📚 Lecture 4 · Genomics & Bioinformatics Prof. Quanhua Mu · 23 Sep 2026
Genomics asks three questions — read, understand, edit. Every platform, file format and tool belongs to exactly one of them.
101 · L4 Genomics & Bioinformatics — From Reads to Gene Therapy
Master Logic · 主線邏輯
Genomics asks three questions — read, understand, edit. Every platform, file format and tool in this lecture belongs to exactly one of them; keep the three questions in mind and the tool zoo becomes a map.
Lecture 4 · Genomics & Bioinformatics (Prof. Quanhua Mu)
The fourth lecture of ABCT5101, delivered by Prof. Quanhua Mu (ABCT, quanhua.mu@polyu.edu.hk). This is the lecture that maps most directly onto sequencing work as it is actually done — which platforms, which file formats, which software, and what the data can and cannot tell you.
The lecture opens by asking what bioinformatics can answer. The five questions on the slide are worth remembering because each one maps to a technique introduced later in the deck:
| Question asked | Technique it maps to |
|---|---|
| Where did human beings originate? | Phylogenetics, comparative genomics |
| Why do monkeys have long tails while humans do not? | Comparative genomics, gene annotation |
| Why do some people get sickle cell anemia? | Germline variants, single-gene inheritance |
| How can we diagnose Down syndrome? | Copy number variation, non-invasive screening |
| What are the drivers of chronic myeloid leukemia, and how can we treat it? | Gene fusions, targeted therapy |
Genomics = three questions: read, understand, edit
Every genomics tool answers one of three questions: read / understand / edit.
The lecture defines genomics as the study of genomes, organised around exactly three questions. Everything in the rest of the session sits under one of these three:
- What is the sequence of the genome? — to read
- What does the genome sequence mean? — to understand
- How do we modify the genome sequence? — to edit
The Human Genome Project and the cost collapse
HGP: 3 Gb, $3B, 13 years; now a genome costs a few hundred dollars — the cost collapsed faster than Moore′s law.
| Fact | Figure |
|---|---|
| Scale of the genome | 3 billion base pairs (3 Gb) |
| Cost of the Human Genome Project | 3 billion US dollars |
| Where the result was published | Nature, 2001 |
The lecture then puts up a single slide titled "Sharp decrease of sequencing cost" — and that slide is the whole economic story of this field. The $3 billion first genome is the reference point; everything since is a fall from it.
Sequencing technologies — and how to choose
Choose by read length vs accuracy: Illumina short/accurate; PacBio/Nanopore long/noisier.
| Technology | Position in the lecture |
|---|---|
| Sanger sequencing | The original method; still the accuracy reference |
| Next generation sequencing (NGS) | Short-read, massively parallel — the workhorse |
| PacBio long-read sequencing | Long reads |
| Nanopore long-read sequencing | Long reads |
| BGI DNB-Seq | Named on the "features of different sequencing technologies" slide |
| BGI Cyclone-seq | Named on the same slide |
The lecture does not stop at listing platforms. It sets a discussion exercise: "Choose the most proper sequencing platform(s) — discuss with your neighbour classmate." And then it supplies the answer that matters in practice:
The FASTQ file and the quality check
FASTQ: 4 lines per read; line 4 must equal line 2 in length; Q30 = 99.9%.
Whatever platform produced the data, the raw output has to be read. The lecture walks through the FASTQ format field by field — four fields per record:
| Field | Content |
|---|---|
| 1 | Begins with a @ character, followed by a sequence identifier and an optional description |
| 2 | The raw sequence letters |
| 3 | Begins with a + character, optionally followed by any description |
| 4 | Encodes the quality values for the sequence in Field 2, and must contain the same number of symbols as letters in the sequence |
The tool named for the first look at this file is FastQC — described on the slide simply as "a simple and useful quality check tool".
Two workflows: de novo vs re-sequencing
De novo assembly (no reference) vs re-sequencing (map to a reference).
