⚡ Core Concept: Deciphering Primary Nucleotide Architecture
DNA sequencing is the determination of the precise linear order of adenine, cytosine, guanine, and thymine nucleotides across a DNA molecule. It spans first-generation chain-termination chemistry (Frederick Sanger, 1977), second-generation massive parallel sequencing (Illumina), and third-generation single-molecule long-read platforms (Oxford Nanopore, PacBio).
1. Sanger Dideoxy Chain-Termination Sequencing
The hallmark of first-generation sequencing is the incorporation of 2′,3′-dideoxynucleoside triphosphates (ddNTPs). Lacking a 3′-hydroxyl group (−OH), ddNTP incorporation halts phosphodiester bond formation, resulting in terminated fragments.
Figure 1: Mechanism of Dideoxynucleotide (ddNTP) Chain Termination
Genomic Chemistry
Structural comparison: dNTP contains a 3′-OH required for nucleophilic attack on the next incoming 5′-triphosphate, whereas ddNTP possesses only a 3′-H, terminating extension.
2. Generations of DNA Sequencing Technologies
| Platform Generation | Representative Technology | Read Length | Major Advantages & Limitations |
|---|---|---|---|
| 1st Generation | Sanger Capillary Electrophoresis | ~800–1,000 bp | High accuracy (>99.99%); Gold standard for single-gene validation; Low throughput. |
| 2nd Generation (NGS) | Illumina (Sequencing-by-Synthesis) | 150–300 bp | Massively parallel (gigabases to terabases per run); Low cost per base; Short read length struggles with repetitive elements. |
| 3rd Generation (Long-read) | Oxford Nanopore & PacBio HiFi | 10 kb – >100 kb | Single-molecule real-time sequencing; Direct RNA & methylation detection; Solves complex structural variations and telomere-to-telomere assemblies. |
🎓 High-Yield Exam Checkpoint
- Automated Sanger Sequencing: Uses 4 distinct fluorescent dye terminators (ddATP, ddCTP, ddGTP, ddTTP) loaded into a single capillary, detected by laser excitation to produce a 4-color electropherogram.
- Human Genome Project (HGP): Relied on bacterial artificial chromosome (BAC) hierarchical shotgun Sanger sequencing, completing the initial draft in 2001 and final draft in 2003.
