Next-generation sequencing (NGS) encompasses a set of modern sequencing technologies that can sequence millions of DNA fragments in parallel, making it possible to sequence entire genomes quickly and affordably. Platforms such as Illumina, Ion Torrent, and PacBio dominate the field.
How NGS Works (Illumina Platform)
- Library Preparation
The DNA is fragmented into small pieces, typically 200–600 base pairs. Adapters—short known sequences—are ligated to both ends of each fragment. These adapters allow the fragments to bind to the flow cell and serve as priming sites for sequencing.
- Cluster Generation
The library is loaded onto a flow cell whose surface is coated with complementary oligonucleotides. Each fragment binds to the flow cell and undergoes bridge amplification: the fragment bends over, a new strand is synthesized, and the process repeats, creating a cluster of thousands of identical copies of each fragment.
- Sequencing by Synthesis
Fluorescently labeled nucleotides are flowed over the flow cell one at a time. Each nucleotide has a reversible terminator that allows only one base to be added per cycle. After each cycle, the fluorescence is imaged to identify which base was added to each cluster.
- Data Analysis
Millions of sequencing reads are generated. These reads are aligned to a reference genome or assembled de novo. Bioinformatics tools identify genetic variants, gene expression levels, or epigenetic modifications depending on the application.
- Applications
NGS is used for whole-genome sequencing, targeted gene panels, RNA sequencing, epigenetic analysis, and metagenomics, among many other applications.
Practical NGS Library Preparation Workflow
Start with 10–100 ng of high-quality genomic DNA (A260/A280 > 1.8, A260/A230 > 2.0). Fragment the DNA using a Covaris sonicator or enzymatic fragmentation to achieve a target size of 200–600 bp. Perform end repair to generate blunt ends, followed by A-tailing (addition of a single adenosine to the 3’ ends) using the Klenow fragment. Ligate Illumina-compatible adapters containing a 5’ T overhang to the A-tailed fragments using T4 DNA ligase. Purify the ligation products using AMPure XP beads at a 0.8× bead-to-sample ratio to remove unligated adapters. Amplify the adapter-ligated library by PCR for 8–12 cycles using primers complementary to the adapter sequences; include unique dual-index barcodes for sample multiplexing. Clean up the PCR product with AMPure XP beads at 0.8× ratio. Assess library quality and quantity using a Bioanalyzer or TapeStation (expected size distribution: 250–700 bp) and Qubit fluorometric quantification. Normalize libraries to 4 nM, pool equimolarly, and denature with 0.2 N NaOH before loading onto the Illumina flow cell at 1.8 pM for 2 × 150 bp paired-end sequencing. Monitor cluster density (target: 800–1,200 K/mm² for HiSeq 4000) and Q30 scores (>80% for high-quality runs). Demultiplex the output using bcl2fastq and assess per-base quality with FastQC. Trim adapters and low-quality bases using Trimmomatic before alignment.
Real-World Application
In clinical cancer genomics, a targeted NGS panel covering 500 cancer-related genes is used to profile a patient’s tumor biopsy. Library preparation from 50 ng of FFPE DNA yields 94% of targets covered at >100× depth. The analysis identifies a KRAS G12C mutation (0.46 VAF) and an EGFR amplification, guiding treatment selection with a MEK inhibitor.
