AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Some gene mutations change only one triplet code.
Full specification wording
Alteration of the sequence of bases in DNA can alter the: Some gene mutations change only one triplet code.
The genetic code is non-overlapping and is read in triplets from a fixed starting point, so each base belongs to one triplet only.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 5 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The genetic code is non-overlapping and is read in triplets from a fixed starting point, so each base belongs to one triplet only. A substitution replaces one base with another and therefore alters exactly one triplet; every triplet before and after it is unchanged and the reading frame is preserved. A deletion or addition of three bases also leaves the frame intact, but it loses or gains one whole triplet only when it falls on a triplet boundary: a three-base deletion that straddles a boundary removes one amino acid and alters the triplet at the junction as well, and a three-base addition inserts an amino acid rather than changing one. Provided the new triplet still codes for an amino acid, the consequence is limited to a single amino acid changed.
What earns the marks
- Stating that the code is non-overlapping and read in triplets, so a substitution affects only one triplet.
- Stating that the reading frame and all other triplets are unchanged.
- Only one amino acid in the polypeptide being changed, provided the new triplet is not a stop triplet.
- The effect ranging from none, because the code is degenerate, to a non-functional protein where the amino acid is at a critical position.
- A correct named example, such as a substitution causing sickle cell anaemia.
Common mistakes and how to put them right
Assuming any single-base change alters the whole polypeptide, which is true of a frame shift but not of a substitution.
Describing a deletion of one base as changing only one triplet.
Saying the code is universal when the point required is that it is non-overlapping or degenerate, a confusion mark schemes penalise.
Claiming a single amino acid change can never matter.
Mixing up triplet on DNA, codon on mRNA and anticodon on tRNA.
Exam tips
- State explicitly which mutations preserve the reading frame: substitution, and additions or deletions of three bases.
- Keep the vocabulary separate, using triplet for DNA, codon for mRNA and anticodon for tRNA.
- Give sickle cell anaemia as your named example, since it is the standard single-substitution case.
Try the lesson
The start of the Alteration of the sequence of bases in DNA can alter the: Some gene mutations change only one triplet code. lesson, made from checked teaching notes for this specification point.
Have a go first
What name is given to a gene mutation that changes only one triplet code?
Listen
The genetic code is non-overlapping and is read in triplets from a fixed starting point, so each base belongs to one triplet only. A substitution replaces one base with another and therefore alters exactly one triplet; every triplet before and after it is unchanged and the reading frame is preserved. A deletion or addition of three bases also leaves the frame intact, but it loses or gains one whole triplet only when it falls on a triplet boundary: a three-base deletion that straddles a boundary removes one amino acid and alters the triplet at the junction as well, and a three-base addition inserts an amino acid rather than changing one. Provided the new triplet still codes for an amino acid, the consequence is limited to a single amino acid changed. A strong answer earns marks for these points. stating that the code is non-overlapping and read in triplets, so a substitution affects only one triplet. stating that the reading frame and all other triplets are unchanged. only one amino acid in the polypeptide being changed, provided the new triplet is not a stop triplet. the effect ranging from none, because the code is degenerate, to a non-functional protein where the amino acid is at a critical position. Exam tip: State explicitly which mutations preserve the reading frame: substitution, and additions or deletions of three bases.
Key words
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substitution
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non-overlapping code
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reading frame preserved
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one amino acid changed
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See every definition
- substitution
- A change that replaces one base with another and therefore alters exactly one triplet, with every triplet before and after it unchanged.
- non-overlapping code
- The code is read in triplets from a fixed starting point, so each base belongs to one triplet only.
- reading frame preserved
- The reading frame is intact because every triplet before and after the altered one is unchanged.
- one amino acid changed
- Provided the new triplet still codes for an amino acid, the consequence is limited to a single amino acid changed.
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