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Following the initial isolation of insulin in 1921, diabetic patients could be treated with insulin obtained from the pancreases of cattle and pigs. Unfortunately, some patients developed an allergic reaction to this insulin because its amino acid sequence was not identical to that of human insulin. In the 1970s, an intense research effort began that eventually led to the production of genetically engineered human insulin—the first genetically engineered product to be approved for medical use. To accomplish this feat, researchers first had to determine how insulin is made in the body and then find a way of causing the same process to occur in nonhuman organisms, such as bacteria or yeast cells. Many aspects of these discoveries are presented in this chapter on nucleic acids.
Figure 19.1 Human Insulin Products Now Being Used
Source: Photo courtesy of Mr. Hyde, http://commons.wikimedia.org/wiki/File:Inzul%C3%ADn.jpg.
Dogs have puppies that grow up to be dogs. Foxes have kits that grow up to be foxes. From viruses to humans, each species reproduces after its own kind. Furthermore, within each multicellular organism, every tissue is composed of cells specific to that tissue. What accounts for this specificity at all levels of reproduction? How does a fertilized egg “know” that it should develop into a kangaroo and not a koala? What makes stomach cells produce gastric acid, whereas pancreatic cells produce insulin? The blueprint for the reproduction and the maintenance of each organism is found in the nuclei of its cells, concentrated in elongated, threadlike structures called chromosomesAn elongated, threadlike structure composed of protein and DNA that contains the genetic blueprint.. These complex structures, consisting of DNA and proteins, contain the basic units of heredity, called genesThe basic unit of heredity.. The number of chromosomes (and genes) varies with each species. Human body cells have 23 pairs of chromosomes having 20,000–40,000 different genes.
Sperm and egg cells contain only a single copy of each chromosome; that is, they contain only one member of each chromosome pair. Thus, in sexual reproduction, the entire complement of chromosomes is achieved only when an egg and sperm combine. A new individual receives half its hereditary material from each parent.
Calling the unit of heredity a “gene” merely gives it a name. But what really are genes and how is the information they contain expressed? One definition of a gene is that it is a segment of DNA that constitutes the code for a specific polypeptide. If genes are segments of DNA, we need to learn more about the structure and physiological function of DNA. We begin by looking at the small molecules needed to form DNA and RNA (ribonucleic acid)—the nucleotides.