Chapter 1 - Using pea plants, Mendel revealed the...

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Unformatted text preview: Using pea plants, Mendel revealed the fundamental principles of transmission genetics. Work by others showed that genes are on chromosomes, and that mutant strains can be used to map genes on chromosomes. The discovery that DNA encodes genetic informationaand solving the structure of DNA and the mechanism of gene expression form the foundation of molecular genetics. 5-5 '3‘ The development of recombinant DNA technology revolutionized genetics and is the foundation for _'genome sequencing, including the Human fienome Project. o Biotechnology uses recombinant DNA technology to produce goods and services in a wide range of areas, including agriculture, medicine, and industry. The use of biotechnology has raised many legal and ethical issues involving the patenting of genetically modified organisms, and the use of gene therapy. {c‘an‘ilric‘isecr‘ \. " ' . tiny-.4tiffsjoirlen' ,-:'.T:3'f.‘€'i "‘ (Téi'l‘i'f'fii'fiifl'fi‘. - Model organisms have been used in genetics since the early part of the twentieth century. The extensive genetic knowledge gained from these organisms coupled with recombinant DNA technology and genomics makes these organisms useful as models to study human diseases. c Genetic technology is developing faster than policy, law, and convention in the use of this technology. Education and participation are key elements in the wise use of this technology. 3N0 saidvnj 2 Chapter 1 Introduction to GenetiCS rom our perspective early in the twentysfirst century, we can look back and ask when the interaction between ge— netic technology and society began to affect our lives. Did it begin when recombinant DNA technology was used to produce insulin, when the first food produced by genetic engi- neering reached the marketplace, or when gene therapy was first used to treat a genetic disorder? While each of these is an incremental step in using genetic knowledge in ways affecting society, we will focus on a case where the use of genetic tech— nology directly affects the citizens of an entire country. This case captures how genetics has a significant impact on society and provides a glimpse of future implications as more advances and applications are developed. In December 1998, a controversy affecting the 270,000 resi- dents of the island nation of Iceland was coming to a head. Fol- lowing months of heated debate, the Icelandic Parliament passed a law granting deCODE, a biotechnology company with headquarters in Iceland, a license to create and operate a data- base called IHD (Icelandic Health Sector Database) containing detailed information drawn from coded {to ensure anonymity) medical records of all Iceland residents. The law also allows de- CODE to cross-reference medical information from the IHD with a comprehensive genealogical database from the National ‘Archiyes. In additioggdigCQDEgan correlate information in - these two databases with results of DNA profiles collected from Icelandic donors. The combination of disease, genealogical, and genetic information is a powerful resource available ex- clusively to deCODE, which can market this information to re- searchers and companies for a period of 12 years. This is not a scenario from a movie such as Gottaca, but a real example of the interaction underway between genetics and society as we begin a new century. The development and use of these databases in Iceland has spurred the creation of simiw lar projects in other countries as well. The largest is the “UK Biobank“ effort launched in Great Britain in 2003. There, a huge database containing the genetic information of 500,000 Britons will be compiled from an initial group of [.2 million residents. The database will be used to search for susceptibil- ity genes that control complex traits. Similar projects to develop nationwide databases have been an“ nounced in Estonia, Latvia, Sweden. Singapore, and the Kingdom of Tonga, illustrating the global impact of genetic technology. In the United States, smaller-scale programs involving tens of thousands of individuals are underway at the Marshfield Clinic in Marshfield. Wisconsin; Northwestern University in Chicago, Illinois; and Howard University in Washington, DC. Why did deCODE select Iceland for such a project? Becauso for several reasons, the people of Iceland represent a unique case of genetic uniformity seldom seen or accessible to scientific investi- gation. This high degree of genetic relatedness results from the peopling of Iceland about 1,000 years ago by a small founding population drawn mainly front Scandinavian and Celtic sources, periodic population reductions by disease and natural disasters that further reduced genetic diversity and, until the least few decades, a lack of immigrants bringing new genes into the population. Thus, for geneticists trying to identify genes that control complex dis- orders, the Icelandic population is a tremendous asset. Because of the state—supported health care system, medical records exist for l cocled th all residents as far back as the early 1900s. Genealogical infor- mation is available in the National Archives and church records for almost every resident and for more than 500,000 of the estimated 750,000 individuals who have ever lived in Iceland. In spite of the associated controversies, the project already has a number of suc— cesses to its credit. Scientists at deCODE have identified genes associated with more than 25 of the most common diseases, iri— eluding asthma, heart disease, stroke, and osteoporosis. On the flip side of these successes are issues of privacy, con- sent, and commercialization—issues at the heart of many controversies arising from the applications of genetic technol- ogy. Scientists and nonscientists alike are considering the fate and control of genetic information as it is acquired, and the role of law and society in decisions about how and when genetic tech— nology is used. For example, how will knowledge of the complete nucleotide sequence of the human genome be used? More than at any other time in the history of science, addressing the ethi— cal questions surrounding an emerging technology is now as im— portant as the information gained from that technology. As you launch your study of genetics, remain sensitive to questions and issues like those just described. There has never been a more exciting time to be part of this science, but never has the need for caution and awareness of social issues been more apparent. This text will enable you to achieve a thorough understanding of modern—day genetics and its underlying prin- ciples. Along the way, enjoy your studies, but take your re- sponsibilities as a novice geneticist very seriously. From Mendel to DNA in Less Than a Century Because genetic processes are fundamental to the comprehen- sion of life itself, the discipline of genetics is thought by many to sit at the center ofbioiogy. Genetic information directs cel- lular function, largely determines an organism’s external ap- pearance, and serves as the link between generations in every species. As such, knowledge of genetics is essential to the thor- ough understanding of other disciplines, including molecular biology, cell biology, physiology. evolution. ecology, system— atics, and