Sanger Sequencing

Overview

Sanger sequencing, also called chain-termination or dideoxy sequencing, determines the exact order of bases in a stretch of DNA. For roughly three decades it was the only practical way to read a sequence, and it remains the reference method against which newer technologies are judged. The technique is built on a single elegant trick: deliberately interrupting DNA synthesis at specific bases so that the lengths of the interrupted products reveal the sequence.

This lesson assumes familiarity with how a DNA polymerase extends a primer — it adds each new nucleotide to the free 3’-hydroxyl (3’-OH) group at the growing end of the strand, so an intact 3’-OH is required for the chain to keep growing. That requirement, covered in the polymerases lesson, is the hinge on which the whole method turns. The lesson also assumes the capillary electrophoresis fundamentals — charge- and size-based separation in a polymer- filled capillary, on-line fluorescence detection, and the resulting electropherogram — and does not re-teach them; here the focus is the reaction chemistry that feeds that readout ¹.

Chain termination with dideoxynucleotides

The reagent at the heart of the method is the dideoxynucleotide triphosphate (ddNTP). A normal deoxynucleotide (dNTP) has a 3’-OH group that lets the next nucleotide attach. A ddNTP is identical except that it lacks the 3’-OH — the 3’ carbon carries only a hydrogen. When a polymerase incorporates a ddNTP, the base pairs correctly and the molecule is added to the growing strand, but because there is no 3’-OH, the next nucleotide has nothing to attach to. Synthesis of that strand terminates at that point ².

A Sanger reaction therefore contains the usual sequencing ingredients — a DNA template, a single primer to set the start point, a DNA polymerase, and the four normal dNTPs — plus a small amount of ddNTPs. Because ddNTPs are present at a low ratio relative to dNTPs, termination is a chance event: at any given position the polymerase usually adds a normal dNTP and continues, but occasionally adds a ddNTP and stops. Across the millions of template copies in the tube, termination happens at every possible position for a given base.

The result is a nested set of newly synthesized fragments. All fragments share the same 5’ starting point (the primer) but end at different positions, each ending in a ddNTP. If the terminator is ddA, for example, the reaction produces a fragment ending at every position in the product where an adenine occurs. Sorting those fragments by length reveals exactly where each base sits in the sequence ¹.

Template (3'->5'):   3'- T A C G G A T C A G ... -5'
Primer + extension (5'->3'), * = ddNTP terminator:

5'- A T G *                 (stops at first ddC)
5'- A T G C C *             (stops at next ddC)
5'- A T G C C T A *         (stops at ddG)
5'- A T G C C T A G *       (stops at next ddG)
5'- A T G C C T A G T *     (stops at ddT)
        ...
   shortest --------------------------> longest

Dye-terminator chemistry in one tube

Early Sanger protocols ran four separate reactions, one for each ddNTP, and resolved them in four lanes of a slab gel — the sequence was read by comparing which lane had a band at each successive position. Modern clinical sequencing instead uses dye-terminator chemistry, which collapses the assay into a single tube.

In dye-terminator sequencing each of the four ddNTPs carries a different fluorescent dye: one color for ddA, another for ddC, another for ddG, and another for ddT. Because the identity of the terminating base is now encoded by color rather than by which tube it came from, all four terminators can be combined in one reaction. Every fragment in the nested set ends in a labeled ddNTP whose color names its final base. This one-tube format is what makes Sanger sequencing automatable and is the form used in essentially all clinical Sanger testing today ¹.

Separation and readout by capillary electrophoresis

The labeled, nested fragment set is then separated by capillary electrophoresis (CE), the same platform introduced in the capillary electrophoresis lesson. CE resolves the fragments at single-base resolution: fragments differing in length by a single nucleotide form distinct peaks. Because smaller fragments migrate faster, they reach the detector first, so the fragments arrive in order of increasing length — which is the same as reading the synthesized strand from its 5’ end onward ³.

As each fragment passes the detector window, a laser excites its terminal dye and the instrument records the color. The output is an electropherogram: a series of fluorescent peaks along the migration axis, each peak colored by the base of the ddNTP that terminated that fragment. The reaction chemistry supplies the four-color nested ladder; the capillary supplies the single-base separation and the color-by-color readout.

Electropherogram (one peak per position, color = terminating base)

         A   C   G   T   T   A   G   C   A
        (G) (A) (T) (C) (C) (T) (A) (G) (T)   <- dye color
signal
  |      .       .           .       .
  |   .  |    .  |    .   .  |    .   |    .
  |___|__|____|__|____|___|__|____|__|____|__> migration (5' -> 3')
       short --------------------------> long

Calling bases and reading variants

Base calling. Software scans the electropherogram peak by peak in migration order. At each successive position it identifies the dominant dye color and assigns the corresponding base, producing the called sequence read 5’->3’. A clean read shows one well-shaped, single-colored peak at each position, evenly spaced from its neighbors ¹.

Heterozygous variants. Sanger sequencing reads both alleles of a diploid sample in the same reaction. When the two alleles differ at one position — a heterozygous variant — the nested set contains fragments of the same length that terminate in two different bases. At that position the electropherogram shows two overlapping peaks of different colors, each at roughly half the height of a normal single peak. This double-peak pattern is the characteristic Sanger signature of a heterozygous single-nucleotide variant, and recognizing it is central to interpreting clinical traces ¹.

Homozygous position        Heterozygous position
(one base, full peak)      (two bases, two half-peaks)

   A (full)                   A   G  (overlapping)
    .                          .   .
    |                          |   |
 ___|___                    ___|___|___

Strengths and limitations

Sanger sequencing is prized for accuracy and for producing long, high-quality reads — commonly several hundred bases of reliable sequence from a single run. Because it reads each base directly and resolves heterozygous positions cleanly, it is regarded as the gold standard for confirming variants ¹.

Its limitations follow from the same design. Throughput is low: each reaction reads a single amplicon, one target at a time, so interrogating many regions means many separate reactions. Sensitivity is also limited — a minor sequence variant must usually make up roughly 15-20% of the alleles in the sample before its peak rises clearly above the background of the trace (this limit of detection is a commonly cited approximate value, not a fixed specification). That makes Sanger sequencing poorly suited to detecting low-level variants, such as a small subpopulation of tumor cells in a mixed specimen ¹.

Continuing role

These limitations are exactly what next-generation sequencing addresses, by reading enormous numbers of templates in parallel and detecting variants present at far lower fractions; that technology is the subject of the next lesson. Yet Sanger sequencing has not disappeared. Its accuracy and direct, single-base readout make it the standard tool for confirming findings from next-generation sequencing — an unexpected or clinically critical variant called on a massively parallel platform is frequently verified by a targeted Sanger reaction over the same position before it is reported ¹.

Summary

Sanger sequencing exploits dideoxynucleotides, which lack the 3’-OH that polymerase extension requires, to terminate synthesis at specific bases. A single reaction produces a nested set of fragments ending at every position of each base; in modern dye-terminator chemistry the four ddNTPs each carry a distinct fluorescent color and share one tube. Capillary electrophoresis separates the ladder at single-base resolution and reads each peak’s color in 5’->3’ order, and software calls the sequence — with heterozygous variants appearing as overlapping double peaks. Accurate, long, and direct but low in throughput and sensitivity, Sanger sequencing remains the gold standard for confirming variants, including those first found by next-generation sequencing.