Chapter 20

1. Chapter 20 Biotechnology

2. Overview: The DNA Toolbox • Sequencing of the human genome was completed by 2007. • DNA sequencing has depended on advances in technology, starting with making recombinant DNA. • In recombinant DNA, nucleotide sequences from two different sources, often two species, are combined in vitro into the same DNA molecule.

3. Methods for making recombinant DNA are central to genetic engineering, the direct manipulation of genes for practical purposes. • DNA technology has revolutionized biotechnology, the manipulation of organisms or their genetic components to make useful products. • An example of DNA technology is the microarray, a measurement of gene expression of thousands of different genes.

4. Microarray

5. DNA Cloning and Its Applications: A Preview • Most methods for cloning pieces of DNA in the laboratory share general features, such as the use of bacteria and their plasmids. • Plasmidsare small circular DNA molecules that replicate separately from the bacterial chromosome. • Cloned genes are useful for making copies of a particular gene and producing a protein product.

6. Recombinant DNA Plasmid & Gene Cloning Cell containing geneof interest Bacterium 1 Gene inserted intoplasmid Bacterialchromosome Plasmid Gene ofinterest RecombinantDNA (plasmid) DNA of chromosome 2 Plasmid put intobacterial cell Recombinantbacterium 3 Host cell grown in cultureto form a clone of cellscontaining the “cloned”gene of interest Gene ofInterest Protein expressedby gene of interest Copies of gene Protein harvested Basic research andvarious applications 4 Basicresearchon protein Basicresearchon gene Gene used to alter bacteria for cleaning up toxic waste Gene for pest resistance inserted into plants Protein dissolvesblood clots in heartattack therapy Human growth hor-mone treats stuntedgrowth

7. Using Restriction Enzymes to Make Recombinant DNA • Bacterial restriction enzymes --cut DNA molecules at specific DNA sequences called restriction sites. • A restriction enzyme usually makes many cuts, yielding restriction fragments. • The most useful restriction enzymes cut DNA in a staggered way, producing fragments with “sticky ends” that bond with complementary sticky ends of other fragments. • DNA ligase is an enzyme that seals the bonds between restriction fragments.

8. To Make Recombinant DNA Restriction site 5 3 3 5 DNA Restriction enzymecuts sugar-phosphatebackbones. 1 Sticky end DNA fragment addedfrom another moleculecut by same enzyme.Base pairing occurs. 2 One possible combination DNA ligaseseals strands. 3 Recombinant DNA molecule

9. Cloning a Eukaryotic Gene in a Bacterial Plasmid • In gene cloning, the original plasmid is called a cloning vector. • A cloning vector is a DNA molecule that can carry foreign DNA into a host cell and replicate there.

10. Cloning Recombinant Genes Hummingbird cell TECHNIQUE Bacterial cell lacZ gene Restrictionsite Stickyends Gene of interest Bacterial plasmid ampR gene Hummingbird DNA fragments Nonrecombinant plasmid Recombinant plasmids Bacteria carryingplasmids RESULTS Colony carrying recombinant plasmid with disrupted lacZ gene Colony carrying non-recombinant plasmidwith intact lacZ gene One of manybacterial clones

11. Storing Cloned Genes in DNA Libraries • A genomic library that is made using bacteria is the collection of recombinant vector clones produced by cloning DNA fragments from an entire genome. • A genomic library that is made using bacteriophages is stored as a collection of phage clones.

12. A complementary DNA (cDNA) library is made by cloning DNA madein vitroby reverse transcription of all the mRNA produced by a particular cell. • A cDNA library represents only part of the genome—only the subset of genes transcribed into mRNA in the original cells.

13. Making cDNA DNA innucleus mRNAs in cytoplasm Reversetranscriptase Poly-A tail mRNA Primer DNAstrand DegradedmRNA DNA polymerase cDNA

14. Screening a Library for Clones Carrying a Gene of Interest • A clone carrying the gene of interest can be identified with a nucleic acid probehaving a sequence complementary to the gene. • This process is called nucleic acid hybridization.

