The flow of genetic information in all living cells is DNA → RNA → protein. This chapter explains the mechanisms by which cells copy DNA into RNA and then use the information in RNA to make protein. Transcription and translation are the means by which cells read out, or express, the instructions in their genes. Many identical RNA copies can be made from the same gene, and each RNA molecule can direct the synthesis of many identical protein molecules. The RNA molecules being transcribed from the two ribosomal genes—ribosomal RNAs—are not translated into protein, but are instead used directly as components of ribosomes, macromolecular machines made of RNA and protein. Most protein-coding genes in eukaryotic cells are composed of a number of coding regions, called exons, interspersed with larger noncoding regions, called introns. When a eukaryotic gene is transcribed from DNA into RNA, both the exons and introns are copied.