Nucleic acids, DNA and RNA, are polymers whose units are nucleotides. A nucleotide is made of three parts: a phosphate group, a sugar with five carbon atoms, and a base that contains nitrogen.
The sugar is deoxyribose, \mathrm{C_5H_{10}O_4}, in DNA (deoxyribonucleic acid), and ribose, \mathrm{C_5H_{10}O_5}, in RNA (ribonucleic acid). Deoxyribose has one oxygen atom fewer than ribose.
DNA has four bases: adenine (A), guanine (G), cytosine (C) and thymine (T). RNA has uracil (U) in place of thymine.
Two nucleotides join when the phosphate group of one and the sugar of the other lose a water molecule. Repeating this gives a long chain called a polynucleotide.
In the chain, sugars and phosphate groups alternate to form a backbone, and a base sticks out from each sugar.
The order of the bases along the chain is the genetic information.
The bases form pairs through hydrogen bonds. Adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three.
Only these combinations fit together, so the base sequence of one chain decides the base sequence of the chain that pairs with it. Such sequences are said to be complementary.
This is why DNA contains equal amounts of A and T, and equal amounts of G and C.
DNA is made of two polynucleotide chains joined by base pairs and twisted into a double helix, with the sugar-phosphate backbones on the outside and the base pairs on the inside.
In 1953, James Watson and Francis Crick proposed this structure, using the X-ray diffraction results obtained by Rosalind Franklin and Maurice Wilkins.
When a cell divides, the two chains separate and each is used to build a new partner chain, so the same genetic information is passed on.
RNA has ribose as its sugar and uracil in place of thymine, and it is usually a single chain.
Part of the base sequence of DNA is copied into RNA, with uracil pairing with adenine. This is called transcription.
Every three bases of the RNA decide one amino acid, and the amino acids are joined in that order to make a protein. This is called translation.