Sanger Sequencing Diagram – The Dideoxy Method Labeled

All 13 parts on the diagram

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The reaction 7

Template strand The single strand being sequenced. The new strand is built to match it.
Primer A short strand that gives the reaction its starting point.
DNA polymerase The enzyme that builds the new strand, one nucleotide at a time.
Normal nucleotides (dNTPs) The four building blocks of DNA: dATP, dGTP, dCTP and dTTP. Chains grow while these are added.
Dideoxy nucleotides (ddNTPs) They lack the group the next nucleotide attaches to. Each of the four carries its own dye.
Terminated strand A new strand that has stopped growing. Its last unit is a dideoxy nucleotide.
Fragments of every length Chains stop at every position, giving fragments of every length. Each ends in the dye of its last base.

Reading the result 6

Capillary A hair-thin tube that takes the place of a slab of gel. An electric field pulls the fragments through it.
Shortest fragments They move fastest, so they reach the laser first.
Laser Its beam makes the dye on each passing fragment glow.
Detector It records the glow of each fragment as it passes the beam.
Chromatogram peak One peak for each length of fragment that passes. The trace is also called an electropherogram.
DNA sequence The bases read off the peaks, in order. It is the sequence of the new strand.

How this list was compiled: Every name checked on 2026-10-03 against Kimball's Biology Pages: DNA Sequencing; Nature Education, Scitable: DNA Sequencing Technologies, and DNA Sequencing Technologies Key to the Human Genome Project; Wikipedia: Sanger sequencing, Dideoxynucleotide, DNA sequencer and Capillary electrophoresis, text only; and an open-access review of the history of DNA sequencing (Genomics, 2016) read in full text through Europe PMC. Licence: own work

The drawing: Drawn for this site on 2026-10-03 from plain geometry; nothing traced. Licence: own work

How it works

Sanger sequencing, also called the dideoxy or chain-termination method, reads the order of bases in a piece of DNA. Label the reaction first, then the capillary, laser and trace that turn the fragments into a sequence.

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The reaction: chains that stop

DNA polymerase starts at a primer and builds a new strand along the template from normal nucleotides. A small share of the nucleotides are dideoxy ones, each with a dye. When one is added the chain stops, so the tube ends up holding fragments of every length.

Reading the result: shortest first

The fragments run through a capillary, and the shortest move fastest. Near the end a laser makes each dye glow and a detector records it. The trace has one peak for each length of fragment, and the bases are read off the peaks in order.

Questions and answers

What are the steps of Sanger sequencing?

DNA polymerase copies a template from a primer. Now and then it adds a dideoxy nucleotide, which stops the chain. The fragments are sorted by length, a laser reads the dye on each, and the sequence is read off the peaks.

How do I label the steps and details relevant to the Sanger dideoxy DNA sequencing method?

Start with the reaction: template strand, primer, DNA polymerase, normal nucleotides and dideoxy nucleotides. Then the readout: capillary, laser, detector and chromatogram. The drawing here is the site's own, so its layout may differ from the one in a course.

Why does a dideoxy nucleotide stop the chain?

It lacks the hydroxyl group on its 3′ carbon, the place where the next nucleotide would attach. Once DNA polymerase adds one, nothing more can be joined to that strand.

How is the sequence read from the chromatogram?

Each peak is the fragments of one length passing the laser, and its dye shows which base ended them. The shortest fragments pass first, so reading the peaks in order gives the sequence of the new strand.