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How We Read DNA: Sanger and the Human Genome Project

The first reliable way to read DNA letters — Sanger sequencing — and the 13-year, multinational marathon it powered: the Human Genome Project that produced our first draft of a human genome.

Sanger sequencing: reading by deliberately stopping

DNA sequencing means determining the exact order of nucleotides — A, C, G, T — along a strand. The first method to do this dependably, invented by Frederick Sanger in the 1970s, is still in use today. The clever idea: copy the DNA with DNA polymerase, but occasionally feed in a special “chain-terminator” letter that halts copying. Do this billions of times and you get copies that stop at every possible position.

  1. Start with many copies of the DNA you want to read, plus a short primer to give the polymerase a place to begin.
  2. Let polymerase build new strands using normal letters mixed with a small amount of terminator letters that stop the chain when added.
  3. Each terminator carries a colour tag for A, C, G, or T, so every fragment is labelled by its last letter.
  4. Sort all the fragments by length with gel electrophoresis; reading the colours from shortest to longest spells out the sequence.
Template strand:   3'- T A C G G A T C ...
New strands stop at each position:
  A                 (length 1)
  A T               (length 2)
  A T G             (length 3)
  A T G C           (length 4)
  ...
Sort by length, read the last letter of each:
  A, T, G, C, C, T, A, G ...  ->  the sequence
Sanger's trick: a ladder of fragments ending at every position spells the sequence out one rung at a time.

The Human Genome Project: a 13-year marathon

Sanger sequencing reads only a few hundred letters at a time. To read 3.2 billion of them, the Human Genome Project (1990–2003) coordinated thousands of scientists across six countries, breaking the genome into manageable pieces, sequencing each, and stitching the overlaps back together. The public effort, alongside a private race led by Celera, produced the first essentially complete human genome sequence.

The Project's deepest legacy was not one person's DNA but a shared map everyone could build on. Its data were released freely as they were generated, a policy that shaped how genomics still works. The patched-together composite it produced became the first version of the reference genome — the subject of guide four.