14–3 Human Molecular Genetics

Watson and Crick took the first step in making genetics amolecular science when they discovered the double-helical

structure of DNA in 1953. Today, the transformation theystarted is complete. The exploration of human genes is now amajor scientific undertaking. Biologists can now read, analyze,and even change the molecular code of genes.

Human DNA AnalysisThe roughly 6 billion base pairs you carry in your DNA are a bitlike an encyclopedia with thousands of volumes. In principle,biologists would like to know everything the volumes contain,but as a practical matter there isn’t enough time to read all ofthem. Nonetheless, if you’ve used an encyclopedia you’vealready learned one of the ways to handle huge amounts ofinformation—you find a way to look up only what you need. Inan encyclopedia, you can use an index or an alphabetical list ofarticles. As you might suspect, biologists search the volumes ofthe human genome using sequences of DNA bases.

Testing for Alleles If two prospective parents suspect theymight be carrying recessive alleles for a genetic disorder such ascystic fibrosis (CF) or Tay-Sachs disease, how could they find outfor sure? Because the Tay-Sachs and CF alleles have slightlydifferent DNA sequences from their normal counterparts, avariety of genetic tests have been developed that can spot thosedifferences. Sometimes these genetic tests use labeled DNAprobes. These are specific DNA base sequences that detect thecomplementary base sequences found in disease-causing alleles.Other tests search for changes in restriction enzyme cuttingsites. Tests also detect differences between the lengths of normaland abnormal alleles.

Genetic tests are now available forhundreds of disorders, making it possi-ble to determine whether prospectiveparents risk passing such alleles to theirchildren. In an increasing number ofsuch cases, DNA testing can pinpointthe exact genetic basis of a disorder,making it possible to develop moreeffective treatment for individualsaffected by genetic disease.

Key Concepts• What is the goal of the Human

Genome Project?• What is gene therapy?

VocabularyDNA fingerprinting

Reading Strategy:Finding Main Ideas As youread, find evidence to supportthe following statement: Theinfluence of human moleculargenetics on society is growingrapidly.

� Figure 14–17 This laboratory workeris preparing a report on DNA evidence.The inset shows vials of DNA lying on aprintout of a DNA analysis chart.

1 FOCUSObjectives14.3.1 Summarize methods of

human DNA analysis.14.3.2 State the goal of the Human

Genome Project.14.3.3 Describe how researchers are

attempting to cure geneticdisorders.

Vocabulary PreviewInvite student volunteers to describewhat a fingerprint is and what it isused for. Then, challenge them tomake inferences about what DNA fin-gerprinting is. List the inferences onthe board, and narrow them down totwo or three. Revisit this list through-out the section, if necessary, until theclass correctly defines the term.

Reading StrategyInstruct students to write the state-ment from the text onto a sheet ofpaper. As they read, students shouldlist under the statement the evidencethat supports it.

2 INSTRUCT

Human DNA AnalysisBuild Science SkillsDesigning Experiments Challengestudents to write the steps in a proto-col in which they test for the allele ofa gene that causes a genetic disorderusing restriction enzymes and gelelectrophoresis. Students who needan extra challenge can list the stepsto testing for alleles using a labeledDNA probe. Encourage students touse what they learned in Chapter 13.They might also wish to use theInternet to find additional resources.

The Human Genome 355

SECTION RESOURCES

Print:

• Teaching Resources, Section Review 14–3,Chapter 14 Real-World Lab

• Reading and Study Workbook A,Section 14–3

• Adapted Reading and Study Workbook B,Section 14–3

• Issues and Decision Making, Issues andDecisions 9, 10, 11, 12

• Biotechnology Manual, Labs 2, 11, 12;Concepts 2, 3, 4, 6

• Lesson Plans, Section 14–3

Technology:

• iText, Section 14–3• Animated Biological Concepts Videotape

Library, 30• Transparencies Plus, Section 14–3

Tim

eSaver

Section 14–3

356 Chapter 14

Use VisualsFigure 14–18 Go over the steps inthe procedure used in DNA finger-printing. Have volunteers describewhat occurs in each step. Make sureall students understand which DNAsequences are being targeted andhow restriction enzymes are used tofind similarities and differencesbetween DNA samples. Ask: Why isDNA fingerprinting more accurateif the samples are cut with morethan one restriction enzyme?(There is a greater chance of findingdifferences in the sequences, whichtranslates to restriction fragments ofvarying sizes.)

