DNA Sequencing
Introduction
DNA sequencing is the process of determining the precise order of nucleotides (A, T, C, G) in a DNA molecule. It is essential for genomics, genetic engineering, and medical research.
Types of DNA Sequencing Methods
1. Sanger Sequencing (Chain Termination Method)
Developed by Frederick Sanger (1977), this method is based on selective incorporation of dideoxynucleotides (ddNTPs) to terminate DNA replication.
Steps:
1 DNA Denaturation – The DNA strand is separated.
2 Primer Binding – A short primer binds to the template strand.
3 DNA Polymerization – DNA polymerase adds dNTPs (A, T, C, G).
4 Chain Termination – ddNTPs (lacking 3' OH) stop strand elongation.
5 Fragment Separation – Gel electrophoresis or capillary electrophoresis
sorts fragments by size.
6 Fluorescent Detection – A laser detects the nucleotide sequence.
Advantages:
High accuracy
Suitable for small genomes
Limitations:
Expensive and slow for large genomes
2. Next-Generation Sequencing (NGS)
Massively parallel sequencing technology that sequences millions of DNA fragments simultaneously.
Popular NGS Platforms:
Illumina Sequencing – Uses fluorescently labeled nucleotides
and bridge amplification on a flow cell.
Roche 454 Pyrosequencing – Detects light emitted during nucleotide
incorporation.
Ion Torrent Sequencing – Measures pH changes caused by
nucleotide additions.
Advantages:
High throughput
Fast and cost-effective for large genomes
Limitations:
Requires complex bioinformatics analysis
3. Third-Generation Sequencing (Single-Molecule Sequencing)
Directly sequences single DNA molecules without PCR amplification.
Popular Techniques:
PacBio SMRT (Single-Molecule Real-Time) Sequencing – Uses real-time
observation of DNA synthesis.
Oxford Nanopore Sequencing – DNA is passed through protein
nanopores, detecting electrical signals.
Advantages:
Ultra-long reads (up to 100 kb or more)
No need for amplification
Limitations:
Higher error rates compared to NGS
Applications of DNA Sequencing
Genome Sequencing – Human
Genome Project, personalized medicine
Disease Diagnosis – Identifying genetic mutations in cancer, inherited
disorders
Forensic Science – DNA fingerprinting in criminal investigations
Agriculture & Evolutionary Biology – GMO research, phylogenetic
studies
Future of DNA Sequencing
Advances in AI, automation, and nanopore sequencing are making DNA sequencing faster, cheaper, and more accurate.