Gene sequencing is the process of determining the order of nucleotide bases in a DNA molecule or a selected region of DNA. The four principal DNA bases are adenine, cytosine, guanine and thymine, commonly abbreviated A, C, G and T. Sequencing reveals the precise or inferred order of these bases, allowing researchers or clinicians to identify genetic variants, compare genomes, study gene function and investigate biological relationships.
The term can refer narrowly to sequencing a particular gene or broadly to sequencing larger collections of DNA. Modern sequencing technologies include Sanger sequencing and next-generation sequencing approaches, while newer platforms can perform long-read sequencing. Sequencing is fundamentally different from simply detecting whether a known variant is present because sequencing can provide information about the underlying nucleotide order across the region being examined.
Traditional Sanger sequencing is based on DNA synthesis and chain termination. A DNA template is copied using DNA polymerase in the presence of normal nucleotides and specially modified chain-terminating nucleotides. When a terminating nucleotide is incorporated, extension stops. A collection of fragments ending at different positions can then be separated and analysed to infer the sequence. Sanger sequencing is highly useful for targeted regions and validation of variants, although it is comparatively less suited to sequencing enormous numbers of genomic regions simultaneously. Its historical importance is substantial because it provided a reliable method for determining DNA sequence and played a major role in early genome sequencing projects.
Next-generation sequencing, often called massively parallel sequencing, changed sequencing by allowing many DNA fragments to be processed simultaneously. DNA is typically fragmented, prepared into a library with additional molecular components, amplified or otherwise prepared for detection, and then sequenced in parallel. Different technologies use different chemistry and detection mechanisms. Short-read platforms can generate very large numbers of relatively short sequences with high throughput, while long-read technologies can produce substantially longer reads. Sequencing data are then processed computationally. Reads may be quality filtered, aligned to a reference genome or assembled without a reference, and analysed for variants or other features. The computational pipeline is therefore an integral part of modern sequencing rather than a separate optional stage.
Sequencing can be performed at different scales. Targeted sequencing examines selected genes or genomic regions and can be useful when a specific set of genes is relevant to a clinical or research question. Whole-exome sequencing focuses primarily on protein-coding regions of the genome, while whole-genome sequencing aims to characterise the genome much more broadly. RNA sequencing is related but measures RNA molecules rather than directly sequencing genomic DNA, providing information about gene expression and transcript structure. Mitochondrial DNA sequencing focuses on genetic material in mitochondria. The appropriate scale depends on the question, available resources, required sensitivity and the type of variants or biological information being sought.
Sequencing requires careful sample preparation and quality control. DNA may be extracted from blood, saliva, tissue or another biological sample depending on the application. Contamination, degradation, insufficient quantity or sample misidentification can compromise results. Library preparation can introduce biases, while sequencing instruments generate raw signal data that must be converted into nucleotide calls. Quality scores estimate confidence in individual base calls, and bioinformatics pipelines can identify low-quality regions, sequencing errors and technical artefacts. Variant calling then compares observed sequence information with a reference or other expected sequence. Validation may be necessary, especially when findings have clinical significance. The final result therefore depends on the entire chain from biological sample to laboratory process to computational interpretation.
Sequencing has become important in medicine and research. Clinical sequencing can help investigate suspected inherited disorders, characterise cancers, identify infectious organisms and sometimes inform treatment selection. In cancer, sequencing can reveal somatic mutations present in tumour cells that may contribute to tumour behaviour or indicate potential therapeutic targets. In inherited disease, sequencing can identify variants that may explain a patient's condition, although interpretation can be difficult when variants are rare or poorly characterised. In research, sequencing supports comparative genomics, evolutionary studies, population genetics, microbial surveillance and investigation of gene regulation. Sequencing does not itself establish that every detected variant is harmful or clinically important; interpretation requires additional evidence.
A major technical distinction concerns read length and coverage. Short reads can provide high accuracy and throughput but may have difficulty resolving repetitive genomic regions, structural variants or complex rearrangements. Long reads can span larger genomic regions and help resolve structural variation and repetitive sequences, although each platform has its own accuracy, error profile and sample requirements. Coverage describes how many sequencing observations support a region or base, and greater coverage can improve confidence in detecting variants under appropriate conditions. Sequencing depth and breadth of coverage are therefore important quality measures. Bioinformatics also includes alignment, assembly, variant calling, annotation and interpretation, and different algorithms can produce different results from the same raw data.
Gene sequencing should not be confused with gene editing, genetic testing or gene expression analysis. Sequencing reads genetic information; editing deliberately changes genetic material; a genetic test may use sequencing or other molecular methods to answer a specific diagnostic question; expression analysis examines which genes are being transcribed and at what levels. Sequencing can reveal inherited and acquired variants, but the meaning of those variants depends on biological and clinical evidence. It has become a foundational technology of genomics because it transforms DNA sequence into analysable digital data, enabling increasingly detailed study of heredity, disease, evolution and biological function while creating corresponding requirements for computational analysis, quality assurance, privacy and responsible interpretation.
