The two strands of a DNA double helix are held together by hydrogen bonds between the bases. The bases pair in a fixed way, called complementary base pairing:• adenine (A) pairs with thymine (T), joined by two hydrogen bonds
• cytosine (C) pairs with guanine (G), joined by three hydrogen bondsEach pair is always one purine (A or G, two rings) with one pyrimidine (T or C, one ring). This keeps the distance between the two backbones the same all along the molecule, so the helix has a constant width.
When RNA pairs with DNA (or with other RNA), uracil (U) replaces thymine, so adenine pairs with uracil.Individually hydrogen bonds are weak, which lets the strands be separated easily for copying. But there are so many of them that the double helix is very stable. DNA with a high proportion of C-G pairs is slightly more stable because each C-G pair has three hydrogen bonds.Complementary pairing means the sequence of one strand fixes the sequence of the other. This is why each strand can act as a template for accurate copying.Worked example: a DNA sample contains 22% adenine. A = T, so thymine = 22%. That leaves 100 − 44 = 56% for C and G, and C = G, so each is 28%.
• cytosine (C) pairs with guanine (G), joined by three hydrogen bondsEach pair is always one purine (A or G, two rings) with one pyrimidine (T or C, one ring). This keeps the distance between the two backbones the same all along the molecule, so the helix has a constant width.
When RNA pairs with DNA (or with other RNA), uracil (U) replaces thymine, so adenine pairs with uracil.Individually hydrogen bonds are weak, which lets the strands be separated easily for copying. But there are so many of them that the double helix is very stable. DNA with a high proportion of C-G pairs is slightly more stable because each C-G pair has three hydrogen bonds.Complementary pairing means the sequence of one strand fixes the sequence of the other. This is why each strand can act as a template for accurate copying.Worked example: a DNA sample contains 22% adenine. A = T, so thymine = 22%. That leaves 100 − 44 = 56% for C and G, and C = G, so each is 28%.