Use Community ResourcesInvite a forensics expert to the classto describe how DNA evidence isused in criminal cases. Suggest thatthe expert describe how DNA evi-dence is collected at the crime sceneand how it is manipulated in the lab-oratory. Before the expert visits theclass, have students brainstorm forquestions to ask the expert. Encouragestudents to ask their questions duringthe expert’s presentation.

� The DNA fragments are separated ac-cording to size using gel electrophoresis. The fragments containing repeats are then labeled using radioactive probes. This produces a series of bands—the DNA fingerprint.

Gel

GeneA

GeneB

12 repeats 4 repeats

9 repeats 6 repeats

GeneC

GeneA

GeneB

GeneC

Restriction enzyme

GeneB

GeneC

GeneB

GeneC

GeneA

GeneA

� Chromosomes contain large amounts of DNA called repeats that do not code for proteins. This DNA varies from person to person. Here, one sample has 12 repeats between genes A and B, while the second sample has 9 repeats.

� Restriction enzymes are used to cut the DNA into fragments containing genes and repeats. Note that the repeat fragments from these two samples are of different lengths.

DNA fingerprinting can be used to determine whether blood, sperm, or othermaterial left at a crime scene matches DNA from a suspect. InterpretingGraphics In the DNA fingerprint below, does the DNA fingerprint from the evidence(E) match suspect 1 (S1) or suspect 2 (S2)?

DNA fingerprint Gel electrophoresis

FIGURE 14–18 DNA FINGERPRINTING

14–3 (continued)

English Language LearnersTo help students have a clearer understandingof what the Human Genome Project signifies,introduce the word genome to them. Explainthat a genome is the genetic content of a cell.Remind students that all of the cells in thehuman body have the same genetic content, orgenome. If they don’t understand why, reviewmitosis and its role in growth and development.

Advanced LearnersSome students might wish to learn about theactual human DNA sample that was sequenced.Have them find out if this DNA came from onlyone person or if it was a mixture from manypeople. Also have them find out how scientistsplan to handle the fact that every human indi-vidual has a different DNA sequence. Encouragestudents to present their findings to the class.

The Human GenomeProjectDemonstrationDemonstrate to students howsequencing the human genome wasmuch like putting together a puzzleor solving a word puzzle. Copy onthe board the X chromosome and itsgenes from Figure 14–12 on page350. Make up short sequences ofDNA on the board and assign themto various places on the X chromo-some. Explain that these sequencesare the “markers” sequenced by government scientists. Then, demon-strate the “shotgun” sequencingmethod by cutting up into very shortpieces a length of yarn that repre-sents the X chromosome. Choosefour or five of these short yarn pieces,and give each a DNA sequence.Write the sequences on the board.Set it up so that students can deter-mine where the DNA segments fromthe yarn fragments are located onthe X chromosome based on themarker sequences.

The Human Genome 357

DNA Fingerprinting The great complexity of the humangenome ensures that no individual is exactly like any othergenetically—except, of course, for identical twins. Molecularbiology has used this biological fact to add a powerful new toolcalled to the identification of individuals.Unlike other forms of testing, DNA fingerprinting does notanalyze the cell’s most important genes, which are largelyidentical among most people. Rather, DNA fingerprintinganalyzes sections of DNA that have little or no known functionbut vary widely from one individual to another.

Figure 14–18 shows how DNA fingerprinting works. A smallsample of human DNA is cut with a restriction enzyme. Theresulting fragments are separated by size using gel electrophore-sis. Fragments containing these highly variable regions are thendetected with a DNA probe, revealing a series of DNA bands ofvarious sizes. If enough combinations of restriction enzymes andprobes are used, a pattern of bands is produced that can bedistinguished statistically from the pattern of any other individ-ual in the world. DNA samples can be obtained from blood,sperm, and even hair strands with tissue at the base.

DNA fingerprinting has been used in the UnitedStates since the late 1980s. The reliability of DNAevidence has helped convict criminals as well as over-turn many convictions. The precision that molecularbiology brings to the justice system is good news notonly for those who are victims of crime but also forthose who have been wrongly convicted.

The Human Genome ProjectAdvances in DNA sequencing technologies at the closeof the twentieth century made it possible, for the firsttime, to sequence entire genomes. At first, biologistsworked on relatively small genomes, such as those ofviruses and bacteria. The DNA sequence of the commonbacterium Escherichia coli, which was determined in1996, contains “only” 4,639,221 base pairs, making itjust about as long as this textbook if it were printed onpaper in a readable typeface. The genomes of even thesimplest eukaryotic organisms are much larger, and thehuman genome, which contains over 6 billion base pairs,is nearly 1400 times as large.

