Here is a cell, the basic unit of all living tissue. In most human cells there is a structure called the nucleus. The nucleus contains the genome. In humans the genome is split between 23 pairs of chromosomes. Each chromosome contains a long strand of DNA, tightly packaged around proteins called histones. Within the DNA are sections called genes. These genes contain the instructions for making proteins. When a gene is switched on, an enzyme called RNA polymerase attaches to the start of the gene. It moves along the DNA making a strand of messenger RNA out of free bases in the nucleus. The DNA code determines the order in which the free bases are added to the messenger RNA. This process is called transcription. Before the messenger RNA can be used as a template for the production of proteins, it needs to be processed. This involves removing and adding sections of RNA. The messenger RNA then moves out of the nucleus into the cytoplasm. Protein factories in the cytoplasm, called ribosomes, bind to the messenger RNA. The ribosome reads the code in the messenger RNA to produce a chain made up of amino acids. There are 20 different types of amino acid. Transfer RNA molecules carry the amino acids to the ribosome. The messenger RNA is read three bases at a time. As each triplet is read, a transfer RNA delivers the corresponding amino acid. This is added to a growing chain of amino acids. Once the last amino acid has been added, the chain folds into a complex 3D shape to form the protein.