The lecture contrasts the two standard pipelines. They share a beginning and diverge at the middle:
| De novo genome sequencing | Re-sequencing against a reference | |
|---|---|---|
| Steps | Sequencing → reads → QC and cleaning → genome assembly → genome sequence → gene annotation → further analysis | Sequencing → reads → QC and cleaning → mapping → variant detection → gene prediction → further analysis |
| Typical use on the slide | Sequencing the genome of a new species | Sequencing the genome of a human tumour |
De novo assembly is framed by two concepts the slides name: K-mer and De Bruijn graph-based assembly. The assembler table is worth memorising because read length is the selection criterion:
| Assembler | Read type | Best suited to |
|---|---|---|
| SOAPdenovo | NGS (short reads) | Multiple species |
| SPADES | NGS (short reads) | Mostly prokaryotes |
| MEGAHIT | NGS (short reads) | Mostly metagenomes |
| Flye | TGS (long reads) | Long-read assembly |
| canu | TGS (long reads) | Long-read assembly |
Reference genomes and the file formats around them
References (hg19/hg38) and the format chain: FASTQ → SAM/BAM → VCF.
| Item | What the slide says |
|---|---|
| FASTA | Files usually end in .fasta, .fa, .fna, .faa — e.g. hg19.fa |
| Human genome assemblies | The lecture distinguishes old (and rarely used) versions from current versions |
| Where to get them | UCSC (hgdownload.soe.ucsc.edu, hg38 bigZips) and NCBI (GCF_000001405.26) |
| Other species | UCSC downloads page and NCBI datasets |
Comparing genomes: phylogenetics, SNP and GWAS
Comparing genomes: phylogenetics (relatedness), SNP (single-letter differences), GWAS (trait associations).
- Phylogenetics — comparing genomes across species. The lecture's worked example is the phylogeny of SARS-CoV-2, which is also a reminder that a phylogeny is built from sequence, not from phenotype.
- SNP and genomic diversity — single nucleotide polymorphisms as the substrate of human variation (cited to Science, 2008).
- GWAS — genome-wide association studies, comparing a case cohort against a control cohort. The slide points to the EBI GWAS catalog as the reference database.
From genome to gene: annotation
Annotation turns coordinates into genes: SnpEff, ANNOVAR.
The lecture makes a historical point that is easy to miss: annotating genes in model organisms started long before NGS existed. Researchers used genetic mapping to locate genes in organisms such as the fruit fly. Genes discovered in that era were mostly named after their function, usually by whoever found them — and some have funny names. The slide's editorial note: gene names are important, and if you discover a new gene you get to propose a name.
- Browsing genes — the lecture uses IGV (Integrative Genomics Viewer, igv.org) to view alignment and genome data, and GeneCards for gene records (the worked examples are PD-1 and PD-L1).
- Annotating a new genome — the method is similarity search: find the most similar gene sequence in an existing database. The tool is BLAST; for large-scale work you run BLAST locally, or use DIAMOND.
- Published annotations — NCBI datasets and the Ensembl FTP server (ftp.ensembl.org/pub/current_gtf/). Human annotation is distributed as GTF and GFF3 from Ensembl and GENCODE.
The lecture's light relief on this topic is a recent paper on "the lost tail of the human" (Nature, 2024) — the molecular answer to the monkey-tail question it opened with.
How much of the genome you actually sequence: WGS, WES, targeted
Breadth vs depth: WGS widest/shallowest; panel narrowest/deepest/cheapest.
| Approach | What is captured |
|---|---|
| Whole genome sequencing (WGS) | Everything — introns, exons and the space between |
| Whole exome sequencing (WES) | The exons only |
| Targeted sequencing | A defined set of regions, isolated by target enrichment before sequencing |
This is the commercial core of the deck: the choice between WGS, WES and a targeted panel is simultaneously a scientific decision, a cost decision and a product decision.
