CH20: DNA Tools and Biotechnology: Principles and Applications

DNA Sequencing and Cloning: Foundations of Genetic Engineering

Nucleic Acid Hybridization: The base pairing of one strand of a nucleic acid to a complementary sequence from another nucleic acid strand (either DNA or RNA). This principle forms the foundation of virtually every technique used in genetic engineering.
Genetic Engineering: The direct manipulation of genes for practical purposes. It has revolutionized criminal law, medicine, and biological research.
DNA Sequencing: A process that exploits complementary base pairing to determine the complete nucleotide sequence of a DNA molecule. * Dideoxy Sequencing: The first automated procedure, developed in the $1970s$ by biochemist Frederick Sanger (Nobel Prize $1980$ ). It is still used for small-scale sequencing. * Next-Generation Sequencing: Developed in the first decade of the $21^{st}$ century. It is rapid and inexpensive. Examples include Sequencing by Synthesis. * Sequencing by Synthesis (Methodology): 1. Genomic DNA is fragmented into segments of roughly $300$ base pairs. 2. Each fragment is placed in a droplet with a bead; $1$ million identical copies are made via PCR and attached by their $5'$ end to the bead. 3. The bead is placed in one of $2$ million wells on a multiwell plate with DNA polymerases and primers. 4. Solutions of dATP, dTTP, dGTP, and dCTP are added and washed off sequentially. 5. If the added nucleotide is complementary to the template, it is joined, releasing pyrophosphate ( $PP_i$ ), which causes a flash of light. 6. The pattern of flashes identifies the sequence. This "high-throughput" method can sequence about $2 \times 10^9$ nucleotides in $24$ hours. * Third-Generation Sequencing: Faster and less expensive than previous methods. It sequences single, very long DNA molecules without fragmentation or amplification. * Nanopore Sequencing: Moving a single strand of DNA through a small protein channel pore in a lipid membrane. Each base interrupts an electrical current for a distinct length of time. The first device (released in $2015$ ) is the size of a small candy bar and connects via USB.
DNA Cloning: Methods used to isolate a specific segment of DNA and make multiple identical copies. * The Challenge: Naturally occurring DNA molecules are very long and contain many genes. In eukaryotes, protein-coding regions are a small proportion; a single human gene might be only $\frac{1}{100,000}$ of a chromosomal DNA molecule. * Plasmids: Small, circular DNA molecules in bacteria (like Escherichia coli) that replicate separately from the bacterial chromosome. They carry a small number of genes. * Recombinant DNA: A molecule containing DNA from two different sources (often different species). * Cloning Vector: A DNA molecule that can carry foreign DNA into a host cell and be replicated there. Bacterial plasmids are common vectors because they are easily obtained, manipulated in vitro, and introduced into bacteria. * Gene Cloning Purposes: To amplify a particular gene and/or to produce a protein product from it (e.g., human growth hormone or pest resistance genes inserted into plants).
Using Restriction Enzymes: * Restriction Endonucleases (Restriction Enzymes): Discovered in the late $1960s$ . They protect bacteria by cutting up foreign DNA at specific restriction sites. * Mechanism: Most restriction sites are symmetrical. The sequence is the same on both strands when read in the $5' \rightarrow 3'$ direction. For example, EcoRI recognizes $5'-GAATTC-3'$ . * Restriction Fragments: The result of a restriction enzyme making many cuts in a long DNA molecule. * Sticky Ends: Staggered cuts create single-stranded ends. These form hydrogen-bonded base pairs with complementary sticky ends from other DNA fragments cut by the same enzyme. * DNA Ligase: An enzyme that catalyzes the formation of covalent bonds to close the sugar-phosphate backbones, making the recombinant DNA molecule permanent.
Gel Electrophoresis: A technique using a polymer gel (e.g., agarose) that acts as a molecular sieve to separate nucleic acid fragments by length. Negatively charged DNA molecules move toward the positive electrode (anode); shorter fragments travel faster through the microscopic holes.
Polymerase Chain Reaction (PCR): * Utility: Can produce billions of copies of a target DNA segment within a few hours, even if the target is less than $0.001\%$ of the sample. * Three-Step Cycle: 1. Denaturation: Heating to separate DNA strands (approx. $95^{\circ}C$ ). 2. Annealing: Cooling to allow hydrogen bonding of primers ( $15-20$ nucleotides long) to sequences at opposite ends of the target. 3. Extension: Heat-stable DNA polymerase adds nucleotides to the $3'$ end of each primer. * Enzymes: Taq polymerase (from Thermus aquaticus) is heat-stable but lacks proofreading. Pfu polymerase (from Pyrococcus furiosus) is more accurate and stable but more expensive. * Mathematical Growth: The number of target molecules doubles each cycle ( $2^n$ , where $n$ is the number of cycles). After $30$ cycles, there are over $10^9$ copies. * Limitations: Errors during replication limit the number of "good" copies. PCR is often used to provide fragments for cloning rather than replacing cloning in cells.
Expressing Cloned Eukaryotic Genes: * Bacterial Systems: Require an expression vector (a cloning vector with a highly active bacterial promoter upstream of the insertion site). * Intron Problem: Bacteria lack RNA-splicing machinery. This is solved by using cDNA (complementary DNA) which contains only exons. * Eukaryotic Systems: Yeasts or cultured mammalian/insect cells are used for RNA splicing and post-translational modifications (e.g., glycosylation) that bacteria cannot perform. * Introduction Methods: Electroporation (electrical pulse creates temporary holes in the plasma membrane) or direct injection with microscopic needles.
Cross-Species Gene Expression: The PAX-6 protein triggers eye development in both vertebrates and flies. Replacing a fly's Pax-6 gene with a mouse's version results in the formation of a compound fly eye, demonstrating shared evolutionary ancestry.

