The document summarizes DNA sequencing methods. It discusses the DNA double helix structure and how the four nitrogenous bases form complementary pairs between strands. It then describes the two main historical DNA sequencing methods: the Maxam-Gilbert method which uses chemical degradation, and the Sanger method which is based on chain termination using dideoxynucleotides. The Sanger method is now the most common approach and involves sequencing in four separate reactions with one of the four ddNTPs added to each.