Alignment and variant calling
Align (BWA/minimap2) → call variants (GATK germline; Mutect2 somatic).
| Step | Tools / formats named |
|---|---|
| DNA alignment | BWA, BOWTIE2, MINIMAP2 |
| Viewing alignment results | File formats SAM and BAM |
Variants are then classified by origin — germline, de novo and somatic — and the lecture uses the inheritance of sickle cell disease as the worked example of a germline single-gene disorder.
Types of genomic variation named on the slide: mutations, insertions, SNVs, deletions, tandem repeats and aneuploidy.
Detection methods are split into germline and somatic, and then into four methodological families: heuristic methods, statistical methods, deep learning methods and graph models.
What variants actually do: coding vs non-coding
Coding mutations are interpretable; non-coding mostly unknown (TERT promoter the exception).
| Coding mutations | Non-coding mutations | |
|---|---|---|
| Effect | Predictable from the reading frame — missense changes, and indels which are often frameshifting | Most non-coding mutations' functions remain unknown, with very few exceptions |
| Named exception | — | TERT promoter mutation, found in many cancer types |
| Annotation tools | SnpEff, ANNOVAR | — |
Two further concepts are introduced here:
- Tumour mutation burden (TMB) — the number of mutations in a tumour, cited to Nature 2013 and NEJM 2017. This is a biomarker, and therefore a clinical decision point.
- The two canonical cancer genes — and the lecture's contrast is the cleanest way to remember loss-of-function vs gain-of-function:
Gene Class Mutation type TP53 Tumour suppressing gene Loss of function KRAS Oncogene Gain of function
Risk alleles, tandem repeats, and the EGFR T790M case
Tandem repeats: HTT <27 normal, 40+ disease; T790M is a resistance mutation.
- Risk allele frequencies and effect sizes — the lecture plots the relationship, which is the reason common variants usually have small effects and rare variants can have large ones.
- Case study: germline EGFR T790M and lung cancer risk (cited to Science, 2026) — a germline variant normally associated with resistance, appearing here as a susceptibility factor.
- Tandem repeat expansions — worked example HTT (Huntington's disease): <27 repeats = normal, 40+ repeats = disease (cited to Lancet, 2007). Note the shape of this: the same locus, the same mechanism, and the phenotype decided purely by repeat count.
Copy number and structural variation
CNV by read depth: log2 ratio 0 = same, +1 = double, −1 = half.
Copy number variation (CNV) is where the lecture connects genomics to diagnostics. The key technical definition on the slide is the measurement itself:
| Citation on the slide | Why it is there |
|---|---|
| Lo et al., Lancet 1997; BMJ 2011 | The foundational work on cell-free DNA in maternal plasma — the line of work behind non-invasive prenatal testing, which is the answer to the lecture's own opening question about diagnosing Down syndrome |
| TCGA, 2013 | Large-scale cancer copy number landscape |
| Nature Genetics, 2022 | Later CNV analysis |
Structural variations are treated as a separate class (cited to Chromosome Research, 2022).
Gene fusions — the clearest drug-targeting success story
Fusions come from translocation; BCR-ABL1 → imatinib is the cleanest drug-target story.
The lecture gives translocation / gene fusion its own multi-slide block, and the reason is commercial: this is where a genomic finding becomes a drug.
- The illustration is Imatinib (Gleevec®) — a fusion-driven cancer treated by inhibiting the fusion product.
- Fusions are classified by tissue context: haematological, soft tissue and epithelial cancers — and within epithelial cancers the slide marks the range as rare to common (cited to Genome Medicine, 2015).
- For targeting, the lecture points to OncoKB as the actionable-gene reference.
| Detection approach | Named methods |
|---|---|
| Sequencing-based | INTEGRATE, STAR-fusion, ARRIBA, CICERO — and "many others" |
| Non-sequencing | PCR-Sanger sequencing, nucleotide hybridisation |
What this lab contributed
This lab: MET exon 14 skipping (Cell 2018); MGMT fusion (Nat Commun 2020).
