Biology • Year 12 • Module 8 • Lesson 8
Environmental Diseases — Smoking, UV Exposure, Asbestos and Lifestyle Factors
Build HSC Band 5–6 extended-response technique: multi-mechanism evaluation, epigenetics synthesis, and exposure-to-disease causal chains under exam-adjacent conditions.
1. Stimulus-based extended response — a Queensland surf lifesaver’s melanoma risk (Band 5–6)
8 marks Band 5–6
Stimulus. Janelle is a 45-year-old woman who grew up in Cairns, Queensland. She worked as a volunteer surf lifesaver from age 15 to 35, spending approximately 6 hours per day outdoors on weekends across all seasons. During that time she experienced at least 20 episodes of severe sunburn. She has a fair complexion (Fitzpatrick Type II skin — burns easily, rarely tans) and a family history: her mother was diagnosed with melanoma of the forearm at age 52. A dermatologist has now diagnosed Janelle with a 2.1 mm thick BRAF V600E-positive melanoma on her left shoulder.
The following data describe annual UV index measurements at Cairns Airport (Bureau of Meteorology, 2023):
Figure 1.1. Average monthly UV Index, Cairns Airport. Adapted from Bureau of Meteorology (2023). UV Index >10 = extreme; >6 = high.
Q1. Evaluate, using lesson content and the data above, why Janelle’s melanoma risk is elevated. In your response you must:
- Explain the molecular mechanism by which UV-B radiation causes mutation in melanocytes, naming the specific DNA lesion and the two genes most commonly mutated in melanoma.
- Use the dose-response relationship and data to quantify why Janelle’s cumulative UV exposure is particularly high.
- Explain how Janelle’s fair skin (Fitzpatrick Type II) modifies her individual risk compared to someone with darker skin.
- Explain the role of her family history and the CDKN2A gene in modifying her risk.
- Reach a justified, integrated evaluation of why this case illustrates the multifactorial and dose-response nature of environmental disease.
2. Source critique — a media claim about asbestos and epigenetics (Band 5–6)
7 marks Band 5–6
“Asbestos is dangerous because it is a powerful chemical carcinogen that directly mutates the DNA of cells it contacts, just like tobacco smoke. Scientists now understand that both asbestos and tobacco cause cancer through the same DNA adduct mechanism, and that epigenetic changes — which permanently alter the DNA code — are the same thing as genetic mutations. Since epigenetic changes cannot be reversed, once a person is exposed to these carcinogens, disease is inevitable.”
Hypothetical media article excerpt, illustrating common public misunderstandings.
Q2. This passage contains four biological errors. For each error: (a) identify what is scientifically incorrect; (b) explain the correct biology using lesson content; (c) explain why the distinction matters for understanding environmental disease risk. Conclude with a brief, scientifically accurate reformulation of the passage’s central claim.
Q1 — Sample Band 6 response (8 marks), annotated
UV-B radiation from sunlight is absorbed by adjacent thymine bases in melanocyte DNA. The absorbed photon energy drives a photochemical reaction forming a covalent cyclobutane ring between the two thymine bases — a thymine dimer (cyclobutane pyrimidine dimer, CPD). This distorts the DNA helix and blocks DNA polymerase during replication. If nucleotide excision repair (NER) fails to excise the dimer before replication (likely when dimer formation rate exceeds repair capacity), a CC→TT or C→T signature mutation occurs. In melanocytes, critical targets include the BRAF oncogene (particularly V600E mutation, present in Janelle’s tumour — this constitutively activates the MAPK/ERK pathway, driving uncontrolled melanocyte proliferation) and the CDKN2A tumour suppressor gene (encoding p16, which normally restrains CDK4/6 and imposes a G1 cell cycle checkpoint). [2 marks — thymine dimer mechanism named + BRAF V600E and CDKN2A both named with their roles]
