To transcribe the mRNA to make the DNA, we use genetic codes called codons. These are composed of three amino acids clumped together. It could be Adenine(A), Guanine(G), Uracil(U) and Cytosine (C). The difference is that first thread contains GUG, while the second one contains AGU. GUG is for Histidine while AGU is for Serine.
