember
- The mRNA is read in codons of three, from a fixed starting point.
- An insertion or deletion of one or two bases shifts every later codon: a frameshift.
- All the amino acids after the change are likely to be different.
- A stop codon often appears early, so the polypeptide is shorter.
- A polypeptide with the wrong sequence usually cannot fold properly and stops working.
Remember it as: Lose one letter and every word after it reads wrong.
Real example: Duchenne muscular dystrophy. Most cases of Duchenne muscular dystrophy are deletions of one or more whole exons from the very long dystrophin gene. Most of these shift the reading frame, so no working dystrophin is made and muscles slowly waste.
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Big changes, small changes: How much the polypeptide changes depends on how many bases are added or removed, and where.
| Change | What happens | Likely effect |
|---|---|---|
| One or two bases added or removed | Frameshift: every later codon changes | Polypeptide very different, often shorter; stops working |
| Three bases added or removed | One codon added or lost; no frameshift | One amino acid more or fewer; may still work |
| A large insertion or deletion | Big parts of the gene added, lost or disrupted | Polypeptide very likely to stop working |
Near the start is worse: A frameshift near the start of a gene changes almost the whole polypeptide; one near the end changes only the last few amino acids.
How it is asked: A 1A item asking for a codon after a deletion, or a few marks on why an insertion or deletion has a bigger effect than a substitution.
Explain why an insertion of one base usually has a greater effect on a polypeptide than a substitution of one base.
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See the answer broken into clear points, with guidance on how each point earns marks.