Transformation
17.1 Biotechnology
Biotechnology: the use of biological agents for technological advancement.
- Historically used for breeding livestock and crops before the scientific basis was understood.
- Since the discovery of DNA in 1953, the field has grown rapidly through academic research and private companies.
- Primary applications: medicine (production of vaccines and antibiotics) and agriculture (genetic modification of crops to increase yields).
- Other applications: industrial uses such as fermentation, oil spill cleanup, and biofuel production.
- Analogy: genomic information creates maps of DNA similar to Google maps for physical locations; helps understand human migration and human genetic diseases, among other things.
Basic Techniques to Manipulate Genetic Material (DNA and RNA)
- Nucleic acids are macromolecules built from nucleotides (sugar, phosphate, nitrogenous base) linked by phosphodiester bonds.
- Phosphate groups carry a net negative charge.
- The genome is the entire set of DNA in the nucleus; DNA has two complementary strands held together by hydrogen bonds between bases.
- DNA strands can be separated by high temperature (DNA denaturation) and reannealed by cooling.
- DNA replication is carried out by DNA polymerase.
- RNA vs DNA:
- RNA is usually single-stranded and leaves the nucleus; mRNA reflects actively expressed protein-coding genes.
- RNA is less stable than DNA, posing additional analysis challenges.
DNA and RNA Extraction
- Cells are lysed using a lysis buffer (detergent) to break membranes.
- Macromolecules not desired are inactivated by enzymes: proteases (protein degradation) and RNases (RNA degradation).
- DNA is precipitated with an alcohol for isolation.
- Harvested human genomic DNA appears as a gelatinous white mass.
- DNA samples can be stored frozen at for several years.
Gel Electrophoresis (overview)
- Nucleic acids are negatively charged and migrate under an electric field.
- Gel separates molecules by size: smaller fragments move faster through pores.
- A DNA ladder or molecular weight standards are run alongside samples for size comparison.
- Nucleic acids in gels can be visualized with fluorescent or colored dyes.
- Genomic DNA typically appears as a smear when digested into fragments; uncut genomic DNA forms a single large band near the top of the gel.
Amplification of Nucleic Acid Fragments by Polymerase Chain Reaction (PCR)
- PCR amplifies specific regions of DNA for downstream analysis.
- Primers are short DNA sequences complementary to the ends of the target region.
- Components include: primers, genomic DNA, a thermostable DNA polymerase (e.g., Taq polymerase), and deoxynucleotides.
- Applications include: cloning gene fragments, detecting contaminant DNA, sequencing prep, paternity testing, and genetic disease detection.
- Note: Taq polymerase is derived from the thermostable bacterium Thermus aquaticus (Yellowstone National Park).
Reverse Transcriptase PCR (RT-PCR)
- RT-PCR starts from RNA.
- Step 1: reverse transcription converts RNA to complementary DNA (cDNA) using reverse transcriptase.
- Step 2: PCR amplifies the resulting cDNA with standard PCR.
- Used to study gene expression by analyzing mRNA-derived cDNA.
Hybridization, Southern Blotting, and Northern Blotting
- Probes: short DNA fragments labeled with radioactivity or fluorophores to detect specific sequences.
- After gel separation, fragments are transferred (blotted) to a nylon membrane.
- Southern blotting: DNA on membrane detected with DNA probes; used to find presence of particular DNA sequences.
- Northern blotting: RNA on membrane detected with RNA probes; used to study gene expression.
- Western blotting: proteins separated on a gel and detected with antibodies.
- Concept: blotting enables detection of specific nucleic acids or proteins after separation.
Molecular Cloning
- Cloning: reproducing a gene fragment or sequence to study its function or produce proteins.
- Plasmid (vector): small circular DNA molecule that replicates independently in bacteria; used to carry and propagate foreign DNA (transgenes).
- Foreign DNA vs host DNA:
- Foreign DNA (transgene) is inserted into bacterial host plasmids for propagation.
- Plasmids naturally occur in bacteria and can carry traits like antibiotic resistance.
- Key vector features:
- Multiple cloning site (MCS): short DNA region with many restriction enzyme sites for inserting DNA fragments.
- Restriction endonucleases cut DNA at specific sequences; create staggered cuts with 2- or 4-base overhangs (sticky ends).
- DNA ligase seals the fragments by forming phosphodiester bonds, producing recombinant plasmids.
- Outcome: any DNA fragment generated by restriction digestion can be ligated into a plasmid cut with the same enzyme.
- Recombinant DNA molecules contain foreign DNA inserted into plasmids; they can be used to express recombinant proteins.
- Not all recombinant plasmids express genes; expression may require a different vector or regulatory elements.
- Modeling example (Figure 17.7): a lab scenario illustrating potential outcomes when cloning is performed with degraded donor DNA vs intact plasmid.
Recombinant DNA Molecules and Expression
- Recombinant plasmids can encode recombinant proteins; expression depends on regulatory context and host choice.
- Some plasmids are engineered to express proteins only under certain environmental stimuli.
Cellular Cloning and Reproductive Cloning
- Cellular cloning: unicellular organisms (e.g., bacteria, yeast) clone themselves by asexual reproduction (binary fission); nuclear DNA duplicates via mitosis to create exact genetic replicas.
- Reproductive cloning: making an identical multicellular organism; often involves somatic cell nuclear transfer (SCNT).
- Parthenogenesis: virgin birth; embryo develops without fertilization; seen in certain insects and reptiles.
- Somatic cell nuclear transfer (SCNT): transfer of a diploid nucleus into an enucleated egg; used for therapeutic or reproductive cloning.
- Dolly the sheep (1996): first mammal cloned; life expectancy concerns due to potential aging of donor DNA; subsequent cloned animals show various abnormalities.