The dose-response relationship links cumulative UV exposure to melanoma risk. Janelle’s exposure is exceptionally high: she worked outdoors 6 hours per day, every weekend, for 20 years (ages 15–35), in Cairns — where the UV Index exceeds 10 (extreme) from October to March (7 months of the year) and peaks at UV Index 14–15 in December. Even winter months in Cairns maintain a UV Index of 7 (high), meaning no season provided recovery from UV mutagenic load. Her 20 sunburn episodes represent acute bursts of intense UV at a rate that overwhelmed NER capacity, producing particularly high thymine dimer formation. The total cumulative thymine dimer burden across 20 years of extreme UV exposure corresponds to a very high probability of BRAF and CDKN2A mutations accumulating in the same melanocyte cell line. [2 marks — dose-response applied specifically, graph data used (UV index values cited), cumulative exposure calculated qualitatively]
Janelle’s Fitzpatrick Type II (fair) skin contains less eumelanin than darker skin types. Eumelanin acts as a UV-absorbing filter in the epidermis, reducing the proportion of UV-B photons that reach melanocyte nuclei. With less melanin, more UV-B photons reach melanocyte DNA per unit time outdoors, so the same UV Index produces more thymine dimers per hour in Janelle than in someone with darker skin (Fitzpatrick Type V or VI). This explains why fair-skinned populations in high-UV environments like Queensland carry a disproportionately elevated melanoma risk. [1 mark — mechanism of skin phototype as UV filter modifier explained]
Janelle’s mother had melanoma — this raises the probability that Janelle carries a germline variant in CDKN2A (which is the most commonly inherited melanoma susceptibility gene) or another melanoma predisposition gene. A germline CDKN2A variant means that one allele of p16 may already be partially non-functional at birth — so fewer UV-induced somatic mutations are required in the remaining allele to eliminate p16 function entirely (loss of heterozygosity). This substantially lowers the mutational threshold needed to initiate melanoma, reducing effective latency and explaining why family history is a recognised independent melanoma risk factor. [1 mark — family history / CDKN2A genetic predisposition correctly mechanised]
Janelle’s case illustrates that melanoma is a multifactorial environmental disease: no single factor alone determines outcome. The environmental exposure (extreme cumulative UV at high Cairns UV Index) is necessary but not sufficient alone — genetic predisposition (fair skin, family history, likely CDKN2A variant) and the stochastic element of which mutations accumulate in which cells all interact. The dose-response relationship is visible across all these factors: more UV (more hours, more months, higher UV Index, lower melanin protection) increases the probability of a sufficient mutation burden in a melanocyte. That this produces a BRAF V600E-positive melanoma 30 years after her peak exposure period is consistent with the latency inherent in the multihit model of cancer. [2 marks — integrated multifactorial evaluation that explicitly links dose-response, latency, genetic predisposition, and environmental exposure into a coherent synthesised judgement]
Marking criteria.
- 1 mark — Names and correctly describes the thymine dimer (CPD) mechanism: UV-B → covalent bond between adjacent thymines → distorts helix → blocks replication → CC→TT mutations if unrepaired.
- 1 mark — Names both BRAF V600E (oncogene activation; MAPK/ERK pathway) and CDKN2A/p16 (tumour suppressor loss; cell cycle checkpoint removal) and explains their roles in melanoma.
- 1 mark — Applies dose-response relationship specifically to Janelle’s exposure (uses UV Index data from the graph, 20 years, 6 hours/day, sunburn episodes, Cairns location).
- 1 mark — Correctly explains how reduced eumelanin in fair skin increases the proportion of UV-B reaching melanocyte nuclei, increasing thymine dimer burden per unit time outdoors.
- 1 mark — Correctly explains how family history suggests a CDKN2A germline predisposition that lowers the somatic mutation threshold for melanoma initiation.
- 2 marks — Integrated, synthesised evaluation explicitly linking environmental exposure (dose), genetic predisposition (modifier), multifactorial disease model, and latency into a coherent judgement rather than a list. Must use precise lesson terminology throughout.
