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AS · Practice questions · Gene mutations

Swap, drop, add.

Six original Cambridge-style questions on gene mutations, from defining a mutation to predicting the effect of a change on a given sequence.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Codon, frame, function.

01
Recall
[2 marks]

State what is meant by a gene mutation and name the three types described in your syllabus.

  • A change in the sequence of base pairs in a DNA molecule (which may result in an altered polypeptide). ✓
  • Substitution, deletion and insertion (of nucleotides). ✓
02
Explain
[4 marks]

Explain how a single base substitution in the β-globin gene leads to sickle cell anaemia.

  • The substitution changes one triplet in the DNA, CTC to CAC on the template strand, so the mRNA codon changes from GAG to GUG. ✓
  • Glutamic acid is replaced by valine in the β-globin polypeptide. ✓
  • Valine is non-polar/hydrophobic whereas glutamic acid is charged, so the haemoglobin molecules stick together at low oxygen concentrations. ✓
  • This forms long fibres that distort the red blood cells into a sickle shape, so they carry less oxygen and can block capillaries. ✓
03
Data
[3 marks]

The mRNA for the start of a polypeptide reads AUG GCU AAA GGC UGG. (AUG Met, GCU Ala, AAA Lys, GGC Gly, UGG Trp, GCA Ala, AAG Lys, GCU Ala, AAG Lys, UAA stop.) The first A of the third codon is deleted. Write the new codon sequence and amino acid sequence, and name the type of effect.

  • New mRNA: AUG GCU AAG GCU GG... ✓
  • Amino acids: Met-Ala-Lys-Ala-... ✓
  • A frameshift: every codon after the deletion is read in a different frame (here the first changed codon still codes for Lys, but those after it differ). ✓
04
Explain
[3 marks]

Explain why some substitutions have no effect on the polypeptide, whereas others make it non-functional.

  • The genetic code is degenerate, so the new codon may code for the same amino acid; then the polypeptide is unchanged. ✓
  • If a different amino acid is coded, the primary structure changes, which can change bonding, folding and the tertiary structure (for example of an active site), so function is lost. ✓
  • If the new codon is a stop codon, the polypeptide is shortened and usually non-functional. ✓
05
Analysis
[2 marks]

An insertion of one nucleotide occurs in the last codon before the stop codon of a gene. Suggest why this may have little effect on the protein.

  • Only the reading frame from that point onwards changes, and all earlier codons are read normally, so almost all of the primary structure is unchanged. ✓
  • The changed or extra amino acids (before a new stop codon is reached) are at the very end of the chain, which may not affect folding or the active site. ✓
06
Explain
[2 marks]

Suggest why deleting three adjacent nucleotides usually has a smaller effect than deleting one nucleotide.

  • Deleting three nucleotides removes one codon, so one amino acid is lost (or one changed), and the reading frame is kept. ✓
  • Deleting one nucleotide causes a frameshift, changing every codon after it. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.