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 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.

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.

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.

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

Sources: DNA Sequencing Fact Sheet (National Human Genome Research Institute (NHGRI), 2026); What are whole exome sequencing and whole genome sequencing?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); What is genetic testing?: MedlinePlus Genetics (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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Representative model, for educational purposes only.