behavior. Genetics therefore unifies biology and serves as its core. Thus, it is not surprising that genetics has a long, rich history. Our starting point for this history is a monastery garden in central Europe in the 18605. Mendel’s Work on Transmi '5 garden (Figure 1—1 , conducted a decad l pea plants I ' f Trats g, 5%.,” . W _ i e further concluded that genes controlling a trait ex s "'1 pairs, and that members of a gene pair separate from each other during gamete formation. His work was published in 1866 but was largely ignored until it was partially duplicated and cited in papers by Carl Correns and others around 1900. Having been confirmed by others, Mendel's findings became FlGURE 1—1 The monastery garden where Gregor Mendel conducted his experiments with garden peas. In 1866, Mendel put forward the major postulates of transmission genetics. recognized as the basis for the transmi ‘ c . En The Chromosome Theory of Inheritance: Uniting Mendel and Meiosis 7_ Mendel did his work before the structure and role. of chromo- isornes lvais known. About twenty years after his work. advances in microscopy allowed researchers to identify chromosomes (Figure 1—2). and to establish that in eukaryotic organisms (those with a nucleus and cellular membrane systems). each species has a characteristic number of chromosomes called the diploid number (2:2). For example, humans have a diploid number of 46 {Figure 1—3). Chromosomes in diploid cells exist in pairs. called homologous chromosomes. Members of a pair are identical in size and location of the centromere. a structure to which spindle fibers attach during division. in addition, researchers in the last decades of the nineteenth century described the behavior of chromosomes during two forms of cell division, mitosis and meiosis. In mitosis (Figure 14). chromosomes are copied and distributed so that the two resulting daughter cells each receive a diploid set of 1.1 From Mendel to DNA in Less Than a Century 3 FIGURE 1—2 Colorized image of human mitotic chromosomes as visualized under the scanning electron microscope. FIGURE 1~3 A colorized image of the human male chromosome set. Arranged in this wayr the set is called a karyotype. FIGURE 1—4 A stage in mitosis (anaphase) when the chromosomes (stained blue) move apart. 4 Chapter‘l Introduction to Genetics I FIGURE 1—5 Chromosome l (the X chromosome) of D. melanogaster, showing the location of many genes. Chromosomes can contain hundreds of genes. scute bristles, 5c ' white eyes, w ruby eyes, rb _ e crossveinless ' wings, cv '5; 7 singed bristles, sn 7 chromosome-s. Meiosis is a form of cell division associated with gamete formation in animals and spore. for- mation in most plants. Cells produced by meiosis receive only one copy of each chromosomecalled the haploid lozenge eyes, lz a vermilion eyes, v i: - sable body, 5 (n_l_num_l_3_er of chromosomes. This reduction in chromosome number is r scalloped essential if the offspring arising from Win95: 50' two gametes are to maintain a con- ga, eyes, 3 stant number of chromosomes char— acteristic of their parents and other members of their species. Early in the twentieth century. Wal- ter Sutton and Theodore Boveri inde— pendently noted that genes and chromosomes have properties in com— mon and that the behavior of chroa mosomes during meiosis is identical to the behavior of genes during gamete formation. For example, genes and chromo- somes exist in pairs and members of a gene pair and members of a chromosome pair separate from each other during gamete formation. Based on these parallels, they each proposed that genes are carried on chromosomes (Figure 175). This proposal is the basis of the chromosome theory of inheritance. which carnation eyes, car little fly, if states that inherited traits are controlled by genes residing on' chromosomes that are faithfully transmitted through gametes. maintaining genetic continuity from generation to generation. Genetic Variation About the same time that the chromosome theory of inheriv lance was proposed. scientists began studying the inheritance of traits in the fruit fly. Drosophr'ln melanogrrsrer. Soon after. a white-eyed fly (Figure [—6) was discovered in a bottle cone taining normal [wild-type] red—eyed flies. This variation was produced by mutation. an. inherited change in the gene con trolling eye color. Chromosomal mutations affect the number and structure of chromosomes. Mutations, whether genetic or chromosomal are defined as any heritable change. and are the source of all genetic variation. The variant gene discovered in Drosophila represents an allele ofthe eye color gene. Alleles are defined as alternative forms of a gene. Different alleles may produce differences in the observable features. or phenotype of an organism. The set of alleles for a given trait carried by an organism is called the (W 5.3“ . FIGURE 1—6 The normal red eye color in Drosophila (bottom) and the white—eyed mutant (top). genotype. Using mutant genes as markers. geneticists were able to map the location of genes on chromosomes. The Search for the Chemical Nature of Genes: DNA or Protein? “fork on white-eyed Dmsophifa showed that the mutant trait had a pattern of inheritance that could be traced to a single chromosome. confirming the idea that genes are carried on chromosomes. Once this was established. investigators turned their attention to identifying which chemical component of f chromosomes carried genetic information. By the [9203, DNA and proteins Were identified as the major chemical components of chromosomes. Proteins are the most abundantcomponent in cells. There are a large number ofdifferent proteins and be- cause of their universal distribution in the nucleus and cyto- plasm. many researchers thought proteins would be shown to be the carrier of genetic information. In 1944. Avery, MacLeod, and McCarty, three researchers at the Rockefeller Institute in New York. provided experimental evidence that DNA was the carrier of genetic information in bacteria. This evidence, although clear—cut. failed to convince many influential scientists. Additional evidence for the role of DNA as a carrier of genetic information came from other re— searchers who worked with viruses lhat infect and kill cells of the bacterium Escherichia coir (Figure 1—7]. One of these viruses. called a bacteriophage. or phage for short, consists of a protein coat surrounding a DNA core. These experiments showed that the protein coat of the virus remains outside the cell. while the DNA enters the cell and directs the synthesis and assembly of more phage. This work was more proof that 1.2 Discovery of the Double Helix Launched the Recombinant DNA Era 5 FIGURE 1-—7 An electron micrograph showing T phage infecting a cell of the bacterium E. coli. DNA carries genetic information. Additional experiments over the next few years provided solid proof that DNA. not protein, is the genetic material. setting the stage for work to establish the structure of DNA. Discovery of the Double Helix Launched the Recombinant DNA Era Once it was accepted that nucleic acid in the form of DNA car- ries genetic information, efforts were focused on deciphering the structure of DNA, and the mechanism by which informa— tion stored in this molecule is expressed to produce an observe able phenotype. in the years