Biology
Heredity, genetic change and infectious disease — 20 full HSC papers with band-6 model answers and marking criteria.
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Causes and Consequences of Mutations
What Is a Mutation?
A mutation is any heritable change in the nucleotide sequence of DNA or in the structure/number of chromosomes. Mutations occur at two scales:
- Gene (point) mutations — changes to one or a few nucleotide bases within a single gene. Subtypes include substitution (one base swapped for another), insertion (extra base/s added) and deletion (base/s removed).
- Chromosomal mutations — large-scale alterations: deletion, duplication, inversion or translocation of chromosome segments, or changes in whole-chromosome number (aneuploidy, e.g. trisomy) or whole genome number (polyploidy).
It is essential to distinguish a somatic mutation (occurring in body cells — not passed to offspring, but can lead to cancer) from a germline mutation (occurring in gametes or cells that will form gametes — heritable and the basis of inherited disease and evolution).
Mutations arise either spontaneously (errors during DNA replication, roughly 1 in 109 base pairs per replication cycle before repair) or are induced by external agents called mutagens.
Mutagens: Physical, Chemical and Biological
A mutagen is any agent that increases the rate of mutation above the spontaneous baseline. The NSW HSC syllabus requires you to classify mutagens into three categories.
| Category | Examples | Mechanism of DNA Damage |
|---|---|---|
| Physical | Ultraviolet (UV) radiation, X-rays and gamma rays (ionising radiation), alpha and beta particles | UV causes adjacent thymine bases to bond together, forming thymine dimers that block normal replication. Ionising radiation breaks phosphodiester bonds, causing single- or double-strand breaks and generating reactive free radicals. |
| Chemical | Benzene (cigarette smoke), formaldehyde, aflatoxin B1 (mould on peanuts), nitrosamines, alkylating agents (e.g. ethyl methanesulfonate), base analogues (e.g. 5-bromouracil) | Chemicals interact with bases directly (alkylation alters pairing), intercalate between bases (causing insertion/deletion errors) or mimic normal bases and mispair during replication. |
| Biological | Human papillomavirus (HPV), Epstein–Barr virus (EBV), Helicobacter pylori, hepatitis B and C viruses | Viral genomes integrate into host DNA, disrupting gene sequences or inserting viral oncogenes. Some bacteria produce toxins or chronic inflammation that increases reactive oxygen species, damaging DNA. |
Worked example — UV and thymine dimers: A skin cell's DNA is exposed to UV-B radiation. UV energy causes two adjacent thymine (T) nucleotides on the same strand to form a covalent bond, creating a T–T dimer. During the next round of replication the polymerase cannot read the dimer accurately and may insert incorrect bases opposite it. If the cell's nucleotide excision repair (NER) system fails to remove the dimer first, a point mutation is incorporated into one daughter cell. This is the molecular explanation for why chronic UV exposure causes most non-melanoma skin cancers (squamous and basal cell carcinomas).
In a pea plant, a true-breeding tall plant (genotype TT) is crossed with a true-breeding short plant (genotype tt), where tall is completely dominant. The F1 plants are then self-pollinated to produce 600 F2 plants. Approximately how many of the F2 plants are expected to be short?
- A. 75
- B. 150
- C. 300
- D. 450
Show the worked answer
Answer: B
F1 are all Tt. Selfing Tt x Tt gives a 3:1 ratio, so short (tt) = 1/4. 1/4 x 600 = 150 short plants.
All 20 practice exams
- Exam 1 — Heredity (dominant weighting); Genetic Change; Infectious Disease
- Exam 2 — Genetic Change; Heredity; Infectious Disease
- Exam 3 — Heredity; Genetic Change; Infectious Disease
- Exam 4 — Heredity; Genetic Change; Infectious Disease
- Exam 5 — Heredity; Genetic Change; Infectious Disease
- Exam 6 — Genetic Change; Heredity; Infectious Disease
- Exam 7 — Heredity; Genetic Change; Infectious Disease
- Exam 8 — Non-infectious Disease and Disorders; Heredity; Genetic Change
- Exam 9 — Heredity — dihybrid crosses, meiosis, Hardy-Weinberg, sex linkage, co-dominance; Genetic Change — mutation types, CRISPR, gel electrophoresis, transgenic organisms; Infectious Disease — immune response, vaccination, epidemiology
- Exam 10 — Genetic Change; Heredity; Infectious Disease
- Exam 11 — Heredity; Genetic Change; Infectious Disease
- Exam 12 — Non-infectious Disease and Disorders; Heredity; Genetic Change
- Exam 13 — Heredity; Genetic Change; Infectious Disease
- Exam 14 — Genetic Change (emphasis); Heredity; Infectious Disease
- Exam 15 — Infectious Disease (emphasis); Heredity; Genetic Change
- Exam 16 — Non-infectious Disease and Disorders; Heredity; Genetic Change
- Exam 17 — Heredity; Genetic Change; Infectious Disease
- Exam 18 — Heredity; Genetic Change; Infectious Disease
- Exam 19 — Heredity; Genetic Change; Infectious Disease (emphasis)
- Exam 20 — Non-infectious Disease and Disorders (emphasis); Heredity; Genetic Change
All 20 revision notes
- Causes and Consequences of Mutations
- Gel Electrophoresis and DNA Profiling
- Genetic Screening, Transgenesis and CRISPR
- Recombinant DNA Technology and Gene Cloning
- Types of Gene and Chromosome Mutations
- Blood Typing and Multiple Alleles
- DNA Replication and the Central Dogma
- Meiosis and Genetic Variation
- Monohybrid and Dihybrid Cross Calculations
- Pedigree Analysis and Inheritance Mode Determination
- Sex Linkage and Codominance
- Transcription, Translation and the Genetic Code
- Antibiotics, Antivirals and Drug Resistance
- Antibody Structure, Function and Clonal Selection
- Innate and Adaptive Immune Response
- Pathogens and the Koch–Henle Postulates
- Vaccination and Herd Immunity
- Categories of Non-infectious Disease
- Epidemiological Measures: Incidence, Prevalence and Risk
- Lifestyle Disease, Cancer Biology and Prevention Strategies