The DNA Helix

Deoxyribonucleic acid, or DNA, is made up of two polynucleotide chains.

Each chain has repeating units called nucleotides, and there are four types of nucleotides.

Each nucleotide has three parts: a phosphate group, a sugar called deoxyribose, and one of four bases—adenine or A, cytosine or C, guanine or G, or thymine or T.

The sugar and phosphate groups form the sugar-phosphate backbone, while the bases extend from this backbone.

Along the backbone of each strand, phosphodiester bonds link the 3′ carbon of one sugar to the phosphate group attached to the 5′ carbon of the next sugar in the chain.

The two strands run in opposite directions, forming an antiparallel arrangement and the bases pair in a specific way: adenine pairs with thymine and cytosine pairs with guanine. These pairs are held together by hydrogen bonds.

The arrangement of base pairs and the sugar-phosphate backbone causes the DNA strands to twist around each other and form a three-dimensional double helix with about ten base pairs per turn.