Despite the problem of size, in 1990, scientists inthe United States and other countries began the HumanGenome Project. The Human Genome Project is anongoing effort to analyze the human DNA sequence.Along the way, investigators completed the genomes of severalother organisms, including yeast—a unicellular eukaryote—andDrosophila melanogaster, the fruit fly. In June 2000, scientistsannounced that a working copy of the human genome wasessentially complete.

DNA fingerprinting

� Figure 14–19 The HumanGenome Project is an ongoingeffort to analyze the human DNAsequence. Dr. Francis Collins and Dr. Craig Venter, who headed thepublic and private portions of theproject, jointly announced the com-pletion of a working draft of thehuman genome sequence.

Answer to . . . Figure 14–18 It matches suspect 2.

The beginning of the genomic raceThe Human Genome Project officially began in1990 with two ultimate goals: identify and mapevery gene to its chromosome and determinethe entire DNA sequence for the humangenome. Research centers, universities, and pri-vate companies in the United States and aroundthe world began work on this multibillion-dollar

project, which was first estimated to take 20years to complete. The first step of the projectwas completed in 1993, when a group inFrance completed a rough map of geneticmarkers for the entire genome. These markerswere used to help researchers map the locationsof various DNA fragments.

HISTORY OF SCIENCE

358 Chapter 14

Address MisconceptionsStudents may think that scientistsnow know everything there is toknow about the human genome.Point out that researchers haveuncovered the “big picture,” butthey still need to learn many details.Ask: About how many bases arepresent in a small human chromo-some, such as chromosome 22?(About 43 million base pairs, as notedon page 349) What does that num-ber indicate about the totalnumber of bases in all the chromo-somes? (There are probably well over1 billion bases.) What are somequestions about the humangenome that researchers still needto investigate? (Samples: Whichbases form genes that code for majortraits? Which genes play a major rolein human health?)

Rapid Sequencing How did they do it? Scientists first deter-mined the sequence of bases in widely separated regions of DNA.These regions were then used as markers, not unlike the milemarkers along a road thousands of miles long. The markers madeit possible to locate and return to specific locations in the genome.

Scientists then used a technique known as “shotgun sequenc-ing.” This method involved cutting DNA into random fragmentsand then determining the sequence of bases in each fragment.Computers found areas of overlap between the fragments and putthe fragments together by linking the overlapping areas. Thecomputers then aligned the fragments relative to the knownmarkers on each chromosome. The entire process is something likeputting a jigsaw puzzle together, but instead of matching shapes,the scientists match identical base sequences.

Searching for Genes Only a small part of a human DNAmolecule is made up of genes. In fact, one of the genome’s scientificsurprises was how few genes it seems to contain—possibly as fewas 25,000. Since the genome of the fruit fly Drosophila containsapproximately 14,000 genes and that of a tiny worm roughly20,000, many researchers had expected to find far more in our ownDNA. The final number, however, is far from certain.

Molecular biologists continue to search for genes, which theycan locate in several ways. In one method, they find genes byfinding DNA sequences that are known to be promoters, which arebinding sites for RNA polymerase. Promoters indicate the start of agene. Shortly behind the promoter, there should be an open readingframe. An open reading frame is a sequence of DNA bases that will

produce an mRNA sequence, which then specifies aseries of amino acids. Recall that for most genes, themRNA coding regions, or exons, are interrupted byintrons, which are noncoding regions. Therefore,investigators have to find the introns as well as theexons in order to follow the gene through its completelength, as shown in Figure 14–20.

Research groups around the world are analyzingthe huge amount of information in the DNAsequence, looking for genes that may provide usefulclues to some of the basic properties of life. In addi-tion to its scientific significance, understanding thestructure and control of key genes may have commer-cial value. Biotechnology companies are rushing tofind genetic information that may be useful in devel-oping new drugs and treatments for diseases.

A Breakthrough for Everyone One of theremarkable things about genome research is theopen availability of nearly all its data. From its verybeginning, data from publicly supported research onthe human genome have been posted on the Interneton a daily basis. You can read the latest genome datathere and, if you wish, analyze it. The Web site forthis textbook links to the Human Genome Project.