The lecture closes the variant section with two slides headed "Our works", citing Cell 2018 and Nature Communications 2020 — the MET exon 14 skipping finding in secondary glioblastoma and the MGMT gene fusion driving temozolomide resistance respectively. Whether or not this appears in an exam, it tells you something useful: the lecturer works on exactly the kind of problem this lecture describes.
Genome editing and gene therapy
Untargeted vs targeted editing; gene therapy $0.85M–$4.25M — one-time cures.
The final block answers the third genomics question — how do we modify the genome.
| Concept | Meaning |
|---|---|
| Untargeted mutagenesis | Random damage — no control over where |
| Targeted genome editing | Cutting or changing a chosen site |
| CRISPR-based genome editing | The mechanism behind targeted editing |
| Designing the guides | sgRNA design — the slide points to the Broad Institute's CRISPick portal |
Then the lecture lands on the commercial endpoint: approved gene therapies, with prices. This table is worth knowing in full, because it is the clearest available picture of what a one-time genomic cure costs.
| Product | Manufacturer | Price (USD) | Disease | Gene |
|---|---|---|---|---|
| ADSTILADRIN | Ferring | — | Bladder cancer | IFNα2b |
| BEQVEZ | Pfizer | $3.5 million | Haemophilia B | F9 |
| CASGEVY | Vertex | $2.2 million | Sickle cell disease | BCL11A |
| ELEVIDYS | Sarepta Therapeutics | $3.2 million | Duchenne muscular dystrophy | DMD |
| HEMGENIX | CSL Behring | $3.5 million | Haemophilia B | F9 |
| LENMELDY | Orchard Therapeutics | $4.25 million | Metachromatic leukodystrophy (MLD) | ARSA |
| LUXTURNA | Spark Therapeutics | $0.85 million | Retinal dystrophy | RPE65 |
| LYFGENIA | bluebird bio | $3.1 million | Sickle cell disease | HBB |
| ROCTAVIAN | BioMarin | $2.9 million | Haemophilia A | F8 |
| SKYSONA | bluebird bio | $3 million | Cerebral adrenoleukodystrophy (CALD) | ABCD1 |
| VYJUVEK | Krystal Biotech | — | Dystrophic epidermolysis bullosa (DEB) | COL7A1 |
| ZYNTEGLO | bluebird bio | $2.8 million | β-thalassaemia | HBB |
| ZOLGENSMA | Novartis | $2.1 million | Spinal muscular atrophy (SMA) | SMN1 |
Exam focus — the numbers and contrasts worth memorising
| Item | Value |
|---|---|
| Human genome size | ~3 Gb |
| Protein-coding genes | ~20,000 |
| Coding genome | ~50 Mb |
| Human Genome Project cost | $3 billion (published Nature 2001) |
| HTT repeats | <27 normal / 40+ disease |
| CNV statistic | Log2 ratio = normalised read depth, case over control |
| TP53 vs KRAS | Tumour suppressor, loss of function vs oncogene, gain of function |
| Germline vs somatic vs de novo | The three variant origins |
| WGS vs WES vs targeted | Everything vs exons only vs enriched regions |
| Short vs long read assemblers | SOAPdenovo / SPADES / MEGAHIT vs Flye / canu |
| Alignment tools | BWA, BOWTIE2, MINIMAP2 |
Note on completeness: this session runs to 78 content slides, and a substantial number are figure or screenshot slides with no extractable text (sequence-trace images, IGV screenshots, CNV plots, pathway diagrams, the sequencing-cost curve). Every citation, tool name and number printed on the slides is reproduced above, so the missing visuals can be traced from those references.
Detail Pass — the fuller tables and figures from the slides
The fuller tables and figures from the slides — the detail layer.
This session is unusually figure-heavy: 79 slides, and a large share of the substance sits in diagrams and tables rather than in body text. The block below recovers material that only becomes visible when the slides are viewed as pages rather than read as text — including one table that is genuinely load-bearing for anyone working in sequencing.
