Transcription, Translation & Gene Expression

Q1. (4 marks)

• The central dogma describes the flow of genetic information within a cell: DNA → RNA → Protein. (1)
• Flow diagram with correct labels: (3) $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA (mRNA)} \xrightarrow{\text{translation}} \text{Protein}$
• 1 mark for DNA→RNA arrow labelled transcription; 1 mark for RNA→Protein arrow labelled translation; 1 mark for correct order/molecules. (Allow mention of DNA→DNA replication.) Why: Tests recall of information flow and the naming of each conversion process.

Q2. (6 marks) — 1 mark per correct cell (name + function).

Why: Distinguishes the three RNA classes by identity and role.

Q3. (3 marks)

• (a) Initiation: RNA polymerase binds the promoter and the DNA double helix unwinds/opens. (1)
• (b) Elongation: RNA polymerase moves along the template strand (3′→5′) adding complementary RNA nucleotides in the 5′→3′ direction. (1)
• (c) Termination: RNA polymerase reaches a terminator sequence and releases the completed RNA transcript. (1)

Q4. (4 marks)

• (a) RNA polymerase: Enzyme that catalyses RNA synthesis; reads the DNA template and joins RNA nucleotides via phosphodiester bonds. (2)
• (b) Promoter: A specific DNA sequence upstream of a gene that RNA polymerase (and transcription factors) recognise and bind to; determines start point and direction of transcription. (2)

Q5. (6 marks) — 2 marks each (modification + purpose).

• 5′ cap (modified guanine): protects mRNA from degradation / aids ribosome binding & export. (2)
• Poly-A tail (string of adenines at 3′ end): stabilises mRNA / aids export from nucleus. (2)
• Splicing (removal of introns, joining of exons by spliceosome): produces mature coding mRNA. (2)

Q6. (5 marks)

• Intron: non-coding region of pre-mRNA that is removed during splicing. (1)
• Exon: coding (expressed) region retained in mature mRNA and translated. (1)
• Alternative splicing: the same pre-mRNA can be spliced in different ways, joining different combinations of exons. (2)
• Benefit: one gene can produce multiple protein variants (increases protein diversity). (1)

Q7. (5 marks)

• Template (3′→5′): TAC GGA TTT ACC ACT
• mRNA is complementary and antiparallel → mRNA (5′→3′): AUG CCU AAA UGG UGA (2)
• Reading codons:
  • AUG → Methionine (Met, start) (0.5)
  • CCU → Proline (Pro) (0.5)
  • AAA → Lysine (Lys) (0.5)
  • UGG → Tryptophan (Trp) (0.5)
  • UGA → STOP (translation ends) (1)
• Amino acid sequence: Met – Pro – Lys – Trp – (Stop) Why: Tests complementary base pairing and codon-table reading including a stop codon.

Q8. (3 marks)

• Degeneracy: more than one codon can code for the same amino acid (there are 64 codons but only 20 amino acids). (2)
• Advantage: reduces harmful effects of point mutations — a change in the third base often gives the same amino acid (silent mutation). (1)

Q9. (4 marks) — 1 mark naming, 1 each for correct function (max 4).

• A site (aminoacyl): accepts the incoming aminoacyl-tRNA whose anticodon matches the mRNA codon. (1.5)
• P site (peptidyl): holds the tRNA carrying the growing polypeptide chain; peptide bond forms here. (1.5)
• E site (exit): the now-empty (deacylated) tRNA leaves the ribosome from here. (1)

[
  {
    "claim": "mRNA from template 3'-TACGGATTTACCACT-5' is 5'-AUGCCUAAAUGGUGA-3'",
    "code": "comp={'A':'U','T':'A','C':'G','G':'C'}; template='TACGGATTTACCACT'; mrna=''.join(comp[b] for b in template); result = (mrna=='AUGCCUAAAUGGUGA')"
  },
  {
    "claim": "The first codon AUG is a start codon (Methionine)",
    "code": "codon='AUG'; start_codons={'AUG'}; result = codon in start_codons"
  },
  {
    "claim": "The final codon UGA is a stop codon",
    "code": "stops={'UAA','UAG','UGA'}; result = 'UGA' in stops"
  },
  {
    "claim": "Genetic code degeneracy: 64 codons map to 20 amino acids plus stop, so redundancy exists",
    "code": "codons=4**3; amino_acids=20; result = (codons==64 and codons>amino_acids)"
  }
]