Sanger sequencing
/ SANG-er /
Reading the exact order of bases along a DNA strand — actually spelling out A-T-G-C-C-A and so on — is the core task of molecular biology. The first method that did this cleanly and reliably was invented by Frederick Sanger in 1977, and for decades it was simply how DNA was read. It is built on a beautiful trick: make copies of the DNA that stop at random, controlled points, then sort them by length.
You copy the unknown strand with DNA polymerase, but you spike the reaction with a small fraction of special 'chain-terminator' bases (dideoxynucleotides) that, once added, prevent any further base from being attached. So the new strands grow until, by chance, a terminator gets inserted, and then they stop. Across many molecules this produces a ladder of fragments of every possible length, each ending in a terminator. When the four terminators carry four different fluorescent colors and the fragments are separated by length (smallest first), reading the colors in order — shortest to longest — spells out the sequence directly: the color of the one-base-longer fragment tells you the next base.
Sanger sequencing reads a single template at a time and produces a high-quality 'read' of roughly 500 to 1000 bases. It was the engine of the original Human Genome Project. Although next-generation methods now do the bulk of large-scale sequencing far more cheaply, Sanger sequencing is far from obsolete: its accuracy and longer per-read length make it the trusted standard for confirming a single gene, checking a clone, or validating a suspicious next-gen result. It is the careful, definitive read rather than the mass-produced one.
To confirm a clone, a lab sends it for Sanger sequencing and gets back a clean trace of colored peaks: green, green, blue, red, blue... read left to right, the peaks spell A-A-C-T-C, an unambiguous reading of that stretch of the DNA.
Ordering fragments by length and reading their colors spells the sequence.
Sanger sequencing is not obsolete despite next-gen methods — its long, highly accurate single reads still make it the gold standard for confirming individual genes, clones, or suspicious variants; the newer methods win on scale and cost, not on the quality of a single read.