Sequencing platforms — the full comparison table
The full platform table: read length + error type per platform.
The lecture organises every major platform into three generations and compares them on seven axes. This is the reference table of the whole lecture.
| Technology | Avg read length | Run time | Advantages | Drawbacks | Error types | Year |
|---|---|---|---|---|---|---|
| First generation | ||||||
| Sanger | ~1000 | ~2 hours | Long individual read length | First sequencing technology, and expensive | Insertion · deletion · substitution | 1977 |
| Second generation | ||||||
| Roche 454 | 700 | 24 hours | Long read length | High error rate and expensive runs | Insertion · deletion | 2005 |
| Illumina / Solexa | 2 × (36 to 100) paired-end | 6 days (2 days in rapid mode) | Low cost and low error rate | Long run time | Substitution | 2006 |
| AB SOLiD | 85 | 8 days | Low cost per base and low error rate | Very long run time | Substitution | 2007 |
| Helicos Biosciences | 55 | Depending on read length | Low error rate | Long run time | Insertion · deletion | 2009 |
| Ion Torrent | 400 | 2 hours | Short run time and less expensive equipment | High error rate | Insertion · deletion | 2010 |
| Third generation | ||||||
| Pacific Biosciences | 3000 (up to 15000) | 20 min to 4 hours | Longest read length and fast run time | High error rate | Insertion · deletion | 2010 |
| Oxford Nanopore (MinION) | 9545 | < 6 hours | Long read; small, low-cost USB device | High error rate | Insertion · deletion · substitution | 2014 |
Read length goes 1000 (Sanger) → 55–700 (second gen) → 3000–9545 (third gen).
Run time goes 2 hours → 8 days → under 6 hours.
Error type is the field that separates the families: the Illumina/Solexa and AB SOLiD platforms make substitution errors, while every other platform listed makes insertion–deletion errors. Sanger and Oxford Nanopore make all three.
Cost is the second-generation advantage — "low cost and low error rate" is why short reads won the market.
Platform choice — the five practice scenarios
Platform choice by scenario: match the question to read length and accuracy.
The lecture then sets a worksheet: for each project below, state which platform(s) you would propose and why. This is the discussion exercise, and these are the five scenarios.
| Project | What the question is really testing |
|---|---|
| De novo sequencing and assembly of a microbial genome | Small genome, no reference → assembly is required, and long reads resolve repeats |
| Sequencing a plasmid | Very small, known backbone → the cheapest sufficient method wins |
| Re-sequencing a human genome (e.g. a cancer sample) to look for novel mutations | Large genome, reference exists, low-frequency variants matter → depth and accuracy over read length |
| Rapid diagnosis of a viral infection by sequencing in the field | Speed and portability dominate → run time and instrument size decide it |
| Targeted amplicon / exome sequencing | Defined regions → enrichment first, then high depth on a small footprint |
"In reality, one sequencing project may combine multiple sequencing techniques."
So the correct exam answer is never "platform X" on its own. It is which combination, and what each part is doing.
Variant detection methods — the tool matrix
The matrix: heuristic / statistical / deep-learning / graph × germline / somatic.
Variant calling is organised two ways at once: by the statistical approach, and by the variant origin. The lecture supplies the tool names for each cell.
| Method family | Germline mutations | Somatic mutations |
|---|---|---|
| Heuristic | — | Varscan |
| Statistical methods | GATK-HaplotypeCaller, Samtools, FreeBayes, Platypus, Octopus | Scomatic, Mutect2, Octopus, Vardict |
| Deep learning methods | DeepVariant, Strelka2 | Strelka2 |
| Graph models | Dragen, Pangenie | — |
2. The families are historical layers. Heuristic → statistical → deep learning → graph models is roughly the order the field moved. Note that DRAGEN, the graph-model entry here, is also the bioinformatics engine bundled with Illumina instruments — a reminder that "which caller" is partly decided by "which machine".
