The Double Helix

• consists of 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 pentose 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'
  
  
• 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 pentose 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 with 10 nucleotide pairs, so each turn takes 34 Å.
  
  
• 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)