D1.3 Mutation and gene editing. Practice questions with markscheme.

• D: the classic sickle-cell substitution: one amino-acid change alters haemoglobin's behaviour (aggregation, sickling of red cells);

• A: frameshift: all subsequent codons are read differently (often producing a premature stop), whereas a substitution changes at most one codon;

• (a) [1]
  • a gamete / a cell that gives rise to gametes (germ-line cell) in the ovary or testis;

  Do not accept 'sex cell' alone.

• (b) [2 max]
  • a lung cell is a body (somatic) cell, and mitosis passes the mutation only to its daughter cells within that tissue;
  • an accumulation of such mutations in a cell can lead to uncontrolled division, i.e. a tumour;
  • somatic cells do not contribute DNA to gametes, so the mutation cannot be passed to the next generation, OWTTE;
• (c) [2]
  • the risk depends on the cumulative dose, because mutations accumulate and are not repaired away between exposures;
  • the dose from a single shift is too small to measure reliably or to relate to risk, so totalling it over weeks gives a usable measure, OWTTE;

• (a) [1]
  • a (base) substitution;

  Accept 'silent/synonymous substitution'.

• (b) [2]
  • only the third base of the codon has changed, and both ACA and ACG code for threonine;
  • the genetic code is degenerate, so the amino acid sequence, and therefore the protein, is unchanged, OWTTE;
• (c) [2]
  • the codon UGG (Trp) has become UGA, a stop codon, so translation ends early (nonsense mutation);
  • only a very short truncated polypeptide is made, which cannot fold correctly and is non-functional, OWTTE;
• (d) [2 max]
  • removing one base shifts the reading frame, so every codon after the mutation is read differently;
  • the whole of the rest of the amino acid sequence is therefore changed, and a stop codon often appears early;
  • a substitution alters at most one amino acid and may alter none, so far less of the protein is affected, OWTTE;

• strength: DNA (structure/replication) is universal, so a chemical that damages bacterial DNA is likely to damage human DNA by the same chemistry;
• strength: bacteria allow huge numbers, rapid generations and easy counting, so the test is fast, cheap and quantitative (a dose–response can be measured) and avoids testing on animals/humans;
• limitation: bacteria lack human metabolism, a chemical may only become mutagenic (or be detoxified) after processing by liver enzymes, so results can be false negatives/positives (unless liver extract is added);
• limitation: uptake, DNA repair and exposure levels differ between bacteria and human tissues, and mutagenicity in bacteria does not prove the chemical causes cancer/disease in humans;
• judgement: a valuable first screen for identifying probable mutagens, but positive/negative results must be confirmed with further (mammalian-cell or epidemiological) evidence, OWTTE;

NOS: evaluating an experimental model system; require at least one strength, one limitation and a judgement.

• substitution: a sequence of the same length (12 bases) in which one base has been replaced by a different base, e.g. ATG CCA GTC AAG, labelled and with the changed base marked;
• insertion: a sequence one base longer (13 bases) in which an extra base has been added between two of the original bases, e.g. ATG CCA GTT TAA G, labelled and with the added base marked;
• deletion: a sequence one base shorter (11 bases) from which one original base has been removed, e.g. ATG CCA GTA AG, labelled and with the position marked;
• triplets after the insertion or deletion shown regrouped to indicate the shift in the reading frame, OWTTE;

Any base may be changed, added or removed; credit only if all the other bases are unchanged and in the original order. Do not accept the insertion or deletion of a whole triplet as the example. Award [3 max].