BOX 4.1 FUNDAMENTALS OF GENOME RESEARCH
''Molecular biology is the discipline that demonstrated the relationship between genes and proteins. Molecular biologists determined that the gene is made of DNA (deoxyribonucleic acid)—that is, DNA is the hereditary material of all species. What is more, in what is now scientific legend, Crick and Watson determined in 1953 that the structure is a double helix and concluded correctly that this specific form is fundamental to DNA's function as the agent of storage and transfer of genetic information. In fact, in biology generally, shape determines properties—that is, structure almost always determines function.
''The DNA double helix is both elegant and simple. Each strand of the DNA double helix is a polymer consisting of four elements called nucleotides: A, T, C, and G (the abbreviations for adenine, thymine, cytosine, and guanine). The two strands of DNA are perfectly complementary: whenever there is a T on one strand, there is an A on the corresponding position on the other strand; whenever there is a G on one strand, there is a C on the corresponding position on the other. That is, T pairs with A, and G pairs with C. This complete redundancy accounts for how a cell can pass on a complete set of genetic information to each of its two daughter cells during cell division: the DNA double helix unravels, and each strand serves as a completely sufficient template upon which a second strand can be synthesized. In addition to providing an easy mechanism for the replication of DNA, the redundancy also provides great resiliency against loss or damage of information during the life of a cell. Such loss or damage of information, when it occurs, is the basis for biological mutations.
"From a computer scientist's point of view, the DNA double helix is a clever and robust information storage and transmission system. As Computer scientists accustomed to dealing with a binary alphabet will immediately recognize, the four-letter alphabet of DNA is sufficient for encoding messages of arbitrary complexity.
"In brief, particular stretches of the DNA are copied directly into an intermediate molecule called RNA (ribonucleic acid, also composed of A, T, C, and G). RNA is then translated into a protein—which is again a linear chain, but one assembled from 20 different building blocks called amino acids. Each Consecutive triplet of DNA elements specifies one amino acid in the protein chain. In this fashion, biology "reads" DNA (actually, the RNA copy of the DNA) ... [as if it were a computer program].
"Once synthesized, the protein chain folds according to laws of physics into a specialized form, based on the particular properties and order of the amino acids (some of which are hydrophobic, some hydrophilic, some positively charged, and some negatively charged). Although this basic coding scheme is well understood, biologists are not yet able to predict accurately the shape in which the protein will fold.
"In total, the human genome (the totality of genetic information in each of us) contains about 3 billion nucleotides. These are distributed among 23 separate strands called chromosomes, each containing about 50 million to 250 million nucleotides. Each chromosome encodes about 10,000 to 50,000 genes.
"With the extraordinary advances in molecular biology over the past 20 years, it is now possible to read the specific sequences of individual genes and to predict (by means of the genetic code) the sequence Of the proteins that they encode. A major challenge for molecular biology in the next decade Will be to use this information to predict the actual biological function of these proteins."
SOURCE: Reprinted, with permission, from Lander et al. (1991), p. 35.