Module section description: The chromosomes of an organism contain genes that encode all of the protein and RNA molecules required to construct an organism. Gene expression is the process through which information in a gene is used to produce the final gene product: a protein or an RNA molecule. The process of gene expression is used by all known life - eukaryotes (including multi cellular organisms), prokaryotes (bacteria, archea) and viruses. Cells can regulate gene expression at every step along the way, from DNA to RNA and the final protein. Genetic information in DNA is first copied to form an RNA molecule, in a process known as transcription. The RNA used to make proteins is called messenger RNA (mRNA) because it carries information from the DNA to the ribosome, where protein synthesis occurs. Many kinds of eukaryotic and only limited number of prokaryotic mRNA molecules are further processed by splicing, which removes intervening sequences called introns. The mRNA serves as a template to guide protein synthesis. Protein synthesis is named "translation" because ribosomes translate an mRNA sequence into a protein sequence. This is the first stage of protein biosynthesis (part of the overall process of gene expression) that occurs in the cytoplasm where the ribosomes are located. During the synthesis, 20 different amino acids can be incorporated in proteins. After translation, the posttranslational modification of amino acids extends the range of functions of the protein by attaching to it other biochemical functional groups such as acetate, phosphate, various lipids and carbohydrates, by changing the chemical nature of an amino acid (e.g. citrullination) or by making structural changes, like the formation of disulfide bridges. Also, most nascent polypeptides start with the amino acid methionine because the "start" codon on mRNA also codes for this amino acid. This amino acid is usually taken off during post-translational modification. Post-translational modification of proteins is detected by mass spectrometry or Eastern blotting.