after this was accomplished. researchers learned how to isolate and make copies of specific regions of DNA molecules. opening the way for the era of re— combinant DNA technology. The Structure of DNA and RNA DNA is a long. ladder-like molecule that forms a double helix. Each strand of the helix is a linear molecule made up of subunits called nucleotides. In DNA. there are four different nucleotides. Each DNA nucleotide contains one of four nitrogenous bases — A (adenine). G (guanine), T (thymine), or C (cytosine). These four bases comprise the genetic alphabet, or genetic code. which-in various combinations ultimately specify the amino acid sequence of proteins. One of the great discoveries of the twentieth century was made in 1953 by James Watson and Fran- cis Crick. who established that the two strands of DNA are exact complements of one another. such lhat' the rungs of the ladder in the double helix always consist of either A=T or GEC base pairs. As we shall see in a later chapter, this complement- ary relationship between adenine and thymine and between guanine and cytosine is critical to genetic function. This rela~ tionship serves as the basis for both the replication of DNA and for the basis of gene expression. During both processes. DNA b... .. c «t. .h _,«-- (36.: 3' I Sugar ‘ E (deoxyribose) h * Nucleotide * Phosphate Complementary base pair (thymine-adenine) FIGURE 1—3 Summary of the structure of DNA, illustrating the nature of the double helix (on the left) and the chemical components making up each strand (on the right). strands serve as templates for the synthesis of complementary molecules. Two depictions of the structure and components of DNA are shown in Figure [—8. RNA, another nucleic acid, is chemically similar to DNA. contains a different sugar (ribose vs. deoxyriboselin its nucleotides, and contains the nitrogenous base uracil in place of thymine. Additionally, in contrast to the double helix of DNA, RNA is generally single stranded. lmpbttantty,-tr can form complementary structures With a strand of DNA;- Gene Expression: From DNA to Phenotype As noted earlier. complementarity is the basis for steps in gene ex" pression. This process begins with the transcription of the Chem- ical information in DNA into RNA (Figure 1—9). OnggniRNA [UQchulecomplementary to one strand of DNA is transcribed. the RNA directs the synthesis of proteins. This is accomplished when the RNA%alled messenger RNA. or mRNA. for short— binds to a ribosome. The synthesis of proteins under the direc- tion omeNA is called translation ( bottom part of Figure 1—9). Proteins, as the end product of genes, are polymers made up of amino acid monomers. There are 20 different amino. acids in living organisms. How can information contained in mRNA direct the inser- tion of specific amino acids into protein chains as they are syn— thesized? The answer is now quite clear. The genetic code consists of a linear series of triplet nucleotides present in mRNA molecules. Each triplet reflects the information stored in DNA and specifies the insertion of a specific amino acid into the growing protein chain. This is accomplished by the action of adapter molecules called transfer RNA (tRNAJ_. Within the rtbosoiiiatRNAuecogtuaethejntormaumacedecl in the LRNA.lflpl€l§_E1Dd specii'thC properjmino acidfiirjascttion into theorem n_c.iur,i_ag,tt=_1_n§lation. As the preceding discussion shows. DNA makes RNA, which most often makes protein. These processes. known as the central dogma of genetics. occur with great specificity. Using Introduction to Genetics 6 Chapter 1 ):‘\ ,1 . . . [r l" l Transcrtptlon inRNA Translation complex Ribosome Translation l? rotei n FIGURE 1—9 Gene expression involves transcription of DNA into mRNA (top) and the translation (center) of mRNA on a ribosome into a protein (bottom). an alphabet of only four letters (A, T, C, and G). genes direct the synthesis of highly specific proteins that collectively serve as the basis for all biological function. Proteins and Biological Function As we have mentioned, proteins are the end products of gene expression. These molecules are responsible for imparting the properties that we attribute to living systems. The diverse na- ture of biological function rests with the fact that proteins are made front 20 different amino acids. In a protein chain that is just 100 amino acids in length, at each position there can be any one of 20 amino acids; the number of different 100 amino acid proteins. each with a unique sequence, is equal to 20lUU Because 2010 exceeds 5 X lOlZ, or more than 5 trillion, imagine how large 20100 is! Obviously. evolution has seized on a class of molecules with the potential for enormous structural divetu sity to serve as the mainstay of biological systems. The__large_s_t category 7 3f 7 proteins includes enzymes (Figure 1:10). Thelserrmoleeulesrserve as biological catalysm sentially allowing biochemical reactions to proceed at rates that FIGURE 1—10 The three—dimensional conformation of a protein. The amino acid sequence of the protein is depicted as a ribbon. sustain life under the conditions that exist on Earth. By lowers ing the energy of activation in reactions, metabolism is able to proceed under the direction of enzymes at body temperature. There are countgssprotiejns other than enzymes that are critical components of cells and organisms. These include hemoglobin, the oxygen-bindin g. pigment in red blood cells: insulin. the pancreatiohormone; collagen. the connective tissue. molecule; keratin, the. structural molecule in hair: histories, the proteins integral to chromosome structure in eukaryotes; actin and myosin. the contractile muscle pro- teins; and immunoglobulins= the antibody molecules of the immune system. The potential for such diverse functions rests with the enormous variations in threerdimensional confore [nation of proteins. This conformation is determined by the linear sequence of amino acids constituting the molecule. To come full circle, this sequence is dictated by the stored in- formation in the DNA of a gene that is transferred to RNA. which then directs the synthesis of a protein. DNA makes RNA that then makes protein. Linking Genotype to Phenotype: Sickle-Cell Anemia Once a protein is made, its action or location in a cell plays a role in producing a phenotype. When mutation alters a gene. it may abolish or alter that protein’s function. and cause an altered phe- notype. To trace the chain of events leading from the synthesis of a protein to a phenotype, we will examine sickie-cell anemia, 21 human genetic disorder. Sickle-cell anemia is caused by a tnuw tant form of hemoglobin, the protein that transports oxygen from the lungs to cells in the body (Figure 1—] Ufimoglobin isa composite molecule made up of twodifferent—preteans-rwglobin fitfifiglohjn, each encoded by a ditt‘erentgene. Each functional hemoglobin molecule contains two o—giobin and two fi-globin proteins. In sickleecell anemia, a mutation in the gene encoding fi—globin causes an amino acid substitution in l of the 146 amino acids in the protein. Figure 1712 shows part of the DNA sequence. 