Q2 — Sample Band 6 response (7 marks), annotated
Error 1: “Asbestos is a powerful chemical carcinogen.” This is wrong. Asbestos is chemically inert — it does not react chemically with DNA or other biomolecules. Asbestos is a physical carcinogen. The mechanism of carcinogenesis is the physical properties of the fibre: its length and durability prevent macrophage degradation (frustrated phagocytosis), which causes chronic ROS release and indirect DNA damage in mesothelial cells. The distinction matters because it shows that carcinogenesis does not require a chemical toxin — physical persistence in tissue is itself sufficient to cause chronic mutagenic damage. [2 marks — error identified; correct biology (physical not chemical, frustrated phagocytosis, ROS); distinction explained]
Error 2: “Both asbestos and tobacco cause cancer through the same DNA adduct mechanism.” This is wrong. Tobacco carcinogens (e.g. benzopyrene) cause cancer by being metabolically activated to electrophilic forms that covalently bond to DNA bases — forming DNA adducts — which cause errors during replication. Asbestos does NOT form DNA adducts; it causes cancer indirectly through frustrated phagocytosis → ROS → oxidative DNA damage in mesothelial cells. These are mechanistically distinct pathways that also target different cell types (bronchial epithelium vs mesothelium) and produce different cancers (lung cancer vs mesothelioma). The distinction matters for risk assessment and policy: reducing chemical carcinogen exposure (tobacco bans) does not reduce asbestos-related mesothelioma risk; prevention requires separate strategies (fibre removal, respiratory protection). [2 marks — error identified; both mechanisms correctly described and contrasted; consequence for disease understanding explained]
Error 3: “Epigenetic changes permanently alter the DNA code.” This is wrong. Epigenetic changes (DNA methylation, histone modification) alter gene expression without changing the DNA nucleotide sequence. A methylated cytosine is still a cytosine; the sequence is intact but the gene is silenced because transcription factors cannot bind the methylated promoter. Crucially, some epigenetic changes can be reversed — for example, demethylating agents (e.g. 5-azacytidine, used clinically) can reactivate epigenetically silenced tumour suppressors. This distinction matters because it opens the possibility of epigenetic therapies that restore tumour suppressor expression without altering the underlying DNA sequence, which is not possible for true genetic mutations. [2 marks — error identified; correct epigenetic mechanism (expression, not sequence); reversibility and clinical relevance]
Error 4: “Disease is inevitable once exposed.” This is wrong. Environmental disease risk is probabilistic, not deterministic. The dose-response relationship shows that higher cumulative exposure increases the probability of disease, not its certainty. Not all heavy smokers develop lung cancer; not all asbestos workers develop mesothelioma. Genetic factors (DNA repair enzyme efficiency, germline tumour suppressor variants), immune surveillance, and stochastic variation in which cells accumulate sufficient mutations all modify individual risk. This distinction matters for public communication: fatalistic messaging (“disease is inevitable”) discourages cessation and screening uptake, whereas probabilistic framing (“reducing exposure reduces your risk”) supports prevention behaviour. [1 mark — error identified + correct probabilistic framework applied]
Accurate reformulation: “Asbestos and tobacco smoke are both classified as Group 1 carcinogens but cause disease through mechanistically distinct pathways: tobacco smoke carcinogens (e.g. benzopyrene) form DNA adducts in bronchial epithelial cells, driving lung cancer through accumulated mutations in TP53 and KRAS; asbestos fibres cause mesothelioma indirectly through the physical process of frustrated phagocytosis and ROS-mediated DNA damage in mesothelial cells. Environmental exposures can also produce epigenetic changes — alterations in gene expression that do not change DNA sequence and may be partially reversible. In all cases, disease risk is probabilistic and dose-dependent, not inevitable, and is modified by genetic predisposition and other factors.”
Marking criteria.
- 2 marks — Error 1 (asbestos physical not chemical): error identified [1]; correct mechanism given (frustrated phagocytosis + ROS, not chemical adducts) with explanation of why distinction matters [1].
- 2 marks — Error 2 (different not same mechanism): error identified [1]; both mechanisms correctly described and contrasted, consequence for disease understanding [1].
- 2 marks — Error 3 (epigenetics does not change sequence): error identified [1]; correct biology (expression not sequence, reversibility) with clinical relevance [1].
- 1 mark — Error 4 (probabilistic not inevitable): error identified and corrected using dose-response and multifactorial reasoning.