So we essentially have the mRNA chain, the ribosome attaches onto the mRNA chain and reads the codons on the mRNA chain and brings those amino acids and builds that polypeptide chain that are essentially complementary to those codons. Now, how exactly does the ribosome actually know where to begin and where to end the process of translation on that mRNA molecule? Well, in prokaryotic cells and eukaryotic cells we have the specific sequences of nucleotides. These codons known as start codons and stop codons. And these are used to essentially initiate and terminate the process of translation. So let's briefly discuss how this takes place in prokaryotes and eukaryotes.