BMS5021 · Introduction to Bioinformatics
Mutation, Evolution & DNA Sequencing
Week 3 of Monash University BMS5021 covers how sequences change and how we read them. It classifies mutations (silent, missense, nonsense and frameshift), frames mutation as the raw material of evolution, and compares the DNA-sequencing technologies — Sanger, Illumina short-read and Oxford Nanopore long-read — that produce the data every later chapter analyses. This is core Topic 1 quiz material: expect to classify a mutation from a codon change and to compare sequencing platforms on read length, accuracy, throughput and error rate.
What this chapter covers
- 01Mutation as the raw material of evolution: can be beneficial, neutral or disease-causing (e.g. lactase persistence)
- 02Mutagens: UV radiation causes thymine-thymine dimers; chemical carcinogens (smoking ~70 carcinogens) raise mutation rate
- 03Four mutation classes: silent (no protein change), missense (one amino-acid swap), nonsense (premature stop), insertion/deletion (frameshift)
- 04Frameshift generally causes the biggest protein change because all downstream codons shift
- 05Sanger sequencing: chain-termination with fluorescent ddNTPs; long reads, very accurate (~99.4%, ~Q20), not scalable
- 06Illumina (short-read) sequencing-by-synthesis: fragment -> adapters -> bridge amplification -> cyclic labelled dNTPs; ~35-350 bp reads
- 07Oxford Nanopore (long-read): DNA through a protein pore reads current changes; real-time, very long reads, high error ~10-15%
- 08Sequencing depth/coverage guidance: WGS ~30-50x, WES >50-100x, targeted >500x; depth vs breadth of coverage
Classify a mutation from a codon change (Topic 1 quiz-style item)
- +1Identify the change. A single-base substitution that replaces an amino-acid codon with a stop codon is, by definition, a NONSENSE mutation (it is a point substitution, but one that creates a premature stop rather than swapping one amino acid for another).
- +1State the protein consequence. Translation terminates early at the new stop codon, so the protein is TRUNCATED — here it ends after Pro (Ser-Val-Pro) and loses the C-terminal tyrosine and anything beyond, which usually disrupts or abolishes function.
- +1Contrast with a deletion. Deleting a single base near the start shifts the reading frame (a FRAMESHIFT / indel mutation), so every downstream codon is re-read; essentially the whole downstream amino-acid sequence changes. Frameshifts generally cause the biggest change of all four classes, whereas a silent substitution would change no amino acid and a missense substitution would swap just one.
Key terms
- Silent mutation
- A single-base substitution that does not change the encoded amino acid (a synonymous codon), so the protein sequence is unchanged.
- Missense mutation
- A single-base substitution that changes one amino acid to another (e.g. Pro -> Thr), altering the protein at one position.
- Nonsense mutation
- A single-base substitution that replaces an amino-acid codon with a stop codon, prematurely terminating translation and truncating the protein.
- Frameshift (indel) mutation
- Insertion or deletion of base(s) that shifts the reading frame, so all downstream codons are re-read and the whole downstream amino-acid sequence changes. Generally the most disruptive class.
- Sanger sequencing
- Chain-termination sequencing using fluorescent dideoxynucleotides and capillary electrophoresis: long, very accurate reads (~99.4%, ~Q20) but not scalable.
- Sequencing depth (coverage)
- How many times, on average, each base is read. Guidance: whole-genome ~30-50x, whole-exome >50-100x, targeted >500x. Depth (reads per position) is distinct from breadth (fraction of the genome covered).
Mutation, Evolution & DNA Sequencing FAQ
How is this chapter assessed in BMS5021?
Week 3 is Topic 1 material, tested by the in-workshop MCQ quizzes (30% across Weeks 1-4). Typical items ask you to classify a mutation from a codon change or to compare sequencing platforms (Sanger vs Illumina vs Nanopore) on read length, accuracy, throughput and error rate. There is no exam — confirm the quiz timing on Moodle.
Which sequencing platform should I choose for a task?
It depends on the trade-off. Sanger gives long, very accurate reads but is not scalable, so it suits validating a single region. Illumina short-read is the workhorse for high-throughput accurate sequencing (RNA-seq, exomes). Oxford Nanopore gives very long, real-time reads at low infrastructure cost but a high ~10-15% error rate, mitigated by sequencing to greater depth. Match read length, accuracy and cost to the biological question.
Why is a frameshift usually worse than a missense mutation?
A missense substitution changes a single amino acid, so the rest of the protein is intact. A frameshift shifts the reading frame from the point of the insertion or deletion onward, so every downstream codon is re-read and the entire downstream sequence — often including a new premature stop — is altered. That is why insertions and deletions generally cause the largest change to the protein.
How much sequencing depth do I need?
The unit gives rough targets: about 30-50x for whole-genome sequencing, more than 50-100x for whole-exome, and more than 500x for targeted sequencing (the most cost-effective for a small region needing high confidence). Higher depth improves the confidence of each base call and variant, which matters more for detecting rare variants. Confirm any specific requirement for your assessment on Moodle.
Exam move
For the Topic 1 quizzes, drill two things until they are automatic: classifying mutations and comparing sequencing platforms. Make a four-row table for silent / missense / nonsense / frameshift with the effect on the reading frame and the protein, then practise classifying fresh codon-change scenarios. Make a second table for Sanger / Illumina / Nanopore with read length, accuracy, throughput and error rate (note Nanopore's ~10-15% error), plus the depth targets (WGS 30-50x, WES >50-100x, targeted >500x). Do the pre-workshop mini-lectures first so the workshop reinforces rather than introduces. Ask Sia to generate practice classification items and platform-comparison questions and to check your answers. Confirm quiz dates on Moodle.
Working through Mutation, Evolution & DNA Sequencing in BMS5021? Sia is AskSia’s AI Biology tutor — ask any BMS5021 Mutation, Evolution & DNA Sequencing question and get a clear, step-by-step explanation grounded in how BMS5021 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
