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VCE · VCE Units 3 & 4

Biology

Molecules, cells, genetics and evolution — full exams with worked solutions and short-answer guides.

20full-length model exams with mark-by-mark answer guides
20detailed note sets — ~120 pages across every topic
64exam-style practice questions with worked solutions
60flashcards for every key term & formula
10official past papers

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Sample revision note

DNA & RNA: Structure, Replication & the Genetic Code

Nucleotide Structure: The Molecular Building Block

All nucleic acids are polymers built from nucleotide monomers. Each nucleotide contains three covalently bonded components:

  • Pentose sugar — deoxyribose (in DNA) or ribose (in RNA). The only structural difference is that ribose carries a hydroxyl (–OH) group at the 2' carbon, whereas deoxyribose has only a hydrogen (–H) there.
  • Phosphate group — a negatively charged PO43− group attached to the 5' carbon of the sugar. This gives the backbone its strong negative charge and is the site of covalent linkage between adjacent nucleotides (phosphodiester bonds).
  • Nitrogenous base — attached to the 1' carbon of the sugar. Bases are grouped as purines (double-ring: adenine A, guanine G) or pyrimidines (single-ring: cytosine C, thymine T in DNA; uracil U replaces thymine in RNA).

Nucleotides polymerise via condensation reactions (releasing water), forming a phosphodiester bond between the 3'–OH of one sugar and the 5'–phosphate of the next. This creates a directional backbone with a free 5'–phosphate at one end and a free 3'–OH at the other — the strand is said to run 5' → 3'.

Applied example: A question might show a partial nucleotide sequence and ask you to identify whether it is DNA or RNA. Look for the base U (uracil) — its presence confirms RNA. If only A, T, C, G appear, it is DNA. If asked to draw a nucleotide, remember to show all three components connected correctly: phosphate to 5' carbon, base to 1' carbon.

DNA Double Helix: Structure and Base Pairing

The DNA molecule is a double-stranded helix, first described by Watson and Crick in 1953 using X-ray crystallography data (primarily from Rosalind Franklin). Its key structural features are:

  • Antiparallel orientation — the two strands run in opposite directions. One runs 5' → 3' and its complement runs 3' → 5'. This antiparallel arrangement is essential for replication and transcription enzymes, which can only read and synthesise in one direction.
  • Complementary base pairing — bases on opposite strands hydrogen-bond in a specific, fixed way: A pairs with T (2 hydrogen bonds) and G pairs with C (3 hydrogen bonds). The G≡C bond is stronger; organisms or DNA regions with high G-C content require more energy to denature.
  • Sugar-phosphate backbone — the alternating deoxyribose and phosphate groups form the outer 'rails' of the helix; the bases stack inward, hydrophobically stabilising the structure.
  • Major and minor grooves — the helical twist creates two grooves of different widths. Transcription factors and regulatory proteins often bind in the major groove, where base-specific contacts are possible.

DNA is condensed by wrapping around histone proteins to form nucleosomes, then further coiled into chromatin. During cell division, chromatin is maximally condensed into visible chromosomes.

Applied example (Chargaff's rules): If a DNA molecule is 22% thymine, what percentage is guanine? Because A = T and G = C, and all percentages sum to 100%: A = T = 22%, so A + T = 44%, leaving G + C = 56%, meaning G = C = 28%. VCAA short-answer questions frequently test this logic — practise applying it quickly.

Sample exam question

A researcher synthesises a single-stranded DNA template with the sequence 3'-TACGAACTTGGG-5'. Which of the following correctly represents the mRNA transcript produced from this template?

  • 5'-AUGCUUGAACCC-3'
  • 5'-AUGGAACUUGGG-3'
  • 5'-UACGAACUUGGG-3'
  • 5'-AUGCUUGAACCC-3' with thymine replacing uracil
Show the worked answer

Answer: A

mRNA is complementary and antiparallel to the template strand, with U replacing T. Reading the template 3' to 5' gives mRNA 5'-AUGCUUGAACCC-3'.

