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VCE Units 1–4 · Biology

VCE Biology — Gene Expression

Gene expression — how the information in DNA is used to build proteins — is the central concept of molecular biology and a core component of VCE Biology Units 3 and 4. The flow of information from DNA to RNA to protein (the central dogma) must be understood with precision, including which strand is used at each step and what each molecule looks like.

Key Concepts & Formulas

  • DNA structure: double helix; two antiparallel strands joined by complementary base pairs — A–T and G–C

  • Sense strand (coding strand): same sequence as mRNA (with T replaced by U)

  • Template strand (antisense): used as the template in transcription; complementary to mRNA

  • Transcription (nucleus): RNA polymerase reads the template strand 3'→5' and produces mRNA 5'→3'; T → U

  • Pre-mRNA processing: introns spliced out; exons joined; 5' cap and poly-A tail added

  • Translation (ribosome): mRNA is read in codons (triplets of bases) by ribosomes; tRNA brings amino acids

  • tRNA anticodon is complementary to the mRNA codon; A–U and G–C base pairing

  • Start codon: AUG (methionine); stop codons: UAA, UAG, UGA — no tRNA binds at stop codons

  • Mutations: point mutation (one base change); insertion/deletion (frameshift); missense, nonsense, silent

  • Gene regulation: promoters, transcription factors and enhancers control when and where genes are expressed

Practice Questions

5 questions

Attempt each question before reading the hint. These are styled to match VCE exam format.

Q1.A section of the template strand of DNA reads: 3' – TACGGATCC – 5'. Write the corresponding mRNA sequence.

2 marks
Show hint

Remember T in DNA → U in mRNA.

Q2.Using the genetic code table, determine the amino acid sequence coded by the mRNA: 5' – AUG – CGU – UCG – UAA – 3'.

2 marks

Q3.Explain the difference between a missense mutation and a nonsense mutation, and predict the effect of each on the resulting protein.

4 marks

Q4.Describe the role of a ribosome in translation, including the role of tRNA molecules.

4 marks

Q5.A gene is transcribed but the protein is not produced in certain cell types. Suggest TWO ways in which gene expression could be regulated.

2 marks

Common Mistakes to Avoid

These are the errors that VCE students most frequently make in Gene Expression — and that examiners are specifically watching for.

  • Using the sense strand instead of the template strand in transcription — the RNA polymerase reads the template strand

  • Writing DNA bases in mRNA (T instead of U) — mRNA contains uracil, not thymine

  • Confusing codons (on mRNA, read 5'→3') with anticodons (on tRNA, antiparallel and complementary to the codon)

  • Stating that stop codons code for a "stop amino acid" — no amino acid is added; the ribosome disassembles

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