THE GENOME
An organism’s genetic recipe is encoded on the strings of DNA that form the chromosomes within each of its cells. The code is written using just four different chemical ‘letters’, and their precise order determines which protein building blocks are manufactured. An organism’s entire genetic code is called a genome. In humans it contains more than three billion letters, though the axolotl salamander’s is 10 times as big. Hundreds of species’ genomes have now been sequenced, providing a wealth of information on evolutionary relationships and biological function.