See Related Article: Systems Biology: New Views of Life
Consider a living cell, the fundamental unit of life. Each human cell contains the entire human genomesome 35,000 genes. But only some genes are expressed within a specific cell, resulting in the production of specific proteins. The genes that turn on in a liver cell, for example, are different from the genes that are expressed in a brain cell.
Under the Human Genome Project funded by the Department of Energy and the National Institutes of Health, various sequencing tools have been used to determine the order of the building blocks of genes. Genes are made of millions of deoxyribonucleic acid (DNA) molecules. A DNA molecule is constructed like a spiral staircase, or a double helix. The rails of the staircase are made of a backbone structure of phosphates and sugars, and the steps are pairs of four nitrogen-containing basesadenine (A), cytosine (C), guanine (G), and thymine (T). Through hydrogen bonds the two rails of the staircase are kept together, A and T pair together, and C and G are partners (see image below). For example, a sequence of TACAT would bond specifically with a sequence of ATGTA.
A gene is made of a unique sequence of DNA bases; it is like a message containing a unique combination of letters. This message is translated into information for protein production. A protein is a folded chain of amino acids in a specific order; up to 20 different amino acids exist. The message for each amino acid within a protein is dictated by a sequence of three DNA bases. If the key word in the message provided by the gene is misspelledsay, part of it is supposed to be CATTAG but instead is spelled CATGAGthen it will have a point mutation (G substitutes for the T that should be there). As a result of this altered DNA base, the gene may produce a protein that has an incorrect shape so it won't dock with another protein (e.g., a receptor), leading to a mistake in the resulting message. In other words, if the message in the gene is misspelled, the protein it encodes may be wrong and its function in the body may be changed, sometimes for the worse.
It was once believed that each gene codes for a single protein. However, experimental and computational evidence (partly obtained at ORNL) shows that many genes produce an average of three different proteins and as many as ten protein products. Genes have protein-coding regions (exons) interspersed with non-coding regions (introns). Through "alternative splicing," a gene's exons can be combined in different ways to make variants of the complete protein.
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