Promoter Startcodon

Intron

Insulin gene

Intron Stopcodon

� Figure 14–20 Researchersexploring the human genome canuse DNA sequences to locate manygenes. Promoters are sequences inwhich RNA polymerase can bind toDNA. A typical gene, such as thegene for insulin shown below, hasother DNA sequences that mayserve as signals for RNA polymeraseto start and stop transcription.Interpreting Graphics In whichdirection would RNA polymerasemove in transcribing the insulingene?

After completing the unit on genetics, I have stu-dents work cooperatively searching the Internetand scientific journals for the current research onone of these topics: the Human Genome Project,genetic engineering, DNA fingerprinting, geneticscreening, gene therapy, genetic counseling, orspecific ethical issues in genetics. Then I haveeach group create a presentation to their class-mates by means of a pamphlet, a multimedia

presentation, or a role-playing activity. This strat-egy successfully helps students develop anawareness of the latest innovations in genetics.

—Tracy SwedlundBiology TeacherMedford Area Senior HighMedford, WI

TEACHER TO TEACHER

14–3 (continued)

• High school students interested ingenetics should take courses inbiology, chemistry, and physics, aswell as English and math. Collegestudents should choose majorssuch as biochemistry, biology, orchemistry.

• Encourage students interested instudying genetics to find out aboutresearch going on in the social sciences, such as anthropology, history, and psychology.

ResourcesStudents can contact a universitygenetics department, the GeneticsSociety of America, the AmericanBoard of Genetic Counseling, or thegenetics department of a local hospital.

Careers in Biology

You can have students write amore extensive job description aswell as list the educational require-ments for a career in this field.

Gene TherapyUse VisualsFigure 14–21 Help students under-stand the procedure of gene therapy.Encourage them to compare it toanimal cell transformation. Ask:What is used to carry the DNA intothe cell instead of a plasmid? (Avirus) Why doesn’t the virus causedisease in an individual? (The viralDNA has been modified to prevent thevirus from causing disease.) Remindstudents about the control of geneexpression in eukaryotes. Ask: Wouldall body cells produce hemoglobin?(No, only cells in which hemoglobin isproduced would express the hemoglo-bin gene.) Why do researchers injectthe hemoglobin gene into bonemarrow cells and not into musclecells or skin cells? (Muscle and skincells do not produce hemoglobin; thehemoglobin gene is not expressed inthese cells. It is expressed only in bonemarrow cells, where red blood cells areproduced.)

The Human Genome 359

Gene TherapyThe Human Genome Project will have an impact on society aswell as on scientific thought. For example, information about thehuman genome might be used to cure genetic disorders by genetherapy. Gene therapy is the process of changing the gene thatcauses a genetic disorder. In gene therapy, an absent orfaulty gene is replaced by a normal, working gene. Thisway, the body can make the correct protein or enzyme it needs,which eliminates the cause of the disorder.

The first authorized attempt to cure a human genetic disor-der by gene transfer occurred in 1990. Then, in 1999, a youngFrench girl was apparently cured of an inherited immune disor-der when cells from her bone marrow were removed, modified inthe laboratory, and then placed back in her body. However,scientists do not yet know how long the beneficial effects of thistreatment will last.

Figure 14–21 on the next page shows one of the ways inwhich researchers have attempted to practice gene therapy.Viruses are often used because of their ability to enter a cell’sDNA. The virus particles are modified so that they cannot causedisease. Then, a DNA fragment containing a replacement geneis spliced to viral DNA. The patient is then infected with themodified virus particles, which should carry the gene into cellsto correct genetic defects.

GeneticistJob Description: work in the laboratories ofuniversities or large companies doing molecular-level research, or work in a clinic collecting familyhistories and counseling people who have agenetic disorder or who may carry a geneticdisorder

Education: a master’s degree or doctorate ingenetics or related field; some medical researchpositions require a medical degree, as well

Skills: good verbal and written communicationskills, analytical, detail-oriented, caring, organ-ized, curious, and able to meet deadlines

Highlights: You have the opportunity todiscover new genes or patterns of heredity thatcan help treat or cure genetic disorders or tomake sure that these types of discoveries areput into practice to improve public health.

Careers in Biology

For: Career links

Visit: PHSchool.com

Web Code: cbb-4143

For: More information on the Human Genome Project

Visit: PHSchool.com

Web Code: cbe-4143

Transforming human cellsThe first federally approved transfer of cells withforeign genes into a human occurred in May1989, at the Clinical Center of the NationalInstitutes of Health in Bethesda, Maryland.

