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The structure of DNA

Genetics · DNA and the genome · note 1 of 3

Spec 3.4Download these notes (PDF)4 pages

The structure of DNASpec 3.4

DNA is a polymer made up of many repeating units called nucleotides. A polymer is a long molecule made of many similar smaller units joined together.

  • DNA is made of two strands coiled to form a double helix.
  • Each nucleotide consists of a sugar and a phosphate group, with one of four different bases attached to the sugar.
  • The two strands are linked by a series of complementary base pairs, joined together by weak hydrogen bonds.
The four bases and how they pair
BasePairs with
Adenine (A)Thymine (T)
Thymine (T)Adenine (A)
Cytosine (C)Guanine (G)
Guanine (G)Cytosine (C)

Because A always pairs with T and C always pairs with G, the base sequence on one strand tells you the sequence on the other. If one strand reads A T G C, the complementary strand reads T A C G.

A DNA double helix, an untwisted section showing complementary base pairs A–T and C–G joined by weak hydrogen bonds, and one nucleotide enlarged to show its phosphate group, sugar and base.Tap to enlarge
DNA is a double helix of nucleotides; the bases pair A with T and C with G, held by weak hydrogen bonds.