Prokaryotic cells amino terminus

The protein synthesis machinery reads the RNA template starting from the 5 end (the end made first) and makes proteins beginning with the amino terminus. These directionalities are set up so that in prokaryotes, protein synthesis can begin even before the RNA synthesis is complete. Simultaneous transcription-translation can t happen in eukaryotic cells because the nuclear membrane separates the ribosome from the nucleus. 

Synthesis of all polypeptide chains In prokaryotic and eukaryotic cells begins with the amino acid methionine. In most mRNAs, the start (initiator) codon specifying this amino-terminal methionine is AUG. In a few bacterial mRNAs, GUG is used as the initiator codon, and CUG occasionally is used as an initiator codon for methionine in eukaryotes. The three codons UAA, UGA, and UAG do not specify amino acids but constitute stop (termination) codons that mark the carboxyl terminus of polypeptide chains in almost all cells. The sequence of codons that runs from a specific... 

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