DNA (Sanger) sequencing using dideoxy chain termination.
  1. A primer is annealed to the target template DNA.

  2. A mixture with DNA polymerase, the four dNTPs, and one dideoxynucleotide (such as ddATP), is added.

  3. During primer extension, the polymerase occasionally inserts a ddNTP instead of a dNTP, which lacks the 3'-OH needed to attach the next nucleotide, terminating the chain.

  4. Gel electrophoresis separates the synthesized fragments that terminate at the dideoxynucleotide.