double helix

Meaning: The double helix is the structure of double-stranded DNA: two polynucleotide strands coiled around one axis. The sugar-phosphate backbones lie outside and the bases point inward, pairing by hydrogen bonds: A with T (two bonds) and G with C (three). Each pair joins a purine to a pyrimidine, so the width stays constant. The strands are antiparallel, one running 5′→3′ and its partner 3′→5′. The common form, B-DNA, is right-handed and has major and minor grooves. Hydrogen bonds give pairing specificity, and base stacking adds much of the stability.

On the MCAT: Chargaff's rules apply: in double-stranded DNA, %A = %T and %G = %C, so purines equal pyrimidines. Given %G, you can find %A. A sequence with more G–C pairs has a higher melting temperature, because heat separates the strands (denaturation) and three hydrogen bonds per pair take more energy to break than two. The structure also explains semiconservative replication, since each strand serves as a template for its complement.

Common confusion: Chargaff's rules hold for double-stranded DNA, not for single-stranded DNA or most RNA, where A need not equal T or U. Also keep the bonds apart: hydrogen bonds hold the two strands together, while covalent phosphodiester bonds hold each strand together. Melting breaks the hydrogen bonds, not the phosphodiester bonds.