Codon: the sequence of three nucleotides that codes for one amino acid.
Ribosomes pull the mRNA strand through itself one codon at a time. As the strand moves, the start codon and its complementary tRNA molecule shift into the 2nd site inside the large subunit. This shift leaves the 1st site empty, which exposes the next mRNA codon.
by hwsparkles January 12, 2012