Methods for Analyzing Gene Expression and Function

Studying Single Genes: * In Situ Hybridization: Uses a fluorescently labeled single-stranded nucleic acid probe complementary to the mRNA of interest to detect its location within an intact organism. * RT-PCR (Reverse Transcriptase-PCR): 1. Reverse Transcriptase creates a DNA copy of mRNA (a reverse transcript). 2. DNA Polymerase synthesizes the second strand to form cDNA. 3. PCR is used to amplify a specific cDNA segment. Bands on a gel indicate if the mRNA was present in the original sample. * qRT-PCR (Quantitative RT-PCR): Uses a fluorescent dye that glows when bound to double-stranded PCR products, allowing for real-time measurement of mRNA levels without electrophoresis.
Studying Groups of Interacting Genes: * RNA Sequencing (RNA-seq): The method of choice for genome-wide studies. mRNA samples are isolated, fragmented, converted to cDNAs, and sequenced. A computer reassembles the sequences to indicate which genes are expressed and at what level. * DNA Microarray Assays: Tiny amounts of single-stranded DNA fragments (representing thousands of genes) are fixed to a glass slide. Labeled cDNAs from different tissues are hybridized to the slide. The color and intensity of fluorescent dots reveal expression patterns.
Determining Gene Function: * In Vitro Mutagenesis: Specific mutations are introduced into a cloned gene, which then knocks out the normal cellular copy to observe the phenotypic consequences. * CRISPR-Cas $9$ System: A gene-editing tool that uses the Cas $9$ nuclease guided by an RNA molecule to cut DNA at a specific sequence. It can be used to knock out genes or repair mutations. * Gene Drive: Engineering a new allele so it is biased for inheritance, rapidly spreading through a population (e.g., to alter disease-transmitting insects). * RNA Interference (RNAi): Uses synthetic double-stranded RNA to trigger the breakdown of specific mRNA, providing a temporary reduction in gene expression.
Genome-Wide Association Studies (GWAS): Analyzing large numbers of people to find shared genetic differences associated with a disease. * Genetic Markers: DNA sequences that vary in a population. * SNP (Single Nucleotide Polymorphism): A single base-pair site where variation is found in at least $1\%$ of the population. Most SNPs are in noncoding regions. * Linkage: A SNP found in all affected people suggests the disease-causing mutation is located very close to that SNP on the chromosome.

