Inquiry Question 1: How does mutation introduce new alleles into a population?
Explain how a range of mutagens operate, including but not limited to: electromagnetic radiation sources, chemicals, naturally occurring mutagens; and classify different types of mutation including point, silent, frameshift and chromosomal mutations
A focused answer to the HSC Biology Module 6 dot point on classifying mutations. Covers point mutations (substitution, insertion, deletion), silent vs missense vs nonsense, frameshift effects on reading frame, and chromosomal mutations (deletion, duplication, inversion, translocation, non-disjunction).
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What this dot point is asking
NESA wants you to classify mutations into the standard categories and explain the structural difference between each type. Most exam questions ask you to compare two types, often with a named example.
The answer
A mutation is a permanent, heritable change to the DNA sequence of an organism. Mutations are classified by scale (point vs chromosomal) and by effect on the protein product (silent, missense, nonsense, frameshift).
Point mutations
A point mutation changes a single base pair in the DNA. There are three structural sub-types.
Substitution. One base is replaced by another (e.g. A to G). The reading frame is unchanged; at most one codon is altered.
Insertion. An extra base is inserted into the sequence.
Deletion. A base is removed from the sequence.
Insertions and deletions of one or two bases shift the reading frame of the ribosome, so all codons downstream are read in the wrong groups of three. This is called a frameshift mutation.
Classifying substitutions by effect
Substitutions are further classified by what they do to the protein.
| Type | Effect on codon | Effect on protein |
|---|---|---|
| Silent | New codon codes for the same amino acid | None (the genetic code is degenerate) |
| Missense | New codon codes for a different amino acid | One amino acid changed |
| Nonsense | New codon is a stop codon (UAA, UAG, UGA) | Truncated, usually non-functional |
Worked example. Sickle cell anaemia is a single substitution (A to T) in the beta-globin gene, changing codon 6 from GAG to GTG. This is a missense mutation: glutamic acid becomes valine. The altered haemoglobin polymerises under low oxygen, deforming red blood cells.
Frameshift mutations
A frameshift is caused by an insertion or deletion of a number of bases not divisible by three. Every codon downstream of the mutation is shifted, so the amino acid sequence past that point is essentially random and a premature stop codon usually appears within a few codons. The resulting protein is truncated and non-functional.
Worked example. Many cystic fibrosis alleles involve deletions in the CFTR gene. The most common, ΔF508, deletes three bases (one codon) and is technically an in-frame deletion, but other CF alleles are true frameshifts that abolish CFTR function entirely.
Chromosomal mutations
A chromosomal mutation changes the structure or number of whole chromosomes. These affect many genes at once.
Structural chromosomal mutations
- Deletion. A segment of the chromosome is lost (e.g. cri-du-chat syndrome, partial deletion of chromosome 5).
- Duplication. A segment is copied so that two copies are present on the same chromosome.
- Inversion. A segment breaks off, flips and rejoins in reverse orientation.
- Translocation. A segment moves from one chromosome to a non-homologous chromosome (e.g. the Philadelphia chromosome in chronic myeloid leukaemia, a translocation between chromosomes 9 and 22).
Numerical chromosomal mutations (aneuploidy)
These arise from non-disjunction during meiosis, where homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate.
- Trisomy 21 (Down syndrome). Three copies of chromosome 21.
- Monosomy X (Turner syndrome). A single X chromosome (XO).
- Trisomy XXY (Klinefelter syndrome). Two X and one Y chromosome.
Germline vs somatic mutations
A mutation in a gamete (egg or sperm) is a germline mutation and is passed to offspring. A mutation in a somatic (body) cell is not inherited but can still cause local effects such as cancer.
Worked example
Compare these two short coding sequences. Original: ATG-CAT-GGA-TAA (Met-His-Gly-Stop).
Sequence A: ATG-CAT-GGC-TAA. The third codon changed from GGA to GGC. Both code for glycine. This is a silent substitution; protein unchanged.
Sequence B: ATG-CAT-TAA (a single deletion of the G at position 7). The codons are now ATG-CAT-TAA (Met-His-Stop). This is a frameshift that introduces a premature stop, producing a truncated dipeptide.
Common traps
Calling every substitution missense. Substitutions can be silent, missense or nonsense. The category depends on the codon's effect.