15. A probe can be synthesized that is complementary to the gene of interest. • For example, if the desired gene is – Then we would synthesize this probe … … 5 G G C T A A C T T A G C 3 C C G A T T G A A T C G 5 3

16. Probes … Locate Gene of Interest • TECHNIQUE Radioactivelylabeled probemolecules ProbeDNA Gene ofinterest Multiwell platesholding library clones Single-strandedDNA from cell Film Nylon membrane Nylonmembrane Location ofDNA with thecomplementarysequence

17. Eukaryotic Cloning and Expression Systems • The use of cultured eukaryotic cells as host cells and yeast artificial chromosomes (YACs) as vectors helps avoid gene expression problems. • YACs behave normally in mitosis and can carry more DNA than a plasmid. • Eukaryotic hosts can provide the post-translational modifications that many proteins require.

18. Amplifying DNAin Vitro: The Polymerase Chain Reaction (PCR) • The polymerase chain reaction, PCR, can produce many copies of a specific target segment of DNA. • A three-step cycle—heating, cooling, and replication—brings about a chain reaction that produces an exponentially growing population of identical DNA molecules.

19. PCR 3 5 TECHNIQUE Targetsequence 3 5 Genomic DNA 1 5 3 Denaturation 5 3 2 Annealing Cycle 1yields 2 molecules Primers 3 Extension Newnucleo-tides Cycle 2yields 4 molecules Cycle 3yields 8 molecules;2 molecules(in whiteboxes)match targetsequence

20. Concept 20.2: DNA technology allows us to study the sequence, expression, and function of a gene • DNA cloning allows researchers to • Compare genes and alleles between individuals. • Locate gene expression in a body. • Determine the role of a gene in an organism. • Several techniques are used to analyze the DNA of genes: Gel Electrophoresis, restriction fragment analysis, Southern blotting, DNA sequencing.

21. Gel Electrophoresis and Southern Blotting • One indirect method of rapidly analyzing and comparing genomes is gel electrophoresis. • This technique uses a gel as a molecular sieve toseparate nucleic acids or proteins bysize. • A current is applied that causes charged molecules to move through the gel. • Molecules are sorted into “bands” by their size.

22. Gel Electrophoresis TECHNIQUE Powersource Mixture ofDNA mol-ecules ofdifferentsizes – Cathode Anode + Gel 1 Powersource – + Longermolecules 2 Shortermolecules RESULTS

23. In restriction fragment analysis, DNA fragments produced by restriction enzyme digestion of a DNA molecule are sorted by gel electrophoresis. • Restriction fragment analysis is useful for comparing two different DNA molecules, such as two alleles for a gene. • The procedure is also used to prepare pure samples of individual fragments.

24. Restriction Fragment Analysis Normal -globin allele Normalallele Sickle-cellallele 175 bp Large fragment 201 bp DdeI DdeI DdeI DdeI Largefragment Sickle-cell mutant -globin allele 376 bp 201 bp175 bp Large fragment 376 bp DdeI DdeI DdeI (a) DdeI restriction sites in normal and sickle-cell alleles of -globin gene (b) Electrophoresis of restriction fragments from normal and sickle-cell alleles

25. A technique called Southern blottingcombines gel electrophoresis of DNA fragments with nucleic acid hybridization. • Specific DNA fragments can be identified by Southern blotting, using labeled probes that hybridize to the DNA immobilized on a “blot” of gel.

