Sequencing Study Guide

📖 Core Concepts Sequencing – Determines the linear order (primary structure) of an unbranched biopolymer and yields a symbolic sequence that summarizes its atomic‑level structure. DNA Sequencing – Determines the nucleotide order of a DNA fragment. Sanger (Chain‑Terminator) Sequencing – Uses a primer, DNA polymerase, normal dNTPs, and low‑conc. dideoxynucleotides (ddNTPs) that stop elongation, producing fragments of every possible length. Dye‑Terminator (Fluorescent) Sanger – Each ddNTP is labeled with a distinct fluorophore, allowing a single reaction and detection by fluorescence. Pyrosequencing – Detects the pyrophosphate released when a nucleotide is incorporated; PPi → ATP (ATP sulfurylase) → light (luciferase). Light intensity ∝ number of bases added in that cycle. No fluorescent labels or gels are required. Carlson Curve – Analogous to Moore’s law; it describes the rapid fall in cost and rise in performance of DNA‑sequencing technologies. Large‑Scale Sequencing – Exome sequencing targets all protein‑coding regions; Whole‑genome sequencing reads the entire nuclear DNA. RNA Sequencing (RNA‑Seq) – RNA is reverse‑transcribed to cDNA, optionally enriched for mRNA, then sequenced. Reveals which genes are actively expressed. Protein Sequencing – Edman degradation, peptide mass fingerprinting, mass spectrometry, and protease digests; DNA‑based inference is usually easier when the gene is known. Genetic Code – The rule set that translates nucleotide triplets (codons) into amino‑acid residues. --- 📌 Must Remember ddNTPs lack both 2′‑OH and 3′‑OH → no further phosphodiester bond formation → chain termination. Dye‑terminator Sanger: single‑tube reaction, four fluorescent labels → read by capillary electrophoresis. Pyrosequencing signal: light peak height = number of identical nucleotides incorporated in that flow (detects homopolymers). Apyrase in pyrosequencing removes unincorporated nucleotides, preventing false peaks. Carlson curve predicts exponential cost decline; newer technologies (pyrosequencing, Illumina, etc.) now out‑produce Sanger. Exome sequencing → focuses on coding DNA only; cheaper & higher coverage for gene‑focused studies. RNA‑Seq requires reverse transcription; enrichment removes rRNA & small RNA to focus on mRNA. --- 🔄 Key Processes Sanger (Chain‑Terminator) Workflow Primer annealing to a defined site on template DNA. DNA polymerase extension with a mixture of dNTPs + low‑conc. ddNTPs. Random incorporation of a ddNTP → termination; creates a population of fragments ending at each possible base. Separation of fragments by polyacrylamide gel or capillary electrophoresis. Detection (traditional: radiolabel; modern: fluorescent dye per base). Dye‑Terminator Improvement Each ddNTP carries a unique fluorophore → a single reaction yields the complete sequence; peaks are read by laser detection. Pyrosequencing Cycle PCR amplification of target strand. Nucleotide flow: add one of the four dNTPs. If the base matches the template, polymerase incorporates it → pyrophosphate (PPi) released. ATP sulfurylase converts PPi + ATP → ATP. Luciferase uses ATP → emits light; intensity recorded as a pyrogram peak. Apyrase degrades any remaining nucleotides → ready for next flow. RNA‑Seq Workflow (outline‑based) Extract total RNA → optionally deplete rRNA and small RNA to enrich mRNA. Reverse transcription → cDNA fragments. Library preparation & sequencing (usually NGS platforms). Map reads back to reference genome; note that introns are spliced out, so reads may be non‑colinear with DNA. --- 🔍 Key Comparisons Sanger vs. Pyrosequencing Labeling: Fluorescent dyes (Sanger) vs. no labels (Pyrosequencing). Separation: Gel/capillary electrophoresis (Sanger) vs. real‑time light detection (Pyrosequencing). Throughput: One fragment per reaction (Sanger) vs. millions of simultaneous reads (Pyrosequencing). Exome vs. Whole‑Genome Sequencing Target: Only coding exons (exome) vs. entire nuclear DNA (whole genome). Cost/Depth: Lower cost, higher coverage for exome; higher cost, broader scope for whole genome. DNA Sequencing vs. RNA Sequencing Template: Genomic DNA vs. cDNA derived from RNA. Information: Static genome vs. dynamic expression profile. --- ⚠️ Common Misunderstandings “Pyrosequencing uses fluorescent nucleotides.” – It relies on light from luciferase, not fluorophores. “All Sanger runs need separate tubes for each base.” – Dye‑terminator chemistry combines all bases in one reaction. “Exome sequencing gives the same data as whole‑genome sequencing.” – It only captures coding regions; regulatory and non‑coding variants are missed. “RNA‑Seq reads the RNA directly.” – RNA must first be reverse‑transcribed to cDNA. --- 🧠 Mental Models / Intuition Chain‑terminator ladder: Imagine building a set of ladders where each rung can be “cut” (by a ddNTP) at every possible step → the collection of ladder lengths spells out the sequence. Pyrosequencing as a “firework display”: Each successful nucleotide addition launches a flash of light; the brighter the flash, the more fireworks (identical bases) went off at once. Carlson curve as a “price‑performance treadmill”: As technology runs faster, cost drops dramatically—just keep up! --- 🚩 Exceptions & Edge Cases Homopolymer detection: Light intensity is proportional, but extremely long homopolymers can saturate the signal, leading to under‑estimation. Second‑ and third‑generation platforms (Illumina, 454, Helicos, Dover) are more cost‑effective than traditional Sanger, but may have different error profiles (e.g., indel errors in homopolymers). Protein sequencing: Direct methods (Edman, MS) are used when the gene is unknown; otherwise, DNA‑based inference is preferred. --- 📍 When to Use Which Sanger (dye‑terminator) – Small‑scale projects, validation of NGS variants, or when ultra‑high accuracy is needed. Pyrosequencing – Rapid bacterial genome coverage, detection of homopolymer stretches, or when fluorescence‑free setups are desired. Exome sequencing – Gene‑focused disease studies, cost‑constrained projects needing high coverage of coding regions. Whole‑genome sequencing – Comprehensive variant discovery, structural variant analysis, de‑novo assembly. RNA‑Seq – Quantifying gene expression, discovering novel transcripts, studying splicing patterns. Protein sequencing (Edman/MS) – When the protein product must be confirmed directly, especially for unknown genes or post‑translational modifications. --- 👀 Patterns to Recognize Sanger electropherogram: Even peak heights → good chemistry; uneven peaks → old dyes or incorporation bias. Pyrogram: Single, sharp peak → incorporation of one base; taller peak → multiple identical bases (homopolymer). RNA‑Seq read mapping: Gaps in alignment often signal introns; reads spanning exon‑exon junctions indicate splicing. Exome capture: Enrichment bias shows as high coverage in coding regions and low/zero coverage elsewhere. --- 🗂️ Exam Traps “All sequencing methods require electrophoresis.” – Pyrosequencing and modern NGS platforms do not. “Dideoxynucleotides are just normal nucleotides with a missing phosphate.” – They lack the 2′‑OH and 3′‑OH, which is why they terminate synthesis. “Higher light intensity always means a longer read.” – In pyrosequencing, intensity reflects how many of the same base were added in that single flow, not overall read length. “RNA‑Seq provides DNA sequence information.” – It reveals the transcriptome; the underlying DNA must be inferred, not directly read. “Exome sequencing can detect regulatory variants.” – It only captures exonic (coding) sequences; regulatory regions are missed. ---