You may recall from Chapter 3 that double-stranded DNA consists of a pair of deoxyribonucleotide polymers wound around each other in antiparallel helical coils in such a way that a purine base (A or G) in one strand is paired with a pyrimidine base (T or C, respectively) in the other strand. To say that the strands are antiparallel means that they run in opposite directions: One strand runs in the 5′-to-3′ direction and the other runs in the 3′-to-5′ direction. These key elements of DNA structure are the only essential pieces of information needed to understand the mechanism of DNA replication.