The Double Helix

• It contains two polynucleotide strands wound around each other.
  
  
• The backbone of each consists of alternating deoxyribose and phosphate groups.
  
  
• The phosphate group bonded to the 5' carbon atom of one deoxyribose is covalently bonded to the 3' carbon of the next.
  
  
• The two strands are "antiparallel"; that is, one strand runs 5′ to 3′ while the other runs 3′ to 5′.
  
  
• The DNA strands are assembled in the 5′ to 3′ direction and, by convention, we "read" them the same way.
  
  
• The purine or pyrimidine attached to each deoxyribose projects in toward the axis of the helix.
  
  
• Each base forms hydrogen bonds with the one directly opposite it, forming base pairs (also called nucleotide pairs).

  Discussion of base pairing in DNA

  
  
• 3.4 Å separate the planes in which adjacent base pairs are located.
  
  
• The double helix makes a complete turn in just over 10 nucleotide pairs, so each turn takes a little more (35.7 Å to be exact) than the 34 Å shown in the diagram.
  
  
• There is an average of 25 hydrogen bonds within each complete turn of the double helix providing a stability of binding about as strong as what a covalent bond would provide.
  
  
• The diameter of the helix is 20 Å.
  
  
• The helix can be virtually any length; when fully stretched, some DNA molecules are as much as 5 cm (2 inches!) long.
  
  
• The path taken by the two backbones forms a major (wider) groove (from "34 A" to the top of the arrow) and a minor (narrower) groove (the one below).