1.3 Genomics Grew Out of Recombinant DNA Technology 7 FlGURE1—11 The hemoglobin molecule, showing the two alpha chains and the two beta chains. A mutation in the gene for the beta chain produces abnormal hemoglobin molecules and sickle cell anemia. mRNA codons. and amino acid sequence for the normal and 1111.17 rant forms of B-globin. Notice that the mutation in sickle-cell anemia involves a change in one DNA nucleotide. leading to a change in codon 6 in the itiRNA from GAG to GUG. which in turn changes amino acid number 6 in B—globin from glutamic acid to valine. The other I45 amino acids in the protein are not changed by this mutation. Individuals with two mutant copies of the B-globin gene have sickle-cell anemia. The mutation causes hemoglobin molecules in red blood cells to polymerize when oxygen concentration is low. formng long chains that distort the shape of red blood cells (Figure 1—13). When th_e b_1_oog1_cells ate sickle shaped, llfiyulglggkfltho flowof blood in capilladesand small Noodles sols. causing severe pain and damage to tissues includingT the heart, brain. muscles. and kidneys. Sicklecei! anemia can re- sult in heart attacks and stroke, and can be fatal if left untreated. In addition. the deformed blood cells break easily. causing ane- mia by reducing the number of red cells in circulation. Thus. NORMAL B-GLOBIN DNA ......................... ..TCA GCA CTC CTC .......... .. ITIRNA ...................... ..ACU CCU GAG GAG .......... .. Amino acid ............ we "till—J pro glu ’qu ‘— ....... .. MUTANT [3-6 LOBIN DNA ......................... .TGA GOA CAC CTC .......... .. mRNA ...................... ..ACU GUG CTC .......... .. CCU Aminoacid ............ arm—Ear} ....... .. FIGURE 1—12 A single nucleotide change in the DNA encoding the ,B-globin gene (CTC % CAC} leads to an altered mRNA codon (GAG a» GUG) and the insertion of a different amino acid (glu —> val), producing an altered version of the fi-globin protein, causing sickle cell anemia. FIGURE 1-13 Normal red blood cells (round) and sickled red blood cells. The sickied cells block capillaries and small blood vessels. all the symptoms of this disorder are caused by a change in a single nucleotidein.ageneihat changes one amino acid otlt of 146 in the ,B-globin molecule. emphasizing the close relation— ship between genotype and phenotype. The Beglobin gene is not expressed until a few days after birth. so the mutant protein cannot be detected prenatally. How~ ever. by using recombinant DNA technology. the mutant gene can be detected prenatally. In addition. the genotypes of fam- ily members and others can also be determined. making it pos~ siblc for people to know if they carry a mutant copy of the gene and are at risk for having an affected child. Genomics Grew Out of Recombinant DNA Technology The era of recombinant DNA began in the early 1970s when re— searchers discovered that bacteria protect themselves from viral infection by making enzymes that restrict or prevent infection by cutting viral DNA at specific sites. When cut. the viral DNA cannot direct the synthesis of more phage particles. which when released. kill the infected bacterial cell. Scientists quickly rc- alized that such enzymes. called restriction enzymes, could be used to cttt DNA from any organism at specific nucleotide sequences. producing a reproducible set of fragments. This set the stage for the development ofcloning. or making large num— bers of copies of these DNA fragments. Making Recombinant DNA Molecules and Cloning DNA Soon after it was discovered that restriction enzymes could be used to produce specific DNA fragments. methods were do vclopcd to insert these fragments into carrier DNA molecules called vectors and transfer the combinetl vector and DNA ling- mcnt [a recombinant DNA molecule) into bacterial cells where hundreds or thousands of copies. or clones. of the. vector and 8 Chapter 1 Introduction to Genetics “T m .r 2’" fl” [1/ DNA fragment 1' . Vector [ ( \i Recombinant ‘ Insert into bacterial cell Clones produced FIGURE 1—14 In cloning, a vector and a DNA fragment produced by cutting with a restriction enzyme are joined to produce a recombinant DNA molecule that is transferred into a bacterial cell, where it is cloned into many copies by replication of the recombinant molecule and by division of the bacterial cell. DNA fragments are made {Figure l—l4). These cloned copies can be recovered from the bacterial cells and large amounts of the cloned DNA fragment can be isolated. Once large amounts of specific,D_NA.fragmentsbecame available__by cloning, they were used in many different ways: to isolate genes, to study their organization and ex-. pression, and to study their nucleotide sequence and evo— lution. In addition to preparing large amounts of specific DNA for research. recombinant DNA techniques were the foundation for the biotechnology industry (described in the next section of this chapter). As techniques became more refined, it became possible to clone larger and larger DNA fragments, paving the way to clone an organism’s genome. which includes all the DNA carried by that organism. Collections ofclones that contain an entire genome are called genomic libraries. Genomic Ii— braries are now available for hundreds of organisms. Sequencing Genomes: The Human Genome Project Once genomic libraries became available. scientists began to consider ways to sequence all theclones in a genomic library in an organized way to obtain the. nucleotide. se— quence of an organism’s genome. The Human Genome Project began in 1990 as a federally sponsored interna— tional effort to sequence the human genome and the genomes of several model organisms used in genetics re— search. At about the same time. other genome projects. sponsored by industry, got underway. The first genome from a free-living organism. a bacterium [Figure 1715). was sequenced and reported in 1995 by scientists at a biotechnology company. In 200] _. the publicly funded Human Genome Project and a private genome project undertaken by Celera Corpora- tion reported the first draft of the human genome sequence. covering about 96 percent ofthe gene—containing portion of the genome. In 2003, the remaining portion of the gene— coding sequence was completed and published. Work is now focused on sequencing the noneoding regions of the genome. Along the way, the genomes of five organisms used in genetic research, Escherichia t‘oli (bacterium). Stit‘cltaivmyces ceret-‘isiae (yeast), C nenor‘liribditis elegtms (a roundworm), the fruit fly (D. irreicurogtister), and the mouse (Mus muscnlirs) were also sequenced. As genome projects multiplied and more genome sew qucnces were deposited in databases, :1 new discipline called genomics, the study of genomes. came into exis- tence. Genomics uses nucleotide sequence information in databases to study the structure, function. and evolu- tion of genes and genomes. Genomics is drastically changing biology from a laboratory-based science to one combining lab experiments with information technology. Geneticists and other biologists can use information in databases containing nucleic acid sequences. protein se— quences. and gene interaction networks to answer exper- imental questions in a matter of minutes instead of months and years. FIGURE 1-15 A colorized electron micrograph of Haemophr’lus i'nfluenzae, a bacterium that was the first free-living organism to have its genome sequenced. This bacterium causes respiratory infections and bacterial meningitis in humans. Recombinant DNA technology has