DNA Sequencing

DNA Sequencing - Chain Terminator Classic

• Principle: Employs dideoxynucleotides ($ddNTPs$) which lack the $3'-OH$ group essential for phosphodiester bond formation, leading to premature termination of DNA synthesis. $ddNTPs \rightarrow 3'-OH \text{ missing} \rightarrow \text{Chain Termination}$. 📌 Sanger Stops Synthesis.
• Key Components:
  • Single-stranded DNA template
  • Specific primer
  • DNA polymerase (e.g., Taq polymerase)
  • All four deoxynucleotide triphosphates ($dNTPs$)
  • Small amounts of four fluorescently labelled dideoxynucleotide triphosphates ($ddNTPs$), each with a distinct dye.
• Process Overview:

![Sanger sequencing chromatogram interpretation guide](https://ylbwdadhbcjolwylidja.supabase.co/storage/v1/object/public/notes/L1/Biochemistry_Biochemical_Techniques_DNA_Sequencing/af2b661d-5c2d-4f02-86d7-22ae3ec17124.png)

• Reading Chromatogram: The sequence of bases is read from the order of fluorescent peaks, each color corresponding to a specific $ddNTP$.

⭐ Sanger sequencing is highly accurate and often used to confirm results from Next-Generation Sequencing (NGS) methods.

DNA Sequencing - Massive Parallel Power

Next-Generation Sequencing (NGS), also known as massively parallel sequencing, allows for simultaneous sequencing of millions to billions of DNA fragments. This high-throughput technology offers a significant leap from traditional methods.

Core Workflow:

Key Advantages over Sanger Sequencing:

• Massive Throughput: Generates vast amounts of sequence data in a single run.
• Increased Speed: Rapidly sequences entire genomes or targeted regions.
• Reduced Cost: Significantly ↓ cost per sequenced base, enabling large-scale projects.
• Broader Applications: Facilitates diverse studies like transcriptomics (RNA-Seq), epigenomics (ChIP-Seq), and metagenomics.

⭐ NGS can detect somatic mutations with allele frequencies as low as 1-5%, vital for early cancer detection and monitoring.

DNA Sequencing - Tech Titans Tangle

• Key Platforms & Principles:

  • Illumina (SBS): Fluorescent reversible terminators. Dominant for short reads.
  • Pyrosequencing: Detects pyrophosphate (PPi) release.
  • Ion Torrent: Detects H+ ion release (pH change).
  • PacBio SMRT: Single-molecule real-time sequencing in Zero-Mode Waveguides (ZMWs); long reads.
  • Oxford Nanopore: DNA through protein nanopore; measures ionic current changes; long reads, portable.

• Sanger vs. NGS Comparison:

⭐ Illumina sequencing, utilizing sequencing-by-synthesis (SBS), is the most prevalent NGS technology due to its high throughput and accuracy for short reads.

DNA Sequencing - Gene Scene Unveiled

Unveils genetic blueprints. Vital in diagnostics, research, personalized medicine.

• Clinical Diagnostics:
  • Genetic Disorders: Identifies inherited conditions (e.g., cystic fibrosis).
  • Cancer Genomics: Detects mutations (EGFR, BRCA), CNVs, fusions for targeted therapy.
  • Infectious Diseases: Rapid pathogen ID (MTB, HIV), outbreak tracking, AMR detection.
• Pharmacogenomics: Guides drug choice & dose (CYP2C19 & clopidogrel).
• Forensics: Individual ID.
• Research: WGS, WES, RNA-Seq (transcriptome), ChIP-Seq (DNA-protein interaction).
• Advanced: Third-Gen Sequencing (TGS) - long reads (PacBio, Nanopore), real-time.

⭐ Sanger sequencing, the first-generation method, remains crucial for validating Next-Generation Sequencing (NGS) findings and for targeted sequencing of specific DNA regions.

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High‑Yield Points - ⚡ Biggest Takeaways

• Sanger sequencing (dideoxy method) uses ddNTPs as chain terminators; a gold standard.
• Next-Generation Sequencing (NGS) enables massively parallel sequencing, high throughput, and cost-effectiveness.
• Pyrosequencing detects pyrophosphate (PPi) release upon nucleotide incorporation, generating light.
• Shotgun sequencing is key for whole genome sequencing by assembling random DNA fragments.
• Applications: mutation detection, genetic disorder diagnosis, forensics, and pharmacogenomics.
• Automated Sanger employs fluorescently labeled ddNTPs and capillary electrophoresis.