Gene mutations
Gene mutations
- A gene mutation is a change in the base sequence of DNA.
- A change in the code can change the polypeptide that is made.
- How much it changes depends on the type of mutation.
Practice
A gene mutation is:
A gene mutation is a change to the sequence of bases in a DNA molecule.
Substitution
- A substitution swaps one base for a different base.
- It changes at most one amino acid — and sometimes none at all, because the code is degenerate (the new triplet may still code for the same amino acid).
Practice
A substitution mutation changes:
Only one triplet is altered, so at most one amino acid changes — and the degenerate code means sometimes none does.
Practice
A substitution can sometimes leave the amino acid unchanged.
Because the code is degenerate, the new triplet may still code for the same amino acid — no change.
Deletion and insertion: frameshift
- A deletion removes a base; an insertion adds an extra base.
- Either one shifts the reading frame — every later triplet is read wrongly.
- So these usually change many amino acids after that point, with a large effect.
original: CAT-GGA-TCC-ATG
substitution: CAT-GCA-TCC-ATG one codon changed
deletion: CAT-GAT-CCA-TG? frame shifts → all later codons change
Practice
Why do deletions and insertions usually have a large effect?
Adding or removing a base shifts the reading frame, changing all the triplets that follow (a frameshift).
You've got it
Key idea
- a mutation = a change in the DNA base sequence
- substitution: one base swapped → changes ≤ 1 amino acid (sometimes none — degenerate code)
- deletion / insertion: shift the reading frame → change many later amino acids (frameshift)
- frameshift mutations usually have a much larger effect than a substitution