mRNA stands for messenger RNA and it serves as the bridge between the two phases of protein biosynthesis. In the transcription phase, a sequence of DNA unbinds and the sequence of nucleotides is used to make a complementary strand of messenger RNA. The mRNA then exits the nucleus and goes to a ribosome for the translation phase, where the mRNA is converted to a polypeptide sequence of amino acids to make a new protein. A codon refers to a sequence of three nucleotide that in turn codes for a specific amino acid. In addition, there are stop and start codons which tell the ribosome to either start or stop a new polypeptide sequence. So a codon is made up of three nucleotides.