All 20 practice exams

  1. Exam 1 — U3 AOS1: nucleic acids, gene expression, CRISPR molecular mechanism; U3 AOS2: enzymes, biotechnology, gel electrophoresis, PCR; U4 AOS1: innate and adaptive immunity, immunotherapy, CAR-T, autoimmunity
  2. Exam 2 — U3 AOS2 – Photosynthesis, enzymes & cellular respiration; U3 AOS1 – Nucleic acids, gene expression & biotechnology; U4 AOS1 – Innate & adaptive immunity, immunotherapy
  3. Exam 3 — U3 AOS1: nucleic acids, protein synthesis, mutation; U3 AOS2: enzyme inhibition, biotechnology (PCR, gel electrophoresis); U4 AOS1: innate and adaptive immunity, immune dysfunction
  4. Exam 4 — U3 AOS1 gene expression & protein secretory pathway; U3 AOS2 photosynthesis, cellular respiration & biotechnology; U4 AOS1 innate and adaptive immunity & immunotherapy
  5. Exam 5 — U3 AOS1 – nucleic acids, gene expression, protein structure; U3 AOS2 – biotechnology, CRISPR, enzyme function; U4 AOS1 – innate/adaptive immunity, immunotherapy, dysfunction
  6. Exam 6 — U3 AOS1 nucleic acids, gene expression and regulation, protein secretory pathway; U3 AOS2 photosynthesis (C3/C4), cellular respiration, enzymes, biotechnology (CRISPR); U4 AOS1 innate and adaptive immunity, cell signalling, immune dysfunction, vaccines
  7. Exam 7 — U3 AOS1: nucleic acids, protein synthesis, gene expression; U3 AOS2: photosynthesis, cellular respiration, biotechnology; U4 AOS1: innate and adaptive immunity, dysfunction, immunotherapy
  8. Exam 8 — U3 AOS1: nucleic acids, gene expression, protein secretory pathway; U3 AOS2: enzymes, cellular respiration, biotechnology (CRISPR, PCR); U4 AOS1: innate and adaptive immunity, vaccines, immunotherapy
  9. Exam 9 — U3 AOS1: nucleic acids, protein synthesis; U3 AOS2: enzyme biotechnology, photosynthesis, cellular respiration; U4 AOS1: innate and adaptive immunity, immunotherapy, immune dysfunction
  10. Exam 10 — U3 AOS1: nucleic acids, gene expression, protein secretory pathway; U3 AOS2: enzymes, photosynthesis, cellular respiration, biotechnology; U4 AOS1: innate and adaptive immunity, immune dysfunction, vaccines
  11. Exam 11 — U3 AOS1: nucleic acids, gene expression, protein structure; U3 AOS2: enzymes, cellular processes, biotechnology; U4 AOS1: innate and adaptive immunity, autoimmune disease, immunotherapy
  12. Exam 12 — U3 AOS1 nucleic acids and protein synthesis; U3 AOS2 photosynthesis, biotechnology and GMO crop transformation; U4 AOS1 innate and adaptive immunity including immunotherapy
  13. Exam 13 — U3 AOS1: gene expression and protein secretory pathway; U3 AOS2: enzyme kinetics and cellular respiration; U4 AOS1: adaptive immunity and immune dysfunction (allergy/hypersensitivity focus)
  14. Exam 14 — U3 AOS1: nucleic acids, protein synthesis, DNA replication; U3 AOS2: photosynthesis, cellular respiration, biotechnology; U4 AOS1: innate and adaptive immunity, dysfunction, immunotherapy
  15. Exam 15 — U3 AOS1: gene expression, protein secretory pathway, gene regulation; U3 AOS2: cellular respiration (anaerobic fermentation), enzymes, biotechnology (CRISPR, recombinant DNA, PCR, gel electrophoresis); U4 AOS1: innate and adaptive immunity, cell signalling, vaccines
  16. Exam 16 — U3 AOS1: nucleic acids, protein synthesis, gene expression; U3 AOS2: enzymes, cellular respiration, biotechnology (PCR/gel electrophoresis/CRISPR); U4 AOS1: innate and adaptive immunity, transplant rejection, immunosuppression, immunotherapy
  17. Exam 17 — molecular phylogenetics; nucleic acids and proteins; immunity
  18. Exam 18 — U3 AOS1: nucleic acids and protein synthesis; U3 AOS2: photosynthesis, cellular respiration, biotechnology; U4 AOS1: innate and adaptive immunity, immunotherapy
  19. Exam 19 — U4 AOS1 — immunity, vaccination, herd immunity, immunotherapy; U4 AOS2 — natural selection, genetic change, speciation, evidence for evolution; U3 AOS1 — nucleic acids, protein synthesis, gene expression
  20. Exam 20 — U3 AOS1: nucleic acids, gene expression, protein secretory pathway; U3 AOS2: enzymes, photosynthesis, cellular respiration, biotechnology; U4 AOS1: innate/adaptive immunity, immune dysfunction, immunotherapy, vaccines

All 20 revision notes

  • DNA & RNA: Structure, Replication & the Genetic Code
  • Gene Expression: Transcription, RNA Processing & Translation
  • Proteins & the Proteome
  • Regulation of Gene Expression: The trp Operon
  • The Protein Secretory Pathway
  • Biotechnology: CRISPR-Cas9, Recombinant DNA, PCR & Electrophoresis
  • C3, C4 & CAM Photosynthesis
  • Cellular Respiration: Aerobic & Anaerobic
  • Enzymes, Coenzymes & Factors Affecting Activity
  • Photosynthesis: Light-Dependent & Light-Independent Stages
  • Adaptive Immunity: Humoral & Cell-Mediated
  • Cell Signalling & Signal Transduction
  • Immune Dysfunction: Allergies, Autoimmunity & Immunodeficiency
  • Immunotherapies, Vaccines & Emerging Pathogens
  • Innate Immunity: Barriers & the Inflammatory Response
  • Evidence for Evolution: Homology, Fossils & Molecular Data
  • Genetic Variation & Mutation
  • Human Evolution & Determining Relatedness
  • Natural Selection & Changes in Allele Frequency
  • Speciation & Isolating Mechanisms