DNA with base pairs. Computer representation of a segment of the molecule Deoxyribonucleic Acid (DNA), showing the pairing of nucleotide bases. The DNA molecule contains inherited instructions needed for the development of a living organism. DNA is composed of two strands twisted into a helical shape. Each strand consists of an outer sugar-phosphate backbone (light blue) from which nucleotide bases project inwardly (spheres). The base guanine (orange) pairs with cytosine (green), and adenine (blue) with thymine (red). Hydrogen bonds between base pairs are yellow and lilac. The sequence of these pairs, like rungs on a ladder, make up the genetic code in the form of genes.