Hydrogen Bonding between DNA Bases

DNA consists of two strands twisted around each other to form a double helix. The sugar-phosphate backbones are on the outside, and the bases point inwards, where each base on one strand pairs with a base on the other.The two strands are held together by hydrogen bonds between the bases. The bases have N-H groups (H bonded to electronegative N) and lone pairs on N and O atoms, arranged so that only particular pairs line up correctly:
• adenine pairs with thymine (A-T), forming two hydrogen bonds
• guanine pairs with cytosine (G-C), forming three hydrogen bondsThis is called complementary base pairing. In each pair one base is large (two rings: A or G) and one is small (one ring: T or C), so the distance between the two backbones stays the same along the helix. The positions of the hydrogen bond donors and acceptors mean that A cannot pair properly with C, nor G with T.So the sequence of one strand fixes the sequence of the other. This is what allows DNA to be copied: the strands separate, and each acts as a template for a new strand. Hydrogen bonds are weak compared with covalent bonds, so the strands can separate without breaking the backbones.Worked example. One strand reads A-G-C-T-T. Give the complementary strand and count the hydrogen bonds. The complementary strand is T-C-G-A-A. Pairs: A-T (2), G-C (3), C-G (3), T-A (2), T-A (2), so there are 12 hydrogen bonds in total.