26. Southern Blotting TECHNIQUE Heavyweight Restrictionfragments I II III Nitrocellulosemembrane (blot) DNA + restriction enzyme Gel Sponge I Normal-globinallele II Sickle-cellallele III Heterozygote Papertowels Alkalinesolution 2 1 3 Preparation of restriction fragments DNA transfer (blotting) Gel electrophoresis Radioactively labeledprobe for -globin gene Probe base-pairswith fragments I II III I II III Fragment fromsickle-cell-globin allele Film overblot Fragment fromnormal -globin allele Nitrocellulose blot 4 5 Probe detection Hybridization with radioactive probe

27. DNA Sequencing • Relatively short DNA fragments can be sequenced by the dideoxy chain termination method. • Modified nucleotides called dideoxyribonucleotides (ddNTP) attach to synthesized DNA strands of different lengths. • Each type of ddNTP is tagged with a distinct fluorescent label that identifies the nucleotide at the end of each DNA fragment. • The DNA sequence can be read from the resulting spectrogram.

28. DNA Sequencing TECHNIQUE Primer Deoxyribonucleotides Dideoxyribonucleotides(fluorescently tagged) DNA(template strand) dATP ddATP dCTP ddCTP dTTP ddTTP DNA polymerase dGTP ddGTP DNA (template strand) Labeled strands Shortest Longest Directionof movementof strands Longest labeled strand Detector Laser Shortest labeled strand RESULTS Last baseof longestlabeledstrand Last baseof shortestlabeledstrand

29. Studying the Expression of Single Genes • Changes in the expression of a gene during embryonic development can be tested using • Northern blotting • Reverse transcriptase-polymerase chain reaction. • Both methods are used to compare mRNA from different developmental stages.

30. Northern Blotting TECHNIQUE 1 cDNA synthesis mRNAs cDNAs Primers 2 PCR amplification -globingene 3 Gel electrophoresis Embryonic stages RESULTS 1 2 3 4 5 6

31. Studying the Expression of Interacting Groups of Genes • Automation has allowed scientists to measure expression of thousands of genes at one time using DNA microarray assays. • DNA microarray assayscompare patterns of gene expression in different tissues, at different times, or under different conditions.

32. Fig. 20-15 TECHNIQUE Tissue sample 1 Isolate mRNA. 2 Make cDNA by reversetranscription, usingfluorescently labelednucleotides. mRNA molecules Labeled cDNA molecules(single strands) DNA fragmentsrepresentingspecific genes 3 Apply the cDNA mixture to amicroarray, a different gene ineach spot. The cDNA hybridizeswith any complementary DNA onthe microarray. DNA microarray DNA microarraywith 2,400human genes 4 Rinse off excess cDNA; scanmicroarray for fluorescence.Each fluorescent spot represents agene expressed in the tissue sample.

33. Determining Gene Function • One way to determine function is to disable the gene and observe the consequences. • Using in vitro mutagenesis, mutations are introduced into a cloned gene, altering or destroying its function. • When the mutated gene is returned to the cell, the normal gene’s function might be determined by examining the mutant’s phenotype.

34. Cloning Plants: Single-Cell Cultures • One experimental approach for testing genomic equivalence is to see whether a differentiated cell can generate a whole organism. • A totipotent cell is one that can generate a complete new organism.

35. Fig. 20-16 EXPERIMENT RESULTS Transversesection ofcarrot root 2-mgfragments A singlesomaticcarrot celldevelopedinto a maturecarrot plant. Fragments werecultured in nu-trient medium;stirring causedsingle cells toshear off intothe liquid. Singlecellsfree insuspensionbegan todivide. Embryonicplant developedfrom a culturedsingle cell. Plantlet wascultured onagar medium. Later it wasplantedin soil.

36. Cloning Animals: Nuclear Transplantation • In nuclear transplantation, the nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell. • Experiments with frog embryos have shown that a transplanted nucleus can often support normal development of the egg. • However, the older the donor nucleus, the lower the percentage of normally developing tadpoles.

