DNA double helix structure

Summary

DNA most commonly exists as a double-stranded molecule whose two complementary strands twist around each other to form the characteristic double helix. Each strand is a chain of nucleotides made from a deoxyribose sugar, a phosphate group, and one of four nucleobases: adenine (A), thymine (T), cytosine (C), or guanine (G). The bases point inward and pair with hydrogen bonds—A with T (two hydrogen bonds) and G with C (three hydrogen bonds)—while the sugar and phosphate form a backbone on the outside of the helix. The two DNA strands run in opposite directions (anti-parallel): one strand is oriented 5′→3′ and the other 3′→5′. This directionality matters because DNA polymerase can add new nucleotides only to the 3′ end of a growing strand. Within a strand, nucleotides are linked by stronger phosphodiester bonds, whereas hydrogen bonds stabilize base pairing across the helix. Because the base-pairing information is complementary, separating the strands allows each original strand to serve as a template for synthesizing a new partner strand during replication (semiconservative replication).

Key Points

• 1

  DNA’s double helix consists of two complementary strands held together by base pairing (A–T and G–C) via hydrogen bonds.

• 2

  Nucleotides form a sugar-phosphate backbone through phosphodiester bonds, while bases point inward to form hydrogen-bonded base pairs.

• 3

  DNA strands are anti-parallel (5′→3′ vs 3′→5′), and polymerase synthesis depends on adding nucleotides to the 3′ end.

• 4

  During replication, strand separation enables each original strand to template synthesis of a new complementary strand (semiconservative replication).