Some gene mutations change the nature of all base triplets downstream from…

The idea

The ribosome reads mRNA in consecutive, non-overlapping codons from a fixed start codon. Adding or deleting a base, or any number of bases not a multiple of three, shifts that reading frame, so every codon after the mutation is regrouped from different bases. From that point onwards, the codons specify a completely different sequence of amino acids, and the polypeptide is usually non-functional, because its primary structure, and therefore its R group interactions and tertiary structure, bear no relation to the original. Very often the new reading frame throws up a stop codon earlier than normal, so translation halts and a short, truncated polypeptide is released; less often a stop codon is lost and the chain runs on beyond its normal end. The earlier in the gene the frame shift occurs, the more codons are affected and the more damaging it is.

What earns the marks

• An addition or deletion of a base shifts the reading frame.
• All codons downstream of the mutation are changed.
• A completely different sequence of amino acids from the point of the mutation onwards.
• The polypeptide is non-functional because its tertiary structure is altered, or a premature stop codon produces a shortened polypeptide.
• A frame shift near the start of a gene has a greater effect than one near the end.
• Use codon for mRNA triplets and triplet for DNA base triplets.

Common mistakes and how to put them right

• Writing that the whole polypeptide is changed, when only the part downstream of the mutation is affected.

  Put it right: Specify that only codons after the mutation are altered.

• Believing every deletion causes a frame shift, forgetting that a deletion of three bases does not.

  Put it right: Only insertions or deletions not in multiples of three cause a frame shift.

• Omitting the word downstream, so the answer never identifies which codons change.

  Put it right: State that all codons downstream of the mutation are affected.

• Describing a substitution as causing a frame shift.

  Put it right: A substitution changes one codon and does not shift the reading frame.

• Saying the protein is denatured rather than that a different polypeptide has been made.

  Put it right: A frame shift produces a different amino acid sequence, not a denatured version of the original.

Exam tips

• Count the bases added or deleted: if the number divides by three, there is no frame shift.
• When given a sequence, rewrite it in codons after the mutation so the examiner can see the new reading frame.
• Say how many amino acids are affected rather than just saying the protein changes.