DNA contains the genetic code for building and maintaining organisms. It is made up of nucleotides that pair together in a double helix structure. Rosalind Franklin discovered through X-ray diffraction that DNA has a spiral shape, which Watson and Crick used to build a double helix model showing that DNA is made of two strands of alternating sugar-phosphate units connected by complementary nucleotide base pairs. DNA replicates through splitting and using each strand as a template to form two new double helices.