The DNA is the blueprint for protein synthesis. The nucleotide sequence on the DNA that codes for the eventual protein is what is read by the RNA polymerase (note this is the non-coding or opposite side of the DNA strand) which converts or transcribes this first blueprint into mRNA (message RNA). This mRNA is then subsequently read by a ribosome which translates the nucleotide sequence on the mRNA into a series of amino acids giving rise to the polypeptide (or protein). The ribosome reads the mRNA nucleotide sequence in a series of 3 nucleotides (called a codon), and charged tRNAs bring the specific amino acids corresponding to each codon to the ribosome where the polypeptide is made.
If you don't have the right DNA sequence to begin with - you don't get the right protein!
