How did you know which bases to use when you transcribed the dna sequence to mrna codons?

Nucleotide bases are adenine, thymine, guanine, cytosine, and uracil (unique in RNA). Adenine is always paired with thymine or uracil and guanine is always paired with cytosine. In transcription, a single strand of DNA is transcribed to RNA with the respective base pairing except RNA does not contain thymine instead it uses uracil. 

For example:
AAACGTTACCGACATTAC

Transcribing the following DNA sequence to RNA will yield:
UUUGCAAUGGCUGUAAUG