Protein biosynthesis is a two step process. The first step is called transcription. This is when the DNA in the nucleus unfolds and a portion of the double helix separates into single strands. One of the strands is used to create a complementary strand of messenger RNA. The messenger RNA then exits the nucleus and the second step of the process called translation begins. The messenger RNA travels through the cytoplasm to the ribosome on the rough endoplasmic reticulum. In the ribosome, the mRNA is used to create a sequence of amino acids. Groups of three nucleotides on the mRNA strand are called codons and each codon corresponds to a particular amino acid. The amino acids are brought into the ribosome by transfer RNA and join together via amide bond formation to become a polypeptide chain. The polypeptide chain then folds into a completed protein and exits the ribosome.