The genome is the store of biological information possessed by every organism on the planet. The vast majority of genomes are made of DNA, the few exceptions being those viruses that have RNA genomes. Genome expression is the process by which the information contained in the genome is released to the cell. The first product of genome expression is the transcriptome, the collection of RNAs derived from those genes that are active at a particular time. The second product is the proteome, the cell’s repertoire of proteins that specify the nature of the biochemical reactions that the cell is able to carry out. Experimental evidence showing that genes are made of DNA was first obtained in 1945–1952, but it was the discovery of the double-helix structure by Watson and Crick in 1953 that convinced biologists that DNA is indeed the genetic material. A DNA polynucleotide is an unbranched polymer made up of multiple copies of four chemically diferent nucleotides. In the double helix, two polynucleotides are wound around one another, with the nucleotide bases on the inside of the molecule. The polynucleotides are linked by hydrogen bonding between the bases, with A always base-paired to T and G always base-paired to C. RNA is also a polynucleotide but the individual nucleotides have diferent structures compared with those found in DNA, and RNA is usually single-stranded. A cell contains various types of RNA, including mRNAs, which are transcripts of protein-coding genes, and several types of noncoding RNA. Many RNAs are initially synthesized as precursor molecules, which are processed by cutting and joining reactions and by chemical modifcation to give the mature forms. Proteins are also unbranched polymers, but in proteins the units are amino acids linked by peptide bonds. The amino acid sequence is the primary structure of a protein, the higher levels of structure—secondary, tertiary, and quaternary—being formed by folding of the primary structure into three-dimensional conformations and by association of individual polypeptides into multiprotein structures. Proteins are functionally diverse because individual amino acids have diferent chemical properties that, when combined in diferent ways, result in proteins with a range of chemical features. Proteins are synthesized by translation of mRNAs, with the rules of the genetic code specifying which triplet of nucleotides codes for which amino acid. Te genetic code is not universal: variations occur in mitochondria and in lower eukaryotes, and some codons can have two diferent meanings in a single gene.