Deoxyribonucleic acid (DNA), the fundamental genetic material of life, specifies the amino acid sequence of the large number of proteins that make up cells. Nucleotides are the building blocks of DNA. DNA consists of nucleotide chains, and each nucleotide consists of a nitrogenous base (purine or pyrimidine), a pentose sugar (2-deoxyribose in DNA and ribose in ribonucleic acid [RNA]), and a phosphate group.The four bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T); uridine (U) replaces thymine in RNA (Fig.1).The nucleotide sequence determines the amino acids encoded. It is the sequence-specific pairing of these nucleotides that is the basis for inheritance of the genetic code. A binds to T, and G binds to C: it is from this pairing that the double helical structure of DNA is derived, with its unique ability to reproduce very accurately over many generations (each chain acting as a unique template to which complementary base pairs bind).The genetic information in DNA is transferred to RNA through a process called transcription and from RNA to peptides (proteins) through translation.