The simplest definition of a gene is a "unit of DNA that contains the information to specify synthesis of a single polypep-tide chain or functional RNA (such as a tRNA)." The vast majority of genes carry information to build protein molecules, and it is the RNA copies of such protein-coding genes that constitute the mRNA molecules of cells. 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.
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 formation of functional mRNAs from protein-coding genes (see Figure 4-1, step 1). A similar process yields the precursors of rRNAs and tRNAs encoded by rRNA and tRNA genes; these precursors are then further modified to yield functional rRNAs and tRNAs (Chapter 12).
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