SACE Stage 2 Biology deep dive: all four topics for the 2026 exam

How to use this deep dive

The SACE Stage 2 Biology exam can draw on all four topics, and the subject outline notes that questions may combine them. This guide takes one pass through each topic, links every dot point on the site and adds worked examples. For the exam format, criteria and inquiry-skills practice, see the exam strategy guide.

Topic 1: DNA and proteins

Dot points: DNA structure and replication, gene expression: transcription and translation, the genetic code, protein structure and function, enzymes and factors affecting activity, gene regulation, mutations and mutagens, DNA technologies: PCR, electrophoresis and GMOs.

• DNA: a double helix of nucleotides, with complementary base pairing (A with T, C with G) held by weak hydrogen bonds, which lets the strands separate for semi-conservative replication. Eukaryotic DNA is linear and bound to histones; prokaryotic, mitochondrial and chloroplast DNA is circular and unbound.
• Genes: in eukaryotes, both exons and introns are transcribed, but introns are removed and only exon information is translated.
• Proteins: the amino acid sequence (primary structure) determines folding into secondary, tertiary and quaternary structure, and the 3D shape determines function. Enzymes lower activation energy; the induced-fit model describes the active site moulding around the substrate. Temperature, pH and inhibitors change activity.
• Gene expression is controlled by transcription factors and by epigenetic changes such as DNA methylation, which can explain differences between identical twins and can contribute to cancer.
• Mutations (point, frameshift, chromosomal) come from replication errors or mutagens (ionising radiation, chemicals, viruses). Frameshifts usually change every codon after the mutation.

Biotechnology. PCR amplifies DNA through cycles of heating (strand separation), cooling (primer binding) and extension by a heat-resistant polymerase using free nucleotides; each cycle doubles the target, so 30 cycles give about $2^{30} \approx 10^9$ copies. Electrophoresis separates fragments by size (smaller fragments move further towards the positive electrode), and the results can be read as an electropherogram for DNA sequencing and profiling. Genes can be cut with restriction enzymes, found with probes, transferred with plasmid or viral vectors, and edited with CRISPR-Cas9.

Topic 2: Cells as the basis of life

Dot points: the cell membrane and transport, surface area to volume ratio, prokaryotic vs eukaryotic cells, cell organelles and their functions, cellular respiration, photosynthesis, the cell cycle and mitosis.

• Fluid mosaic model: a phospholipid bilayer with embedded proteins, cholesterol and glycoproteins. Small non-polar molecules diffuse through the bilayer; ions and polar molecules need channel (such as aquaporins) or carrier proteins.
• Transport: diffusion, facilitated diffusion and osmosis are passive; active transport uses ATP to move substances against a gradient; endocytosis and exocytosis move large particles in vesicles.
• Energy: aerobic respiration releases energy from glucose to make ATP from ADP and phosphate; fermentation is the anaerobic alternative (ethanol and carbon dioxide in yeast and plants, lactic acid in animals) and yields far less ATP.
• Division: prokaryotes divide by binary fission; eukaryotic cells by mitosis, producing genetically identical daughter cells.

Topic 3: Homeostasis

Dot points: principles of homeostasis and negative feedback, the nervous system and nerve impulses, the endocrine system and hormones, blood glucose regulation, osmoregulation and the kidney, thermoregulation, homeostasis in plants and transpiration.

• Organisms survive best within tolerance limits for temperature, water, blood glucose and carbon dioxide.
• Stimulus-response model: receptor, control centre, effector, response; negative feedback reverses the change.
• Nervous pathway: receptor, sensory neuron, interneuron, motor neuron, effector; synapses use neurotransmitters; reflexes bypass conscious control.
• Hormones (amino acid derivatives, peptides, proteins or steroids) travel in the blood to target cells.
• Named examples to know: insulin and glucagon (blood glucose; diabetes mellitus), ADH (water reabsorption in the nephron), TSH and thyroxine (metabolic rate), adrenaline (fight or flight), and chemoreceptors detecting pH changes from carbon dioxide to adjust breathing.

Topic 4: Evolution

Dot points: evidence for evolution, natural selection and adaptation, population genetics and allele frequencies, genetic drift and gene flow, speciation and reproductive isolation, antibiotic resistance as evolution.

• Early life: life has existed for about 3.5 billion years; the first cells may have used RNA, with ribozymes as catalysts; prokaryotes appear before eukaryotes, and mitochondria and chloroplasts probably arose by endosymbiosis.
• Comparative genomics: protein sequencing (such as cytochromes), DNA-DNA hybridisation and DNA sequencing show relatedness; fewer differences mean a more recent common ancestor, and a known mutation rate works as a molecular clock.
• Species: for sexual organisms, members that can interbreed to produce fertile offspring. Pre-zygotic isolating mechanisms (temporal, behavioural, mechanical, gamete) and post-zygotic ones (hybrid inviability, hybrid sterility) keep species distinct.
• Changing gene pools: natural selection, genetic drift (including founder and bottleneck effects), gene flow and mutation change allele frequencies.

Common mistakes

Check your knowledge

1. A mutation deletes one base near the start of a gene. What type of mutation is this and why is it usually serious? (Answer: a frameshift, which changes every codon after it.)
2. Which transport process needs ATP? (Answer: active transport, plus endocytosis and exocytosis.)
3. Two populations of frogs call at different pitches and females respond only to their own. Which isolating mechanism is this? (Answer: behavioural, a pre-zygotic mechanism.)

Then try the topics quiz.