- Human cloning attempts exist for embryonic stem cell production (therapeutic cloning); raises bioethical concerns and regulatory debates.
Genetic Engineering and Genetically Modified Organisms (GMOs)
- Genetic engineering: altering an organism’s genotype via recombinant DNA to achieve desirable traits.
- GMOs: organisms that carry foreign DNA; host organisms receiving foreign DNA are called transgenic.
- Historical context: GMOs have been developed since the early 1970s for medicine, agriculture, and industry.
- Examples in the US:
- Roundup-ready crops and pest-resistant varieties are common GM crops used in food products.
- Gene Targeting and Reverse Genetics
- Traditional genetics started with observing phenotypes to infer gene function (forward genetics).
- Reverse genetics starts with a DNA sequence and asks what the gene does; aims to disrupt or mutate specific genes to study function.
- Gene targeting uses recombinant DNA vectors to alter gene expression (mutations, deletions, or regulatory changes).
Biotechnology in Medicine and Agriculture
- Medicine: leveraging genetic knowledge to diagnose, treat, and prevent disease.
- Agriculture: enhancing disease resistance, pest resistance, environmental stress tolerance, yield, and nutritional value.
- Genetic Diagnosis and Gene Therapy
- Genetic diagnosis: testing to identify disease-causing genes; informs treatment choices and preventive strategies for families.
- Gene therapy: introducing a healthy gene to cure disease; vectors (often viral) deliver the gene to diseased cells.
- Example: adenovirus vector in gene therapy (Figure 17.9).
- Some therapies target mutations at the original genomic site (e.g., SCID).
- Production of Vaccines, Antibiotics, and Hormones
- Vaccines: traditional live/attenuated strategies vs. gene-based strategies; cloning pathogen genes to produce antigens.
- Antibiotics: originally produced by fungi/bacteria; biotechnology enables large-scale production and optimization.
- Recombinant insulin produced in E. coli (as early as 1978) replaced animal-derived insulin to avoid allergic reactions.
- Human Growth Hormone (HGH) produced by cloning the HGH gene into bacterial hosts.
- Transgenic Animals
- Genes inserted into animals (e.g., sheep, goats, chickens, mice) to express recombinant proteins (often in milk or eggs).
- Transgenic Plants and Agricultural Biotechnology
- Plants modified for disease resistance, herbicide/pesticide tolerance, improved nutrition, and shelf-life.
- Transformation often uses Agrobacterium tumefaciens to transfer DNA into plant genomes.
- Ti plasmids from A. tumefaciens carry tumor-inducing genes; researchers remove tumor genes and insert desired DNA fragments for plant gene transfer.
- Ti plasmids can carry antibiotic resistance markers to aid selection; propagate in E. coli as well.
- Bt Toxin and GM Crops
- Bacillus thuringiensis (Bt) produces protein crystals toxic to certain insects.
- Bt toxin genes cloned into plants provide intrinsic insect resistance while remaining safe for humans and mammals; approved for organic farming.
- Flavr Savr Tomato (1994)
- First GM crop marketed; used antisense RNA to slow softening and fungal rotting, increasing shelf life; later market issues due to shipping/production challenges.
17.2 Mapping Genomes
Genomics and Genome Mapping
- Genomics: study of entire genomes, including the full set of genes, their nucleotide sequences, organization, and interactions within and between species.
- Genome mapping: process of finding the locations of genes on chromosomes.
- Maps are analogous to navigation maps: assist researchers in locating genes and understanding disease relationships.
Types of Maps
- Genetic maps: illustrate gene locations on chromosomes using genetic markers; based on genetic linkage and recombination data.
- Physical maps: represent the actual physical distance between genes or markers in nucleotides; provides a high-resolution view of chromosome structure.
- Both map types are needed for a complete view of the genome.
Genetic Maps and Linkage Analysis
- Linkage analysis: analyzes recombination frequencies between genes to determine if they are linked (on the same chromosome) or assort independently.
- Historical context: linkage analysis predates the discovery of DNA; phenotypic observations guided early map development.
- Mendelian observations in crosses (e.g., garden peas) showed certain traits were inherited together, indicating physical proximity of genes on the same chromosome.
- Classic example: color of flower and shape of pollen were linked traits, suggesting close proximity of their genes on the same chromosome.
- The concept of linkage led to the first genetic maps before DNA sequencing.
- Recombination during meiosis (crossing over) exchanges DNA between homologous chromosomes, generating new allele combinations and enabling linkage-based distance estimates.
- Distances on genetic maps are measured in centimorgans (cM), which reflect recombination frequency between markers.
Physical Maps and Genomic Applications
- Physical maps provide exact nucleotide distances between genes/markers; crucial for detailed genome navigation and gene discovery.
- Genome maps facilitate identification of disease genes (e.g., cancer, heart disease, cystic fibrosis) and guide research into medical and agricultural applications.
- Beyond human health, genome mapping supports environmental and agricultural applications, such as bioremediation and crop improvement under climate change.
Summary connection to prior principles and real-world relevance
- Mapping and genomics extend Mendelian genetics into a molecular framework, integrating DNA sequence data with trait inheritance.
- The ability to locate genes and understand their regulation underpins modern diagnostics, gene therapies, and the development of GM crops.
- Ethical, philosophical, and practical implications arise from genetic modification, cloning, gene therapy, and GM foods, including safety, ecological impact, and equity considerations.
Key formulas and notation (LaTeX)
- Recombination frequency (rf) relates genetic distance to observed crossovers:
- Distances on genetic maps are measured in centimorgans (cM): 1 cM approximates 1% recombination frequency.
- Physical distance between bases: the distance between two loci on a physical map is measured in nucleotides, e.g.,