An organism’s genome is the ordered set of base-pairs making up all the genetic material in its chromosomes. (In phrases like ‘the human genome’, the genome is the set of genes normally present in organisms of a particular species.) The base-pairs — which in DNA are adenine, guanine, cytosine and thymine — link two chains of nucleotides in a double helix structure. Genome size is generally measured in terms of the total number of base pairs. There are 3.2 billion base pairs in the human genome and approximately 30,000 genes.