not only greatly acceler- ated the pace of research, generating new fields of study in- cluding genome projects and genomics, but has also given rise to the biotechnology industry. which has grown over the last 25 years to become a major component of the US. economy. The Impact of Biotechnology Is Growing Quietly and without much notice in the United States. biotech- nology products and services have moved into and revolution— ized many aspects of everyday life. Humans have used microorganisms. plants. and animals for thousands of years. but the development of recombinant DNA technology and associ- ated techniques allows us to genetically modify organisms in new ways and use them or their products to enhance our lives. Biotech— nology is the commercial use of these modified organisms or their products. It is found at the supermarket. in doctors’ offices. drug stores. department stores. hospitals and clinics. on farms. in or- chards. law enforcement. court—ordered child support, and even industrial chemicals. We will examine the impact ofhiotechnol- ogy on a small cross~section of everyday life. Plants, Animals, and the Food Supply The genetic modification of crop plants is one of the most rapidly expanding areas of biotechnology. Attention has been focused on traits such as resistance to herbicides. insects, and viruses. enhancement of oil content. and others (Table |.ll. Currently. over a dozen genetically modified crop plants have been approved for commercial use in the United States. with dozens more in field trials. Herbicide-resistant corn and soy— beans were first planted in the mid-19905. and now about 40 percent of the corn crop and 80 percent of the soybean crop is genetically modified. in addition. more than 60 percent of the canola crop and 70 percent of the cotton crop are grown from genetically modified strains. It is estimated that more than 60 1.4 The Impact of Biotechnology Is Growing 9 SOME GENETlCALLY ALTERED TRAITS IN CROP PLANTS Herbicide Resistance Corn, Soybeans, Rice, Cotton, Sugarbeets. Canola Insect Resistance Corn, Cotton, Potato Virus Resistance Potato, Yellow Squash, Papaya Altered Oil Content Soybeans. Canola Delayed Ripening Tomato percent of the processed food in the United States contains in- gredients from genetically modified crop plants. This agricultural transformation is not without controversy. Critics are concerned that the use of herbicide-resistant crop plants will lead to dependence on chemical weed management and may eventually lead to herbicide—resistant weeds. Others are concerned that traits in genetically engineered crops could be transferred to wild plants in a way that leads to irreversible changes in the ecosystem. We will examine these concerns in Chapter 22. Biotechnology is also being used to nutritionally enhance crop plants. More than one third ofthe world's population uses rice as a dietary staple. but most varieties of rice contain little or no vitamin A. Vita-min A deficiency causes more than 500.000 cases of blindness in children each year. A genetically engineered strain. called golden rice. has high levels of two compounds that the body converts to vitamin A. Golden rice is now in resting, and should be available for planting in the near future, its aim to reduce or eliminate this burden of disease. Other crops ineludin g wheat. corn. beans, and cassava are. also being modified to enhance nutritional value by increasing their vitamin and mineral content. Livestock such as sheep and cattle have been commercially cloned for more than 25 years. mainly by a method called em- bryo splitting. This melhod is used to produce two prize animals instead of one. In 1996. Dolly the sheep {Figure 146) was cloned by a new method in which the nucleus of a differenti— ated adult cell was transferred into an egg that had its nucleus removed. This nuclear transfer method makes it possible to produce hundreds or even thousands of offspring with desir- able traits. Cloning by nuclear transfer has many applications in agriculture. sports. and medicine. Some desirable traits. such as high milk production. or speed in race horses. do not appear until adulthood; animals with these traits can now be cloned Using differentiated cells. In medical applications. researchers have transferred human genes into animals so that as adults. they produce human proteins in their milk. By selecting and cloning animals with high levels of human protein production. hiopharmaceutical companies can produce a herd with uni- formly high rates of protein production. Human proteins are used as drugs. and proteins from transgenic animals are now being tested as treatments for diseases such as emphysema. if successful. these proteins will soon be commercially available. Introduction to Genetics 10 Chapter 1 FIGURE 1—16 Dollyr a Finn Dorset sheep cloned from the genetic material of an adult mammary cell, shown next to her first-born lamb, Bonnie. Who Owns Transgenic Organisms? Once produced. can a transgenic plant or animal be patented? The answer is_ yes. The United States Supreme Court ruled in 1980 that livirigorganisms can be patented. and the first organ- ism modified by recombinant DNA technology was patented in 1988 (Figure i—l7). Since then. dozens of plants and animals have been patented. The ethics of patenting living organisms is a contentious issue. Supporters of patentng argue that without the ability to patent the products of research to recover their costs. biotechnology companies will not invest in large-scale research and development. They further argue that patents represent an in— centive to develop new products because companies will reap the benefits from taking risks to bring new products to market. Eritics arguethat patents for organisms suchamopplanlsmu concentrate ownership of food production in the hands of a small number ofbiotechnology companies. making farmers econom- FIGURE 1-17 The first geneticaliy altered organism to be patented, mice from the cor strain, genetically engineered to be susceptible to many forms of war. These mice were designed for studying cancer development and the design of new anticancer drugs. ically dependent on seeds and pesticides produced by thesecome panics. and reducing the genetic diversity ofcrop plants as farm- ers discard local crops that might harbor important genes for registance to pe_sts_and disease. To resolve these and other issues about biotechnology and its uses. a combination of public awaue- ness. education, enlightened social policy. and legislation are needed. Biotechnology in Genetics and Medicine Biotechnology in the form of genetic testing and gene therapy has already become an important part of medicine. This tech« nology will shape medical practice in the twenty-first century. The importance of developing tests and treatments for genetic diseases is underscored by the estimate that more than it] mil- lion children or adults in the United States suffer from some form of genetic disorder and that every childbearing couple stands an approximately 3 percent risk of having a child with sotne form of genetic anomaly. The molecular basis for hun- dreds of genetic disorders is now. known (Figure 1—18). For example. the genes for disorders such as sickle-ceil anemia. cystic fibrosis, hemophilia. muscular dystrophy. phenylke— tonuria. and many other