What is capillary sequencing?

By the mid-1990s, when the Human Genome Project was in full swing, scientists were sequencing DNA using capillary sequencers.  


In capillary sequencing machines, DNA fragments are separated by size through a long, thin, acrylic-fibre capillary (instead of an electrophoresis gel, as with the Sanger method).

  1. A sample containing fragments of DNA is injected into the capillary. This is done by dipping the capillary and an electrode into a solution of the sample, and briefly applying an electric current. This causes the DNA fragments to migrate on to the end of the capillary.
  2. Once the sample has been injected, the electric field is reapplied, to drive the DNA fragments through the capillary.
  3. A fluorescence-detecting laser, built into the machine, then shoots through the capillary fibre, causing the coloured tags on the DNA fragments, to fluoresce. Each base terminator is labelled with a different colour: A = Green, C = Blue,  G = Yellow and T = Red. 
  4. The colour of the fluorescent bases is detected by a camera, and recorded by the sequencing machine.
  5. The colours of the bases are then displayed on a computer as a graph of different coloured peaks. Like this one:
The output from a capillary sequencing machine.

Image credit: Genome Research Limited

The small diameter of the capillary allows for the use of extremely high electric fields, and consequently, very rapid separation of DNA sequencing fragments. 

This page was last updated on 2021-07-21