Organismal Cloning and Stem Cell Technology

Plant Cloning: F. C. Steward demonstrated in the $1950s$ that differentiated carrot root cells could grow into normal adult plants. * Totipotent: The potential for a mature cell to "dedifferentiate" and give rise to all specialized cell types of the organism.
Animal Cloning: * Nuclear Transplantation (Somatic Cell Nuclear Transfer): Replacing the nucleus of an egg with the nucleus of a differentiated cell. * Inquiry (Gurdon, $1970s$ ): Found that the ability of a transplanted nucleus to direct development into a tadpole decreased as the donor cell became more differentiated ( $2\%$ success with intestinal cells vs. high success with embryonic cells). * Dolly the Sheep ( $1997$ ): The first mammal cloned from an adult cell. She developed premature arthritis and died at age $6$ . * CC (Carbon Copy): The first cloned cat; her coat pattern differed from her parent due to random X chromosome inactivation. * Low Efficiency: Few cloned embryos develop normally due to incomplete epigenetic reprogramming (e.g., abnormal DNA methylation in donor nuclei).
Stem Cells: Relatively unspecialized cells that can reproduce indefinitely and differentiate into specialized cells. * Embryonic Stem (ES) Cells: Isolated from the blastula/blastocyst stage. They are pluripotent (capable of differentiating into many different cell types). * Adult Stem Cells: Found in bone marrow, brain, skin, etc. They are less versatile than ES cells; bone marrow stem cells can generate all blood cell types. * Therapeutic Cloning: Producing ES cells to treat disease using a patient's own donor nucleus to avoid immune rejection. * Induced Pluripotent Stem (iPS) Cells: Shinya Yamanaka ( $2007$ , Nobel Prize $2012$ ) deprogrammed differentiated human skin cells by using a retrovirus to introduce four "stem cell" master regulatory genes (Oct $3/4$ , Sox $2$ , c-Myc, Klf $4$ ). * Uses of iPS Cells: Modeling diseases (e.g., Type $1$ diabetes, Parkinson's) and regenerative medicine (e.g., replacing retinal cells in patients with macular degeneration).

Practical Applications of DNA-Based Biotechnology

Medical Applications: * Pathogen Detection: RT-PCR is used to quantify HIV RNA in blood. * Genetic Disorder Diagnosis: PCR and sequencing detect mutations for sickle-cell disease, cystic fibrosis, and Huntington's disease. * Personalized Medicine: Analyzing gene expression to determine cancer recurrence risk or using pharmacogenetics to predict medication risks for over $300$ FDA-recommended drugs. * Gene Therapy: Inserting a normal allele into somatic cells. Successful for SCID using retroviral vectors, though some patients developed leukemia due to vector insertion near cell-proliferation genes.
Pharmaceuticals: * Small Molecules: Imatinib (Gleevec) inhibits a tyrosine kinase causing chronic myelogenous leukemia (CML). * Protein Production: Human insulin, HGH (Human Growth Hormone), and TPA (tissue plasminogen activator) are produced in cell cultures. * Transgenic "Pharm" Animals: A transgene (e.g., for human antithrombin) is inserted into a goat's genome so the protein is secreted in its milk for easy purification.
Forensic Evidence: * Genetic Profile: A unique set of genetic markers for an individual. * STRs (Short Tandem Repeats): Tandemly repeated units of $2-5$ nucleotides. Forensic scientists typically analyze $13$ STR markers. * Sensitivity: PCR allows analysis of samples as small as $20$ cells. Probability of two people matching at all $13$ markers is $1$ in $10$ billion to $1$ in several trillion.
Environmental and Agricultural Uses: * Bioremediation: Engineering bacteria to extract heavy metals (copper, lead, nickel) or degrade chlorinated hydrocarbons. * Agriculture: Creating GMOs with herbicidal/pest resistance or delayed ripening. * $C_4$ Rice Project: Aiming to engineer rice with more efficient photosynthesis to meet a predicted $70\%$ increase in food demand by $2050$ .
Safety and Ethics: * Regulations: Strains are "genetically crippled" so they cannot survive outside laboratories. * GMO Controversy: Concerns in Europe regarding "super weeds" (pollen transfer to wild relatives) and human allergens. * Human Germ-Line Editing: Condemned by the biological community. A Chinese researcher's $2018$ claim of editing CCR $5$ genes in twin embryos led to international outcry and calls for a WHO registry.

Questions & Discussion

Concept 20.1 Q1: What type of bonds are cleaved by the restriction enzyme Pvul (cutting between T and C)? * Answer: Covalent phosphodiester bonds (sugar-phosphate backbone) and hydrogen bonds (between base pairs).
Concept 20.1 Q2: Will Pvul ( $5'-CGATCG-3'$ ) cut the sequence $5'-CTTGACGATCGTTACCG-3'$ ? * Answer: Yes, because the complementary strand contains the same sequence in reverse, forming the restriction site.
Concept 20.2 Q2: In a microarray where normal tissue is green and cancer is red, which spots are interesting? * Answer: Red spots (genes expressed only in cancer) and green spots (genes silenced in cancer).
Concept 20.3 Q2: Should clients expect a cloned pet to look identical to the original? * Answer: No, because of environmental influences and random phenomena like X chromosome inactivation (as seen in CC the cat).
Concept 20.4 Q3: If viral proteins aren't found for suspected Hepatitis A (an RNA virus), what test identifies it? * Answer: RT-PCR to amplify and detect the viral RNA genome.