Coding mutations — the full classification
Synonymous / missense / nonsense / frameshift — know the consequence of each.
This is the slide with the worked example, and it defines four mutation outcomes from one codon. Starting from the original sequence 5'-AAG-3' / 3'-TTC-5', transcribed to mRNA 5'-AAG-3', translated to Lys:
| Mutated DNA | Mutated mRNA | Protein result | Classification |
|---|---|---|---|
| AAA | AAA | Lys (unchanged) | Synonymous / silent |
| TAG | UAG | STOP | Nonsense (a subset of non-synonymous) |
| AGG | AGG | Arg | Non-synonymous → missense |
| ACG | ACG | Thr | Non-synonymous → missense |
The slide draws the classification as a hierarchy, which is the cleanest way to remember it:
| Category | Contains |
|---|---|
| Synonymous / silent | The codon changes but the amino acid does not |
| Non-synonymous | → Nonsense (creates a premature STOP, truncating the protein) → Missense (substitutes a different amino acid) |
ATG CAT TGC), then a single T inserted — and the note reads "Reading frame has been changed."Substitutions change one codon. Indels change every codon downstream. That is why the consequence of a one-base insertion is usually worse than a one-base substitution.
One slide earlier, the lecture pairs the central dogma (DNA → transcription → RNA → translation → protein) with the full genetic code table, with start and stop codons marked — the reference you need to read the table above.
Genomic variation — the complete taxonomy
Complete taxonomy: SNV, indel, CNV, fusion + chromosomal-level events.
The lecture's variation slide separates two levels, and it is worth learning the separation because exam questions tend to test one level at a time.
| Level | Types |
|---|---|
| Sequence level (the slide labels these "mutations", circled together) | Insertion · SNP · Deletion |
| / (shown on the same sequence line) | Tandem repeats — the slide's example is a run of CAG repeats |
| Chromosome level (seven distinct types, each illustrated) | Aneuploidy · Deletion · Insertion · Duplication · Inversion · Translocation · Extrachromosomal DNA |
Gene fusions — the historical map, 1914 to 2014
1914 Boveri → 1960 Philadelphia chromosome → 2014: a century of fusions.
Rather than listing fusions, the lecture lays them on a century-long timeline and colour-codes them by tissue type — purple = haematological, green = soft tissue. The colour coding is the finding: the same mechanism recurs across unrelated tissues.
| Era | Fusions on the slide |
|---|---|
| 1914 — the concept | Boveri, mitotic aberrations |
| 1960 — the founding observation | Philadelphia chromosome (CML) |
| 1960s–70s — cytogenetics | t(9;22) CML · t(8;21) CML · t(15;17) APL · t(8;14) Burkitt's lymphoma |
| 1980s–90s — molecular cloning | t(11;22) Ewing's sarcoma · BCR-ABL1 CML · PAX3-FKHR alveolar rhabdomyosarcoma · AML1-X AML · ATF-EWS clear cell sarcoma · FUS-ERG AML · PML-RARA APL · CHOP-FUS liposarcoma |
| 2000s–2014 — systematic discovery | MLL-X MLL · FLI1-EWS Ewing's sarcoma · ETV6-NTRK3 Ph-like ALL · ERG-EWS Ewing's sarcoma · SYT-SSX synovial sarcoma · NPM-ALK anaplastic lymphoma · PDGFb-TEL CML · NAB2-STAT6 solitary fibrous tumour |
Also note the direction of travel: 1914 the mechanism was a hypothesis about mitotic errors; by 2014 fusions were being systematically discovered and matched to drugs.
CNV in the clinic — Down syndrome as the worked example
NIPT: Lo et al. found fetal cell-free DNA in maternal plasma → Down syndrome screening.
