Molecules of DNA are in the form of a twisted ladder called a double helix.
The “rungs” of the ladder make up the 4-letter DNA code:  A, T, C, and G.
The 4 DNA bases bond together according to special rules:     A always bonds to T, and C always bonds to G.
A – T       C – G      
The DNA code is read as 3-letter “words” that make up longer sequences called genes.
ACG  GGC  TAA  TGC  ATT  AAA  GGC  CCT  AAG  CCT  ATG  CCC 
GGC  ATT   CTG TCC  TAA  CCC  GCT  GCA  TAC   TCT  GGT   CGA... 
The “rungs” of the DNA double helix ladder are made up of a 4-letter code that represents the DNA bases: A, T, C, G. These four bases bond to each other in specific ways, so A always pairs up with T and C always bonds with C.
When you “read” a DNA code, you read only one side of the double helix. The code letters are read as 3-letter “words.” The words are strung together like sentences to form longer bits of information called “genes.”