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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# DNA sequencing

Sequencing means reading the exact order of DNA bases; today you can read one gene, all exons (exome) or a person's whole genome.

Updated 2026-11-01

## Key facts

- Sanger (dideoxy): ddNTPs lacking a 3'-OH end the chain; fragments are separated by size and the sequence is read. Ideal for one gene or to confirm a variant.
- Next-generation (NGS): millions of fragments sequenced in parallel; enables gene panels, exome and genome.
- The exome (~1–2% of the genome) holds most variants known to cause Mendelian diseases.
- Variants are classified: pathogenic, likely pathogenic, of uncertain significance (VUS), likely benign, benign.
- Long reads (nanopore, single-molecule): better for repeats and rearrangements.

## In detail

- In Sanger sequencing each ddNTP carries a different fluorochrome; capillary electrophoresis orders the fragments and a laser reads the colour at each end.
- In NGS, DNA is fragmented, adaptors are added, it is amplified and read by synthesis; software then aligns the reads to the reference genome.
- Read depth: how many times each base is read; more depth gives more confidence and can reveal mosaicism or somatic variants in tumours.
- Liquid biopsy sequences cell-free tumour DNA in plasma to follow treatment response.
- Incidental findings (unsought risk genes such as BRCA1) require consent and genetic counselling.

**Also called**
Sanger, Next-generation sequencing (NGS), Exome

## Clinical notes

- Gene panels for hereditary cancer (BRCA1/2, Lynch genes) and tumour sequencing to choose targeted therapies.
- Exome sequencing shortens the 'diagnostic odyssey' in children with rare diseases.
- Pathogen sequencing to track viral variants and antimicrobial resistance.

## Related conditions

- [Lynch syndrome (hereditary non-polyposis colorectal cancer)](https://durgical.com/en/conditions/lynch-syndrome/)
- [Hereditary breast and ovarian cancer syndrome (BRCA1/BRCA2)](https://durgical.com/en/conditions/hereditary-breast-and-ovarian-cancer-syndrome/)

## Related

- [Polymerase chain reaction (PCR)](https://durgical.com/en/biochemistry/topics/polymerase-chain-reaction/)
- [DNA microarrays](https://durgical.com/en/biochemistry/topics/dna-microarrays/)
- [DNA polymerases](https://durgical.com/en/biochemistry/enzymes/dna-polymerases/)
- [Types of mutations](https://durgical.com/en/biochemistry/topics/types-of-mutations/)

## Test yourself

Think of the answer, then tap the question to see it.

## Why do dideoxynucleotides end the chain in Sanger sequencing?

They lack the 3'-OH needed to form the next phosphodiester bond.

## What advantage does NGS have over Sanger?

It sequences millions of fragments at once: many genes, the exome or the genome can be studied in one test.

## What does a variant of uncertain significance (VUS) mean?

There is not enough evidence to say whether it causes disease; it should not drive clinical decisions on its own.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [DNA Sequencing Fact Sheet](https://www.genome.gov/about-genomics/fact-sheets/DNA-Sequencing-Fact-Sheet) (National Human Genome Research Institute (NHGRI), 2026); [What are whole exome sequencing and whole genome sequencing?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/testing/sequencing/) (MedlinePlus, US National Library of Medicine, 2026); [What is genetic testing?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/testing/genetictesting/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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