Forgetting the "not divisible by 3" detail for frameshifts. An insertion or deletion of three bases preserves the reading frame and only adds or removes one amino acid (in-frame indel), not a frameshift.
Mixing up translocation and inversion. Translocation moves a segment to a different chromosome; inversion flips a segment within the same chromosome.
Ignoring whether the mutation is germline or somatic. Only germline mutations contribute to allele frequencies in a population.
In one sentence
Mutations are classified as point mutations (substitution, insertion, deletion, with substitutions sub-classified as silent, missense or nonsense), frameshift mutations (indels that shift the reading frame), and chromosomal mutations (deletion, duplication, inversion, translocation and non-disjunction), with each category producing predictable effects on the protein and phenotype.
Past exam questions, worked
Real questions from past NESA papers on this dot point, with our answer explainer.
2022 HSC5 marksCompare point mutations and chromosomal mutations, and explain how each can affect the phenotype of an organism.Show worked answer →
A 5-mark answer needs definitions, a worked example of each and a clear phenotype link.
Point mutation. A change to a single base pair in DNA. Three sub-types:
- Substitution. One base is swapped for another. May be silent (no amino acid change), missense (new amino acid) or nonsense (premature stop codon).
- Insertion. An extra base is added.
- Deletion. A base is removed.
Insertions and deletions of one or two bases cause a frameshift, where every codon downstream is read incorrectly, usually destroying protein function. Worked example: sickle cell anaemia is a single missense substitution (GAG to GTG) at codon 6 of the beta-globin gene, changing glutamic acid to valine and producing the sickled haemoglobin phenotype.
Chromosomal mutation. A change to the structure or number of whole chromosomes. Examples: deletion (a chromosome segment is lost), duplication, inversion (segment reversed), translocation (segment moves to a different chromosome), and non-disjunction (whole chromosome fails to separate at meiosis, producing aneuploidy). Worked example: trisomy 21 (Down syndrome) results from non-disjunction of chromosome 21, giving three copies and a distinctive phenotype.
Phenotype link. Point mutations alter a single protein product; chromosomal mutations alter dosage of dozens to thousands of genes at once, so they typically have larger, more systemic effects on phenotype.
Markers reward (1) a clear definition of each, (2) worked named examples (sickle cell, Down syndrome), and (3) explicit phenotype consequence.
2019 HSC3 marksExplain why a frameshift mutation often has a more severe effect on protein function than a substitution mutation.Show worked answer →
A substitution swaps one base for another and changes at most one codon, so the protein is altered by zero amino acids (silent), one amino acid (missense) or truncated at one point (nonsense).
A frameshift (insertion or deletion of one or two bases) shifts the entire downstream reading frame. Every codon after the mutation is now read in the wrong groups of three, so most or all of the amino acids downstream are wrong. A premature stop codon usually appears soon after the shift, producing a truncated, non-functional protein.
Markers reward (1) the reading-frame argument, (2) explicit comparison of how many codons are affected, and (3) the link to loss of protein function.
Related dot points
- Explain how a range of mutagens operate, including but not limited to: electromagnetic radiation sources, chemicals, naturally occurring mutagens
A focused answer to the HSC Biology Module 6 dot point on mutagens. Physical mutagens (UV, X-rays, gamma rays), chemical mutagens (base analogues, alkylating agents, intercalators) and biological mutagens (viruses, transposons), with named examples and the molecular mechanism by which each damages DNA.
- Assess the significance of 'coding' and 'non-coding' DNA segments in the process of mutation and investigate the effects of different mutations on a protein's amino acid sequence
A focused answer to the HSC Biology Module 6 dot point on how mutations alter protein products. Coding versus non-coding regions, silent missense and nonsense substitutions, frameshift consequences, splice-site mutations, and a worked sickle cell example.
- Evaluate the effects of biotechnology on the genetic diversity of agricultural and natural populations, and the impact on biodiversity
A focused answer to the HSC Biology Module 6 dot point on biotechnology and biodiversity. The narrowing effect of monocultures and cloning, gene flow to wild relatives, herbicide and insecticide resistance, conservation applications (gene banks, de-extinction), and an evaluative judgement on net impact.