metabolic disorders have been cloned. These cloned genes are used for the prenatal detection of af- fected fetuses. In addition. parents can also learn of their status as “carriers” of a large number of inherited disorders. The com- bination of genetic testing and genetic counsciing gives couples objective information on which they can base informed deci- sions about childbearing. At present, genetic testing is available for several hundred inherited disorders. and this number will grow as more genes are identified. isolated. anti cloned. The use of genetic testing and other technologies. including gene ther— apy. have raised ethical concerns that have yet to be resolved. Instead of testing one gene at a time to discover whether someone carries a mutant gene that can produce a disorder in his or her offspring. technology is being developed that will allow screening of an individual’s genome to determine the person's risk of developing a genetic disorder or of having a child with a genetic disorder. This technology uses devices called DNA microarrays or DNA chips (Figure 149). Each chip contains thousands of fields. each carrying a different gene. In fact. chips carrying the human genome are now commer- cially available. making it possible to scan someone's entire genome to see which genetic diseases the individual carries or may develop. DNA chip technology has many other applica- tions as well. such as testing for gene expression in cancer cells to develop therapies tailored to specific forms of cancer. In addition to testing for genetic disorders. clinicians can transfer normal genes into individuals affected with genetic disorders in a procedure known as gene therapy. Although ini- tally suceessfttl, therapeutic failures and patient deaths have slowed its development. Recent advances in methods of gene transfer may reduce the risks involved. and it seems certain that gene therapy will become an important tool in treating inher- ited disorders. in fact. as more is learned about the molecuiar basis of human diseases. more therapies can be developed. Much of the present-day research on human genetic disorders involves the use of model organisms. 1.5 Genetic Studies Rely On the Use of Model Organisms 11 I DNA test currently available Adrenoleukodystrophy (ALB). Muscular Dystrophy ‘ fir“ Progressive deterioration i / of the muscles \ . J Hemophilia A 0 ._ l I. l Clotting deficiency ' l l Glucose-Calactose Malabsorptiou Syndrome. —- Potentially fatal digestive . disorder l Amyotrophic Lateral Sclerosis o r l (A15) Latevonset lethal “ degenerative nerve disease ADA Immune Deficiency- First hereditary condition treated by gene therapy FamiEial Hypercholesterulemia I l .‘ Extremely high cholesterol ‘\ ‘\ l l l l l \ \ 22 x Y 1 2 Myotonic Dyslrophyc —-L%\ H 21 Form oi adult 4 muscular dystrophy 2° 19 H Amyloidosis o mik_7_ _ “man i t . s-c-e. —. Accumulation In the tissues 18 chromosome of an insoluble lilJrillnr protein number Neuroflbromatosis (NH) t 7. Benign tumors oi nerve tissue below the skin / Breast Cancer- 5% of all cases Polycyslic Kidney Disease. J" Cysts resulting in enlarged kidneys and renal failure 1 ' ; Toy-Sachs Disease. ;‘ Fatal hereditary disorder r’ l involving lipid metabolism 3' l often occurring in Ashkenazi i Jews Aixheimer Disease " .‘ Degenerative brain disorder ‘ marked by premature senility ‘ Retinoblastoma I Childhood tumor of the eye // Fatal nerve disease , Azuospermla #— Gaucher Disease 0 A chronic enzyme deficiency occurring frequently among Ashkenazl Jews 9-- Ehlers-Danlos Syndrome Connective tissue disease fietlnitls Pigmentosa 0 Progressive degeneration of the retina /~ Huntington Disease I Lethal, late-onset. nerve degenerative disease .4’" Familial Adenomatous Polyposls (PAP). ' Intestinal polyps leading to colon cancer Hemochromatosisn Abnormain high absorption of iron from the diet " '* Spinocerebeiiar Ataxia I Destroys nerves in the brain and spinal cord, resulting in loss of muscle control 7 Cystic Fibrosis 0 Mucus in lungs, interlering with breathing Werner Syndrome O Premature aging Meianomae Tumors originating in the skin _ ""--' Multiple Endocrine Neoplasia, Type 2 0 Tumors in endocrine gland and other tissues Sickle-Cell Anemia I Chronic inherited anemia, in which red blood cells sickle, clogging arterioles and capillaries "7 Phenylketonuria (Pitui- An inborn error of metabolism; if untreated, results in mental retardation FlGURE 1—18 Diagram of the human chromosome set, showing the location of some genes whose mutant forms cause hereditary diseases. Conditions that can be diagnosed using DNA analysis are indicated by a red dot. FIGURE 1—19 A DNA microarray. The glass plate in the array contains thousands of fields to which DNA molecules are attached Using this microarray, DNA from an individual can be tested to detect mutant copies of genes. Genetic Studies Rely On the Use of Model Organisms After the rediscovery of Mentlcl‘s work in 1900. genetic research on a wide range ofnrgunisms continued that the principch of in— heritance he described were of universal significance among plants and animals. Although work on the gcnctics of many dill l‘crcnt organisms continued. veneticists gradually centered their attention on a small number of organisms. including Df’().\'(}[?rllllcl. the mouse (Mus musculux). and com (26:; ways] [Figure l—Zil). These organisms became popular for two main reasons: First. it was clear that genetic mechanisms were the same in most or— ganismS. and second, these species had several advantages loi- genetic research. They were easy to grow. had relatively short life cycles. produced many offspring. and genetic analysis as fairly struiglul'orward. Over limc. researchers created :1 large catalog of mutant strains for each species. These mutations were carefully studied. characterized. and mapped. Because of their well— devclupcd genetics, these species bccumc model organisms. Which we define as organisms used lor lhc study nl’ husic 12 Chapter 1 Introduction to Genetics biological processes, including normal cellular events as well as genetic disorders and other diseases. Model orgamisms. as we will see in later chapters, we used to study many aspects of biology. including aging. cancer. the immune system. and behavior. The Modern Set of Genetic Model Organisms Gradually. other species also became model organisms for resem'ch in genetics and modem biology. In the middle years of the [wear tieth century, viruses (such as the T phages and lambda phage) and microorganisms (including the bacterium Escherict'n'o mil. the yeast Sriccl'iaromycar t'eret-‘r'sioe. and the fungus Nerimspom consul became models (Figure 1—21). Some of these were chosen for the reasons outlined above. while others were selected because they allowed certain aspects of genetics to be studied more easily. In the last part of the century; three additional organisms were se- lected and developed as model organisms. Each began as a system used to study some aspect ol‘emblyonic development. To study the nervous system and its role in behavior. the nematode Ctmmrlicrbdiris elegcms IFigure 1—22t all was chosen as a model FIGURE 1—20 The first generation of model organisms in genetic analysis included (a) the mouse, (b) corn plants, and (c) the fruit fly. system. it is small. easy to grow, has a nervous