37. Frog Cloning Frog egg cell Frog embryo Frog tadpole EXPERIMENT UV Fully differ- entiated (intestinal) cell Less differ-entiated cell Donornucleustrans-planted Donor nucleus trans- planted Enucleated egg cell Egg with donor nucleus activated to begin development RESULTS Most develop into tadpoles Most stop developing before tadpole stage

38. Mammal Cloning “Dolly” TECHNIQUE Mammarycell donor Egg celldonor 2 1 Egg cellfrom ovary Nucleusremoved Cells fused 3 Culturedmammary cells 3 Nucleus frommammary cell Grown inculture 4 Early embryo Implantedin uterusof a thirdsheep 5 Surrogatemother Embryonicdevelopment 6 Lamb (“Dolly”)genetically identical tomammary cell donor RESULTS

39. Problems Associated with Animal Cloning • In most nuclear transplantation studies, only a small percentage of cloned embryos have developed normally to birth. • Many epigenetic changes, such as acetylation of histones or methylation of DNA, must be reversed in the nucleus from a donor animal in order for genes to be expressed or repressed appropriately for early stages of development.

40. Stem Cells of Animals • A stem cellis a relatively unspecialized cell that can reproduce itself indefinitely and differentiate into specialized cells of one or more types. • Stem cells isolated from early embryos at the blastocyst stage are called embryonic stem cells; these are able to differentiate into all cell types. • The adult body also has stem cells, which replace nonreproducing specialized cells.

41. Stem Cells Embryonic stem cells Adult stem cells From bone marrowin this example Early human embryoat blastocyst stage(mammalian equiva-lent of blastula) Cells generatingall embryoniccell types Cells generatingsome cell types Culturedstem cells Differentcultureconditions Differenttypes ofdifferentiatedcells Blood cells Nerve cells Liver cells

42. Concept 20.4: The practical applications of DNA technology affect our lives in many ways • The aim of stem cell research is to supply cells for the repair of damaged or diseased organs. • Many fields benefit from DNA technology and genetic engineering. • One medical benefit of DNA technology is identification of human genes in which mutation plays a role in genetic diseases.

43. Diagnosis of Diseases • Scientists can diagnose many human genetic disorders by using PCR and primers corresponding to cloned disease genes, then sequencing the amplified product to look for the disease-causing mutation. • Genetic disorders can also be tested for using genetic markers that are linked to the disease-causing allele.

44. Single nucleotide polymorphisms (SNPs) are useful genetic markers. • These are single base-pair sites that vary in a population. • When a restriction enzyme is added, SNPs result in DNA fragments with different lengths, or restriction fragment length polymorphism (RFLP).

45. Useful Genetic Markers DNA T Normal allele SNP C Disease-causingallele

46. Human Gene Therapy • Gene therapyis the alteration of an afflicted individual’s genes. • Gene therapy holds great potential for treating disorders traceable to a single defective gene. • Vectors are used for delivery of genes into specific types of cells, for example bone marrow. • Gene therapy raises ethical questions, such as whether human germ-line cells should be treated to correct the defect in future generations.

47. GeneTherapy Clonedgene Insert RNA version of normal alleleinto retrovirus. 1 Viral RNA Let retrovirus infect bone marrow cellsthat have been removed from thepatient and cultured. 2 Retroviruscapsid Viral DNA carrying the normalallele inserts into chromosome. 3 Bonemarrowcell frompatient Bonemarrow Inject engineeredcells into patient. 4

48. Pharmaceutical Products • Advances in DNA technology and genetic research are important to the development of new drugs to treat diseases • Synthesis of Small Molecules for Use as Drugs • The drug imatinib is a small molecule that inhibits overexpression of a specific leukemia-causing receptor • Pharmaceutical products that are proteins can be synthesized on a large scale

49. Transgenicanimals are made by introducing genes from one species into the genome of another animal. • Transgenic animals are pharmaceutical “factories,” producers of large amounts of otherwise rare substances for medical use. • “Pharm” plants are also being developed to make human proteins for medical use. Protein Production by “Pharm” Animals and Plants:

50. Forensic Evidence and Genetic Profiles • An individual’s unique DNA sequence, or genetic profile,can be obtained by analysis of tissue or body fluids. • Genetic profiles can be used to provide evidence in criminal and paternity cases and to identify human remains. • Genetic profiles can be analyzed using RFLP analysis by Southern blotting.