The lecture answers its own opening question ("How can we diagnose Down syndrome?") in the CNV section, and the answer is simply a copy-number change:
- Trisomy 21 — the karyotype shows three copies of chromosome 21 instead of two. That is the whole molecular definition.
- The slide pairs the karyotype with a clinical-features panel: small nose, mouth and ears; almond-shaped eyes with a skin fold covering the inner corner; white spots on the coloured part of the eyes; flat, round face; slightly bent fifth finger; short, stocky build.
- It also carries a caution worth keeping: intellectual disability and developmental delay vary by individual.
Method note: the tables above were recovered by rendering the slide pages as images and reading them, because a large share of this deck's content exists only as figures. Where a cell was legible on the slide it is reproduced as written; obvious typographical errors in the original (e.g. a manufacturer name misspelt, "expansive" for "expensive") have been corrected silently so the note is safe to revise from.
T2T-CHM13 — why the human reference genome was incomplete until 2022
The Human Genome Project draft (2001) was never actually finished: roughly 8% of the genome — centromeres, telomeres and other long repeats — could not be assembled with short reads. T2T-CHM13 (2022) is the first truly gapless, telomere-to-telomere human reference.
| Reference | Year | Status |
|---|---|---|
| HG19 / GRCh37 | 2009 | Long-standing workhorse; ~8% of the genome missing or collapsed. |
| HG38 / GRCh38 | 2013 | Improved, still contains gaps and false duplications in repeat regions. |
| T2T-CHM13 | 2022 | First complete, gapless assembly — centromeres, telomeres and segmental duplications resolved. Needed long reads (PacBio HiFi / Nanopore) to get there. |
Why it matters commercially and clinically: the missing 8% is not junk — it is enriched for segmental duplications and repeat regions where structural variation lives, which is exactly where several disease-associated variants sit. Short-read resequencing could not see them.
Gene structure — the order, and what each non-coding end actually controls
Read a gene left to right as promoter → 5′ UTR → exon → intron → exon → 3′ UTR. The two UTRs are transcribed but not translated, and they do different jobs: 5′ controls how efficiently translation starts, 3′ controls how long the message lasts.
| Region | Translated? | Function |
|---|---|---|
| Promoter | No (not even transcribed) | Binding site for RNA polymerase and transcription factors; sets whether and how much transcription happens. |
| 5′ UTR | No | Controls translation efficiency — the ribosome loads here and scans. Secondary structure or upstream ORFs here throttle protein output. |
| Exons | Yes | The coding sequence that survives into mature mRNA. |
| Introns | No (spliced out) | Removed by the spliceosome; their boundaries are what splice-site mutations destroy. |
| 3′ UTR | No | Controls mRNA stability and lifetime — AU-rich elements and miRNA binding sites here set the degradation rate, and therefore how much protein a given transcript yields. |
Why this shows up in drug design: mRNA half-life is a design variable, and the 3′ UTR is one of the knobs. It also explains why mRNA level and protein level correlate poorly (the L6 number, r ≈ 0.4): two genes with identical transcript counts can produce very different protein amounts because their UTRs set different translation efficiencies and decay rates.
Three classes of variant — germline, de novo, somatic
The distinction that decides inheritance and who to test: germline is in the egg or sperm and heritable; de novo is new in the child and absent in both parents; somatic is acquired in a body tissue and not passed on at all.
| Class | Where it arises | Heritable? | Typical reading |
|---|---|---|---|
| Germline | Gamete; present in every cell of the child | Yes | Familial disease, carrier screening, predictive testing of relatives. |
| De novo | Early embryo or gametogenesis | Yes for the child, but parents test negative | Explains an affected child with no family history — the key to many severe dominant developmental disorders. |
| Somatic | A single body cell, then its descendants | No | Cancer. Present in the tumour, absent in blood — hence tumour-normal paired sequencing. |
The experimental consequence: germline and de novo variants are found in blood or saliva; somatic tumour variants require tumour tissue (or ctDNA). Comparing tumour against the patient′s own germline is what separates a driver from a harmless inherited polymorphism.
