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One of the major roles of DNA is to encode proteins. This is important because the protein functions are determined by the composition and order of amino acids in the polypeptide chain. The process is basically the following: DNA contains a code within its sequence which is “transcribed” into another information molecule called ribonucleic acid (RNA). The process of transferring the information from DNA to RNA is called “transcription.” Transcription is sometimes referred to as “gene expression.” RNA is similar to DNA except that: (i) it is a single-stranded copy of one of the DNA strands; (ii) its structural backbone contains the sugar “ribose” rather than “deoxyribose”; and (iii) it substitutes the nucleic acid uracil (U) for thymidine (T) wherever thymidine would have occurred based on the sequence of the DNA molecule. In transcription, one of the DNA strands is used as a template to make a complementary RNA strand such that a sequence “ATTCGAAGG” of DNA, for example, is transcribed to an RNA strand with the sequence “UAAGCUUCC.” The transcribed RNA strand is only a section of the DNA representing the gene of interest. It is therefore short and moves easily through the cell to engage the protein-manufacturing complex called a ribosome. Ribosomes travel down the RNA molecule, reading each set of three nucleotides and adding 1 of 20 amino acids according to the instructions from the genetic code. The term “translation” denotes the process of reading the RNA molecule and producing the protein.

Amino acids are small molecules which can be joined in series to create longer molecules called polypeptides, more commonly referred to as proteins. Proteins are the linear arrangement of tens to thousands of amino acids from among the basic set of 20 different amino acids (Table 4.1). The differences between amino acids reside in the side chains attached to the amino and carboxyl core of the molecule. Some of the side chains repel water, some attract water, some are basic or acidic, others have the capacity to form attachments with other amino acids (disulfide bonds). Altogether, the combination of amino acids and their side chains causes the folding of the linear peptide and provides clefts, pockets, and receptor sites that make the protein biologically active as a structure or an enzyme. Examples of proteins include hemoglobin, immunoglobulin, and the diverse molecules making up muscle fibers, as well as the liver enzymes which detoxify blood and blood clotting enzymes which heal wounds. Mammalian genomes contain over 20,000 genes for proteins (Chapter 6).

Table 4.1. The genetic code based on RNA sequences read by the ribosome. The triplet codes for each of the 20 amino acids found in proteins are presented, as well as the codon signals that start and stop protein synthesis.