Biology

1. 1.

   Explain the complementary base pairing that maintains information during DNA replication.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Specific pairing determines the new sequence from the exposed template. Separating the strands then synthesising complements yields two molecules retaining the original base-pair information.

   Marking points

   • A pairs with T and C pairs with G.
   • Each parental strand acts as a template.
   • Each new DNA molecule contains one parental and one newly synthesised strand.

   Examiner tip: Semiconservative refers to strands, not to half of each nucleotide.

2. 2.

   Distinguish the role of mRNA from that of tRNA in translation.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. mRNA is the sequence template, while charged tRNAs are adaptors linking codons to amino acids. Ribosomes coordinate pairing and peptide-bond formation rather than supplying a new genetic code.

   Marking points

   • mRNA supplies the codon sequence read by the ribosome.
   • tRNA carries a particular amino acid.
   • Its anticodon pairs with the complementary mRNA codon.

   Examiner tip: A tRNA anticodon is not the amino-acid coding template itself.

3. 3.

   A coding DNA strand is 5'-ATG GAA TTT TGA-3'. Given AUG=Met, GAA=Glu, UUU=Phe and UGA=stop, write the mRNA and resulting peptide, assuming initiation at the first AUG.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The coding strand matches mRNA apart from replacing T with U. Read successive triplets from the given start and terminate at UGA; do not complement the coding strand as if it were the template.

   Marking points

   • mRNA is 5'-AUG GAA UUU UGA-3'.
   • The peptide is Met-Glu-Phe.
   • The stop codon does not add an amino acid.

   Examiner tip: Identify coding versus template strand before transcribing.

4. 4.

   Explain why deletion of one nucleotide early in a coding region can have a larger effect than substitution of one nucleotide.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Codons are read in groups of three from a fixed start. Removing one character changes how all later characters are grouped, while replacing a character leaves the grouping unchanged.

   Marking points

   • A one-base deletion shifts the downstream reading frame.
   • Many downstream codons can change or a premature stop can appear.
   • A substitution usually affects one codon and may even be synonymous.

   Examiner tip: Say 'can', not 'always'; position and the resulting sequence matter.

5. 5.

   Cells with identical DNA can make different proteins. Explain this through transcriptional regulation and mRNA processing rather than proposing different genetic codes.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The code maps codons to amino acids, but regulation determines which messages exist. Processing can additionally alter a message before translation, giving specialised cells distinct protein profiles.

   Marking points

   • Different transcription factors activate or repress different genes.
   • Chromatin accessibility affects which genes can be transcribed.
   • Alternative splicing can join different exon combinations.
   • Different mRNA populations produce different proteins despite the same genome.

   Examiner tip: Gene expression differs without requiring DNA sequence differences.

6. 6.

   A gene variant is associated with a disease in a case-control study. Evaluate why association alone does not prove that its encoded protein causes the disease, and propose stronger evidence.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. An association locates a statistical relationship, not necessarily a molecular mechanism. Altering the candidate variant while holding background constant and restoring normal function tests that mechanism more directly.

   Marking points

   • The variant may be linked to another causal variant.
   • Population structure or other confounding can generate association.
   • Compare expression/protein function in controlled variant experiments.
   • Replication and rescue of the phenotype strengthen a causal interpretation.

   Examiner tip: Do not equate statistical significance with causation.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.