Reference databases — which one answers which question
Four names carry almost all of the lookup work: NCBI for sequences and literature, UniProt for protein sequence and functional annotation, KEGG and GO for pathways and functional terms, BLAST/DIAMOND for finding similarity.
| Resource | Holds | Reach for it when you need |
|---|---|---|
| NCBI | GenBank sequences, RefSeq, PubMed | A nucleotide or protein sequence, or the paper behind it. |
| UniProt | Protein sequences + curated function, domains, PTMs | “What does this protein do / where is it / what modifies it?” — the proteomics-side counterpart to NCBI. |
| GO (Gene Ontology) | Controlled vocabulary: molecular function, biological process, cellular component | Turning a gene list into functional themes (enrichment analysis). |
| KEGG | Curated pathway and network maps | “Which pathway is this gene set in?” |
| BLAST / DIAMOND | Similarity search (DIAMOND = faster, for big data) | An unknown sequence — find its homologues and infer function. |
None of these replaces the alignment and quantification tools covered earlier; they are the annotation layer you query after you have a result.
TBXT and the AluY insertion — why humans have no tail
A 2024 Nature result: an AluY element inserted into an intron of TBXT causes exon 6 to be skipped during splicing, producing a shorter TBXT isoform — and this alternative splicing is what suppressed tail formation in the primate lineage.
Why it is a good exam illustration: it is a non-coding change with a large phenotypic effect, it acts through splicing rather than through the protein sequence itself, and it shows an Alu mobile element doing real evolutionary work instead of being “junk DNA”.
It also mirrors a therapeutic idea already in the notes: if a disease is caused by exon skipping, then forcing or blocking that skip is a drug strategy — the same logic behind antisense oligonucleotides in splice-modulating therapies.
How sequencing depth is calculated (inputs from the slides, formula derived)
Definition and formula
Depth = the average number of times each base is read.
Depth = (number of reads × read length) ÷ target size; conversely, data needed = depth × target size.
| Symbol | Meaning | Example |
|---|---|---|
| N | Number of reads (or read pairs) | 300 M pairs |
| L | Read length | PE150 → 300 bp per pair |
| G | Target size (genome / exome / panel) | 3 Gb, 50 Mb, 1 Mb |
Three worked examples
| Scenario | Calculation | Answer |
|---|---|---|
| Human WGS at 30× | 30 × 3 Gb = 90 Gb; 90 Gb ÷ 300 bp = 300 M | ~300 million PE150 pairs |
| WES at 100× (50 Mb exome) | 100 × 50 Mb = 5 Gb; ÷ 300 bp ≈ 16.7 M | ~16.7 million pairs |
| Panel at 500× (1 Mb) | 500 × 1 Mb = 0.5 Gb; ÷ 300 bp ≈ 1.67 M | ~1.67 million pairs |
Common traps
| Trap | Correct view |
|---|---|
| Treating depth and breadth / coverage as the same | Depth is the average; breadth is the fraction of bases covered at least once (30× average still leaves some bases at 0×) |
| Forgetting on-target rate | WES capture is typically 60–80%, so you must sequence 25–60% more to hit the nominal depth |
| Ignoring duplicate rate | PCR duplicates consume effective depth, and the fraction rises with sequencing depth |
| Comparing platforms by read count | Convert to total bases: 300 bp × 100 M = 30 Gb, the same as 150 bp × 200 M |
Typical depths to memorise
| Application | Typical depth | Why |
|---|---|---|
| Human WGS (germline) | 30× | Balance point for heterozygous calls |
| WES | 100× | Small target; needs extra depth to offset capture loss |
| Cancer panel | 500–1,000× | Tumour heterogeneity, low-VAF variants |
| Bulk RNA-seq | 20–30 M reads per sample | Enough for transcript quantification |
| Single-cell RNA-seq | 20–50k reads per cell | Limited transcripts per cell |