system with only a few hundred cells. and has an unvarying program of cell specification during development. .I-lrnbict'opsr's rhrriinna [Figure I—ZEth is a small plant with a short life cycle that can be grown in the laboratory. [t was first used to study tlower develop- ment but has become a model organism for the study of many other aspects of plant biology. The zebralish. Douro mm. [Figure lélfltcl] has several advantages for the study oi'veltebrate development: it is small. reproduces rapidly, and the egg. embryo. and lam-ac are all transparent. In each of these species. geneticists collected large numbers ol‘mutants. making these organisms use— ful as model to study not only development but a wide range ol' other biological processes in plant and animal biology. Some ol‘ the early model organisms are now used to study a narrow range of problems. or have been replaced by other model organisms. Nenmsporn. once a central organism in FIGURE 1—21 Microbes that have become model organisms for genetic studies include (a) the yeast Saccharomyces, (b) the bacterium E. coil, and (c) the fungus Neurospora. 1.5 Genetic Studies Rely On the Use of Model Organisms 13 genetics. has been displaced by yeast and is now used mainly for research on specialized topics such as circadian rhythms. Corn has been largely replaced by Ambidopsis as a tnodel or— ganism for the study ot‘ Flowering plants. FIGURE 1—22 The third generation of model organisms in genetics includes (a) the roundworm C. elegans, (b) the plant Arabi'dopsis, and (c) the zebrafish. Model Organisms and Human Diseases The development ol’ recombinant DNA technology and the re- sults from genome sequencing projects have continued that all lil'c has a common on gin. and as a result. genes with similar functions in different organisms are similar or identical in structure and DNA sequence. ln addition. the ability to transfer genes across species has made it possible to develop models of human dis- eases in organisms ranging from bacteria. fungi, plants. and ani- mals [Table 1.2}. For these reasons. the Human Genome Project incorporated projects to sequence the genomes of five model or- ganisms in addition to the human genome. Other genome projects have sequenced the genomes ot’thc remaining model organisms. as well as the genomes of hundreds of other organisms. it may seem strange to study a human disease such as colon cancer by using E. coli. but the basic process of DNA repair {the DNA is defective in some forms of colon cancer} is the same in both organisms. and the gene involved (inan in E. coli and MLHl' in humans) is the same in both organisms. More impor— tant. E. call has the advantage of being easier to grow (the cells divide every 20 minutes) and it is easy to create and study new mutations in the mutL gene to help understand how it works. This knowledge may eventually lead to the development of drugs and other therapies to treat colon cancer in humans. Other model organisms. including the fruit fly. Drni'opi'rilu melmiogusrer. are being used to study specific human diseases. Over several decades. many mutant genes have been identified in Dmsopln'lu that produce phenotypes with abnormalities of the nervous system. including abnormalities of brain structure, adult— onset degeneration of the nervous system, and visual defects such as retinal degeneration. The information from genome sequenc— ing projects indicates that almost all these genes have human counterparts. As an example. genes involved in a complex human disease of the retina called retinitis pigmentosa are identical to Drosoplriln retinal degeneration genes such as rng and mlgC‘. Study of these mutations in Drosapi'zila is helping to dissect this complex disease and identity the function ol'the genes involved. Using recombinant DNA technology. Drosnphila is being used as a model to study diseases of the human nervous system by transferring a human disease gene into flies. In this way. it is pos- sible to create models for specific human diseases. Flies carrying human genes are used to study the effects of these mutant genes MODEL ORGANISMS USED TO STUDY HUMAN DISEASES Organism Human Diseases E. coli DNA repair; colon cancer and other cancers Yeast Cell cycle; cancer, Werner syndrome Drosophila Cell Signaling," cancer C. elegans Cell signaling; diabetes Zebra'fish Developmental pathways; cardiovascular disease Mouse Gene expression; LeschrNyhan disease, cystic fibrosis, fragile—X syndrome, and many other diseases 14 Chapter 1 introduction to Genetics on the development and function of the nervous system and its components. in addition to studying the mutant gene itself. the model system can be used to study genes affecting the expres- sion of the. human disease genes. and to test the effects of thera peutic drugs on the. action ot'tltese genes. studies that we difficult or impossible to do in humans. This gene transfer approach in Drosophr'la is being used to study almost a dozen human neue rodegenerative disorders. including Huntington disease. Machado— Joseph disease, myotonic dystrophy, and Alzheimer’s disease. As you lead through the text. you will encounter these model or- ganisms again and again in the genetic analysis ofbasic biological processes. Remember. they not only have a rich history in genetics. butane at the forefront in the study of human genetic dis— orders and infectious diseases. Keep in mind that understanding how a gene. controls a process in yeast is relevant to understand- ing the same generand the same process in normal human cells. The development and use of model organisms is only one of the ways genetics and biotechnology are rapidly changing many aspects of everyday life. As discussed in the next section. we have yet to reach a consensus on how and when this technol- We Live in the "Age of Genetics" Genetics is no longer just a laboratory science in which researchers study fruit flies or yeast to learn about basic processes in cell function or development. As stated in the beginning of this chapter. it is the core of biology. and the method of choice in dissecting and understanding the functions and malfunctions of biological systems. As knowl- edge has increased. genetics has become involved in many social issues. Genetics and its applications in the form of biotechnology are now developing much faster than social convention, public policy. and law. Although other scientific disciplines are also expanding in knowledge. none has par- alleled the growth of information occurring in genetics. While there has never been a more exciting time to be im- mersed in the study of genetics. the potential impact of this discipline on society has never been more profound. We are confident that by the end of this course you will agree that the present truly represents the “Age of Genetics.” and we urge you to think about and become a participant in the dialogue ogy is acceptable and useful. As you embark on your study of Genetics. we want you to be aware of a special feature of this text found at the con» clusion of most chapters: Genetics. Tech- nology and Society essays. in these essays we provide coverage of a variety of topics derived from genetics that impact on the lives of each of us. and thus on society in general. Genetics touches all aspects of modern life. Genetic technologies are rapidly