Which mutation is usually worst?

A one-base insertion or deletion shifts the frame, so every later amino acid can change.

Why is GC-rich DNA harder to separate?

Each G to C pair has three hydrogen bonds instead of two, so more energy is needed.

S5 Biology: DNA and Protein Synthesis (Paper 3)

2026-11-01

The third paper works on mutation, laboratory techniques and base-ratio reasoning, judging consequences from given data.

Direct answer　A substitution may leave the amino acid unchanged, while insertions shift the reading frame; higher GC content raises the melting temperature, and 20 per cent adenine leaves 30 per cent guanine.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Biology: DNA and Protein Synthesis
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationWhat is meant by a gene mutation?
FoundationDoes replacing one base always change the protein?
FoundationWhat is DNA fingerprinting used for?
FoundationA DNA segment has 1 200 base pairs of 0.34 nm each. What is its total length?
IntermediateWhy does GC-rich DNA need a higher temperature to separate its strands?
IntermediateWhy is a one-base insertion usually worse than a substitution?
IntermediateWhat are the three steps of the polymerase chain reaction?
ChallengeAdenine makes up 20 per cent of the bases in a DNA segment. What percentage is guanine?
ChallengeA gene has 900 base pairs, 300 of them introns. If the mRNA keeps only exons, how many amino acids result?
Error redoA student says each daughter molecule is built from two brand-new strands. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	A change in the base sequence of DNA	The change may be a substitution, an insertion or a deletion, and the message changes with it.
2	Not always, because the code is degenerate and the amino acid may stay the same	The code is degenerate, so several codons can specify the same amino acid.
3	Identifying individuals and establishing family relationships	Repeat lengths differ between individuals, so the band pattern after electrophoresis is unique.
4	408 nm	Twelve hundred times 0.34 nm gives 408 nanometres.
5	G and C share three hydrogen bonds, one more than A with T	More hydrogen bonds mean more energy is needed to separate the strands, so the melting point rises.
6	Every codon after the insertion shifts, so the amino acid sequence changes drastically	The reading frame shifts by one, so nearly every later amino acid is altered.
7	Denaturation, annealing and extension	Heat separates the strands, cooling lets primers bind, then polymerase extends the new strand.
8	Thirty per cent	A and T together take 40 per cent, so the remaining 60 per cent is split equally at 30 per cent each.
9	Four hundred amino acids	The exons hold 600 pairs, that is 1 200 bases, which divide into 400 codons.
10	Each daughter keeps one parental strand plus one new strand	That is what semi-conservative means: each daughter keeps one parental strand.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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