The simplest definition of a gene is “a unit of DNA that contains the information to specify synthesis of a single polypeptide chain or functional RNA (such as a tRNA).” The DNA molecules of small viruses contain only a few genes, whereas the single DNA molecule in each of the chromosomes of higher animals and plants may contain several thousand genes. The vast majority of genes carry information used to build protein molecules, and it is the RNA copies of such protein-coding genes that constitute the mRNA molecules of cells.

During synthesis of RNA, the four-base language of DNA containing A, G, C, and T is simply copied, or transcribed, into the four-base language of RNA, which is identical except that U replaces T. In contrast, during protein synthesis, the four-base language of DNA and RNA is translated into the 20–amino acid language of proteins. In this section, we focus on the formation of functional mRNAs from protein-coding genes (see Figure 5-1, step 1). A similar process yields precursors of rRNAs and tRNAs, encoded by rRNA and tRNA genes; these precursors are then further modified to yield functional rRNAs and tRNAs (see Chapters 9 and 10). Similarly, thousands of micro-RNAs (miRNAs), which regulate the translation and stability of specific target mRNAs, are transcribed into precursors by RNA polymerases and processed into functional miRNAs (see Chapter 10). Other non-protein-coding (or simply noncoding) RNAs help to regulate the transcription of specific protein-coding genes. Regulation of transcription allows distinct sets of genes to be expressed in the multiple different types of cells that make up a multicellular organism. It also allows different amounts of mRNA to be transcribed from different genes, resulting in differences in the amounts of the encoded proteins in a cell. Regulation of transcription is addressed in Chapter 9.