Cancer-fighting cells into which a foreign genehad been inserted were infused into the blood-stream of a cancer patient who had volunteeredfor the experiment. The primary purpose of the

manipulation was to make the cells easily identifi-able so that doctors could track them in thepatient’s body. The patient was not expected tobenefit directly.

The cells, tumor-infiltrating lymphocytes, hadbeen taken from the patient’s cancerous tissueand treated in the laboratory to increase theirnumbers and, thus, their ability to attack the can-cer tissue.

HISTORY OF SCIENCE

Answer to . . .

Figure 14–20 It moves from thepromoter to the stop signal (in thediagram, from left to right).

You can have students visit theWeb site of the Human GenomeProject for additional information.

360 Chapter 14

Ethical Issues inHuman GeneticsBuild Science SkillsMaking Judgments Have groupswork together to devise a set ofguidelines for the use of the humangenome. Make sure students under-stand the consequences of theguidelines they develop.

3 ASSESSEvaluate UnderstandingChallenge students to choose a topicin genetics and briefly describe itsimplications for society. Challengethem to explain how an understand-ing of science will help them makeinformed decisions.

ReteachHave students design a flowchart toshow the steps in gene therapy. Theyshould also include the stepsrequired to engineer the virus thatcarries the human gene. Studentscan use Figure 14–21 as a guide.

Normal hemoglobin gene

Genetically engineered virus

Nucleus

Bonemarrow

Bonemarrow cell

Chromosomes

� Figure 14–21 Genetherapy is the process ofchanging the genes thatcause a genetic disorder. Thisdrawing shows how a virusmight be used to deliver thegene for normal hemoglobininto a person’s bone marrow.

Unfortunately, gene therapy experiments have not always beensuccessful. Attempts to treat cystic fibrosis by spraying geneticallyengineered viruses into the breathing passages have not produced alasting cure. For all the promise it holds, in most cases gene therapyremains a high-risk, experimental procedure.

Ethical Issues in Human GeneticsIt would be marvelous to be able to cure hemophilia or other geneticdiseases. But if human cells can be manipulated to cure disease, shouldbiologists try to engineer taller people or change their eye color, hairtexture, sex, blood group, or appearance? What will happen to thehuman species if we gain the opportunity to design our bodies? Whatwill be the consequences if biologists develop the ability to clone humanbeings by making identical copies of their cells? These are questionswith which society must come to grips.

The goal of biology is to gain a better understanding of the natureof life. As our knowledge increases, however, so does our ability tomanipulate the genetics of living things, including ourselves. In ademocratic nation, all citizens—not just scientists—are responsiblefor ensuring that the tools science has given us are used wisely. Thismeans that you should be prepared to help develop a thoughtful andethical consensus of what should and should not be done with thehuman genome. To do anything less would be to lose control of two ofour most precious gifts: our intellect and our humanity.

1. Key Concept What is theHuman Genome Project?

2. Key Concept Describehow gene therapy works.

3. Name two common uses forDNA testing.

4. Describe how molecular biolo-gists identify genes in sequencesof DNA.

5. Critical Thinking MakingJudgments Evaluate thepotential impact of the HumanGenome Project on both scien-tific thought and society. Howhas it improved our understand-ing of human genetics? Howmight it be used to benefithumankind? What potentialethical problems might it create?

Persuasive ParagraphBiologists may one day be ableto use genetic engineering toalter a child’s inherited traits.Under what circumstances, if atall, should this ability be used?When should it not be used?Write a persuasive paragraphexpressing your opinion. Hint:Use specific examples of traitsto support your ideas.

14–3 Section Assessment

If your class subscribes to the iText,use it to review the Key Concepts inSection 14–3.

14–3 (continued)

Students will have different opin-ions, but their opinions should befully developed and supported byspecific examples.

14–3 Section Assessment1. An ongoing effort to analyze the human

DNA sequence2. An absent or faulty gene is replaced by a nor-

mal, working gene.3. To detect alleles for a genetic disorder and to

identify individuals4. By looking for promoters, which are binding

sites for RNA polymerase; an open readingframe; and introns as well as exons

5. Possible answer: Learn causes of genetic dis-orders and how the inheritance andexpression of human traits is controlled.Helpful in curing diseases, but could causediscrimination and the manipulation ofhuman traits for profit