DNA is normally double‐stranded. The sequences of the two strands are related so that an A on one strand is matched by a T on the other strand; likewise, a G on one strand is matched by a C on the other strand. Thus, the fraction of bases in an organism's DNA that are A is equal to the fraction of bases that are T, and the fraction of bases that are G is equal to the fraction of bases that are C. For example, if one‐third of the bases are A, one‐third must be T, and because the amount of G equals the amount of C, one‐sixth of the bases will be G and one‐sixth will be C. The importance of this relationship, termed Chargraff's rules, was recognized by Watson and Crick, who proposed that the two strands form a double helix with the two strands arranged in an antiparallel fashion, interwound head‐to‐tail, as Figure 1 shows.
You usually read nucleic acid sequences of DNA in a 5′ to 3′ direction, so a DNA dinucleotide of (5 1) adenosine‐guanosine (3 1) is read as AG.
The complementary sequence is CT, because both sequences are read in the 5′ to 3′ direction. The terms 5′ and 3′ refer to the numbers of the carbons on the sugar portion of the nucleotide (the base is attached to the 1′ carbon of the sugar).
Complementarity is determined by base pairing—the formation of hydrogen bonds between two complementary strands of DNA. An A–T base pair forms two H‐bonds, one between the amino group of A and the keto group of T and the second one between the ring nitrogen of A and the hydrogen on the ring nitrogen of T. A G–C base pair forms three H‐bonds, one between the amino group of C and the keto group of G, one between the ring nitrogen of C and the hydrogen on the ring nitrogen of G, and a third between the amino group of G and the keto group of C. DNA's double helix is a result of the two strands winding together, stabilized by the formation of H–bonds, and of the bases stacking on each other, as Figure 2 shows.
Because an A on one strand must base‐pair with a T on the other strand, if the two strands are separated, each single strand can specify the composition of its partner by acting as a template. The DNA template strand does not carry out any enzymatic reaction but simply allows the replication machinery (an enzyme) to synthesize the complementary strand correctly. This dual‐template mechanism is termed semi‐conservative, because each DNA after replication is composed of one parental and one newly synthesized strand. Because the two strands of the DNA double helix are interwound, they also must be separated by the replication machinery to allow synthesis of the new strand. Figure shows this replication.