changing how we approach medi- cine, agriculture, law, the pharmaceutical industry, and biotechnology, We now use hundreds of genetic tests to diagnose and predict the course of disease and to detect genetic defects in utero DNA based meth- ods allow scientists to trace the path of evo- lution taken by many species. including our own. We new design disease resistant and drought resistant crops, as well as more productive farm animals, using gene trans— fer techniques. We apply DNA profiling methods to paternity testing and murder investigations. Biotechnologies base on in- formation from genomics research have had dramatic effects on industry. The biotechnology industry doubles in size every decade. generating over 700,000 jobs and $50 billion in revenue each year. about genetics and its applications in society. Alond with these rapidly changing game based technoligies comes a challenging array of ethical dilemmas. Who owns and controls genetic information? Are gene— enhanced agricultural plants and animals safe for humans and the environment? Do we have the right to patent organism and profit from their commercialization? How can we ensure that genomic tech- nologies will be available to all and not just to the wealthy? What are the social is- sues that accompany the new reproduc— tive technoligies? It is a time when everyone needs to understand genetics in order to make complex personal and so- cietal choices. The goal of the Genetics, Technology and Society essays is to introduce topics that interface with society. including some of the new technologies based on genetics and genomics. As well, we will explore the relevant social and ethical issues. It is our hope that these essays will act as entry points for your exploration of the myriad applications and societal implications of modern genetics, Below, we list the topics that serve as the basis of many of these es- says (including the chapter in which each is found). Even should your Genetics course not cover all chapters. we hope that you GENETICS, TECHNOLOGY, AND SOCIETY will find the essays in those chapters of in- terest. Good reading! Tay-Sachs Disease (3) The Fate of Purebred Dogs (4) Edible Vaccines and Cholera (6) Human Sex Selection (7) Fragile Chromosomes and Cancer (8) Mitochondrial DNA and the Romanov's (9) The DNA Revolution (10) Telomerase, Aging and Cancer (1 ‘I) Antisense Technology (1 3) Mad Cow Disease and Prions (i 4) Chernobyl‘s Legacy (15) Quorum Sensing (16) Genetic Deregulation and Disease (1 7) Breast Cancer (18) DNA Fingerprinting and Forensics (19) Human Cloning (20) Gene Therapy (22) The Stem Cell Debate {23) The Green Revolution Revisited (24) Tracking Humans out of Africa (25) Eugenics (26) Conserving the Florida Panther (27) CHAPTER SUMMARY Problems and Discussion Questions 15 Mendel's work on pea plants established the principles of the transmission of genes front parents to offspring and Established the foundation for the science of genetics. Genes and chromosomes are the fundamental units in the chro- mosomal theory of inheritance. which explains the transmission of genetic information controlling phenotypic traits. Molecular genetics. based on the central dogma that DNA makes RNA—which makes protein—serves as the underpinnings of Mendelian genetics. referred to as transmission genetics. Recombinant DNA technology allows genes from one organism to be spliced into vectors and cloned. serving as the basis for a far-reaching technology used in molecular genetics. Genomics is one application of recombinant DNA technology in which the entire genetic makeup of an organism is sequenced 6. 7. and the structure and function of its genes are explored. The Human Genome Project is one example of genomics. Biotechnology has revolutionized agriculture, the pharmaceuti- cal industry, and medicine. it has made possible the mass pro, duction of medically important gene products. Genetic testing and gene therapy allow detection of individuals with genetic dis— orders and those at risk of having affected children. The use of model organisms in genetics has advanced our basic understanding of genetic mechanisms and. coupled with recom— binant DNA technology. has been used to develop models of human genetic diseases. Genetic technology is affecting many aspects of society. The de— velopment of policy and legislation is lagging behind the inno— vations and uses of biotechnology. PROBLEMS AND DISCUSSION QUESTIONS Describe Mendel’s conclusions about how traits are passed from generation to generation. What is the Chromosome theory of inheritance and how is it re- lated to Mendel's findings? Define genotype and phenotype and describe how they are related. What are alleles? If individuals carry genes in pairs; is it possi- ble for more than two alleles of a gene to exist? Given the state of knowledge at the time. why was it difficult for some scientists to accept that DNA is the carrier of genetic information? Contrast chromosomes and genes. How is genetic information encoded in a DNA molecule? Describe the central dogma of molecular genetics and how it serves as the basis of modern genetics. How many different proteins, each with a unique amino acid se- quence. are possible in a protein that is five amino acids long? 10. ll. Outline the roles played by restriction enzymes and vectors in cloning DNA. What impact has biotechnology had on crop plants in the United States“? Summarize the arguments for and against patenting genetically modified organisms. We all Cilt‘t'y 25,000—30.UOO genes in our genome. So far. patents have been issued for more than 6.000 of these genes. Do you think that companies or individuals should be able to patent human genes? Why or why not? How has the use of model organisms advanced our knowledge of the genes that control human diseases? if you knew that a devastating lute—onset inherited disease runs in your fatnin and you could he tested for it at the age of '20. would you want to know if you are a carrier? Would your an- swers be likely to change when you reach age 40‘? 16 Chapter 1 Introduction to Genetics SELECTED READINGS Barnum. SR. 2005. Biotechnology 2d ed. Belmont, CA: Brooks-Cole. Dale. 9.1.. Clarke. 13.. and Fumes. E,M.G. 2002. Potential for the envi- ronmental impact of transgenic crops. Nature Biotech. 20567674. Forlini. M. and Bonini. NM. 2000. Modeling human neurodegener- utive diseases in Dmmpl‘iila. Trends Genet. I6: 161—] 67. Lurquin, P. 2002. High Ted: Harvest. Boulder. CO: Westview Press. Potter, C..I.. Turenclmlk. (3.3., and Xu. T. 2000. Droxophila in cancer research: An cxpzmding role. Trends Genet. 163349. Pray. C.E.. Huang. J.. Hu. R.. and Rozelle. S. 2002. Five years of BL cotr ton in China—The benefits continue. The Plant Joumu! 3 l:423—430. Primrose. 8.8., and Twyman, RM. 2004. Genrmn'cs: Applications in Human Biology. Oxford, Blackwell Publishing. Weinberg. RA. 1985. The molecules of life. Sci. Am. (00L) 253:48757. Wisniewski. J-P.. Frange. N.. Mussonneau. A.. and Dumas. C. 2002. Between myth and reality: Genetically modified maize, an ex- ample of a sizeable scientific controversy. Biachimie 841095—1103. Yang. X.. Tian. X.C... Dai, Y.. and Wang. B. 2000. Transgenic farm animals: Applications in agriculture and binmedicine. Bimw‘hnol. Anna. Rev. 5:2697292. ...
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