DNA uses a code made up of four letters (actually molecules, ATCG) – we call it Nucleobasis.

  • Adenine
  • Thymine
  • Cytosine
  • Guanine

If you imagine the ladder they form up the rungs (pairs). Watson-Crick base pair rule: A pairs with T, C pairs with G.

Nucleotide is nucleobase + phosphate + d-sugar.