The answer is 'at the very start (5' end) of the mrna. According to the central dogma, dna -> rna -> protein. messenger rna carries genetic code from dna to be translated into protein by the ribosome. One difference between Rna and dna is that rna has uracil while dna has thiamine bases. Therefore the start codon on mrna will be AUG rather ATG.
Because amino acids can be arranged in many different combinations, it's possible for your body to make thousands of different kinds of proteins from just the same 21 amino acids. You may see books that say there are only 20 amino acids.
