Biotechnology: Class 12 — Chapter 4: Applications of Recombinant DNA Technology

• Genome Uniformity & Variation: About $99.9\%$ of the human genome ($3.2 \times 10^9\text{ bp}$) is identical across all individuals. The remaining $0.1\%$ contains inherited variations that make each individual unique.
• Non-Coding Regions: More than $90\%$ of the human genome consists of non-coding DNA. Within these regions are short, tandemly repeated nucleotide sequences.
• VNTRs & STRs: Variable Number Tandem Repeats (VNTRs) (minisatellites, 10 to 100 bp repeats) and Short Tandem Repeats (STRs) (microsatellites, 1 to 9 bp repeats) exhibit high levels of inheritable polymorphism, serving as the basis for DNA profiling and paternity testing.

1. DNA Isolation: Extract genomic DNA from biological samples such as blood, hair follicles, skin, semen, or buccal swabs.
2. Restriction Digestion: Cleave the isolated DNA into fragments of varying sizes using specific restriction endonucleases, a process known as Restriction Fragment Length Polymorphism (RFLP).
3. Electrophoresis: Separate the digested DNA fragments by size using agarose gel electrophoresis.
4. Southern Transfer (Blotting): Transfer the separated DNA fragments from the gel to a synthetic nylon or nitrocellulose membrane.
5. Fixation: Expose the membrane to UV radiation for 3 minutes or bake at $80^\circ\text{C}$ for 2 hours to permanently bind the DNA.
6. Hybridisation: Incubate the membrane with labeled single-stranded VNTR Probes to bind to complementary target sequences.
7. Detection & Analysis: Detect the hybridised DNA bands to reveal highly individual-specific patterns.

Note: Polymerase Chain Reaction (PCR) is integrated to amplify trace amounts of DNA, significantly increasing the sensitivity of the technique.

• Forensics & Criminology: Comparing DNA profiles from crime scenes with suspects to establish guilt or innocence.
• Paternity & Maternity Disputes: Resolving parentage cases since VNTR patterns are inherited directly from biological parents.
• Evolutionary Biology: Comparing DNA extracts from fossils with modern counterparts to study phylogenetic relationships.
• Medical Diagnostics: Identifying DNA patterns associated with inherited genetic disorders.

• 1973: First GMO bacterium engineered by Herbert Boyer and Stanley Cohen.
• 1974: First transgenic animal (mice) created by Rudolf Jaenisch and Beatrice Mintz.
• 1994: Flavr Savr Tomato approved by the USFDA as the first genetically modified food crop.

• Vector-Mediated (Indirect) Gene Transfer:
  • Bacteria-Mediated Transfer: Utilises Agrobacterium tumefaciens (the "Natural Genetic Engineer"), a Gram-negative soil bacterium containing a tumor-inducing (Ti plasmid). The T-DNA region is disarmed by replacing tumor-inducing genes with the gene of interest (e.g., Bt gene) and antibiotic marker genes. Recombinant bacteria infect plant cells, integrating the T-DNA into the plant genome.
  • Plant Virus-Mediated Transfer: Uses plant viruses like caulimoviruses to introduce foreign DNA into intact plant cells.
• Vector-Less (Direct) Gene Transfer:
  • Particle Bombardment (Biolistics): Micro-projectiles of gold or tungsten ($1\text{ \mu m}$ to $3\text{ \mu m}$) coated with transgene DNA are accelerated at high velocity into target plant tissues using a gene gun. Highly effective for monocots like cereals and for chloroplast transformation.
  • Protoplast Transformation & Electroporation: Enzymatic removal of plant cell walls yields naked protoplasts. DNA uptake is facilitated using chemical agents like polyethylene glycol (PEG) or by applying high-voltage electric pulses (electroporation) to create transient pores in the plasma membrane.

1. DNA Pronuclear Microinjection: Direct microinjection of the transgene into the larger male pronucleus of a fertilized egg, followed by in vitro culture and implantation into a pseudopregnant foster mother.
2. Embryonic Stem Cell-Mediated Transfer: Transfecting embryonic stem (ES) cells harvested from the inner cell mass of a blastocyst via electroporation, selecting transformed cells, and microinjecting them back into a blastocyst to generate chimeric mice.
3. Retrovirus-Mediated Transfer: Utilizing disarmed retroviral vectors (e.g., lentivirus) to stably integrate a single copy of the transgene into early embryos or ES cells.

• Agricultural Improvements:
  • Bt Cotton: Engineered by incorporating the cry gene from Bacillus thuringiensis. Encodes an insecticidal crystal protein (Bt toxin) that targets specific pests. cryIAc and cryIIAb control cotton bollworms, while cryIAb controls corn borers.
  • Golden Rice: Engineered to accumulate beta-carotene (provitamin A) to mitigate Vitamin A deficiency.
  • Antisense Technology: Used to silence specific genes. Includes Antisense Oligonucleotides (ASO) (which trigger RNase H-mediated mRNA degradation or block ribosome binding) and RNA interference (RNAi) (where double-stranded RNA is cleaved by Dicer into siRNAs, which guide the RISC complex to cleave target mRNA). Applied to create pest-resistant tobacco plants against the nematode Meloidogyne incognita.
• Molecular Farming: Large-scale production of industrial and therapeutic biomolecules in plants. For example, Elelyso (taliglucerase alfa), an enzyme produced in carrot cell cultures to treat Gaucher's disease.

• Environmental Remediation:
  • Biodegradable Plastics: Production of polyhydroxyalkanoates (PHAs) and polyhydroxybutyrate (PHB) in transgenic sugarcane.
  • Phytoremediation: Using plants to remove environmental pollutants. Transgenic "mercury-breathing" plants expressing bacterial mercuric reductase convert toxic ionic mercury into less toxic elemental forms.

• Molecular Pharming: Using transgenic animals as bioreactors to produce human therapeutic proteins in their milk.
  • Tracy the Ewe: Engineered in the 1990s to produce high levels of human \alpha_1-antitrypsin in her milk to treat emphysema.
  • Rosie the Cow: Created in 1997 to produce human \alpha-lactalbumin-enriched milk, providing a nutritionally balanced alternative for human infants.

• Biomedical Research: Serving as model organisms to study human physiology, development, and diseases like cancer and Alzheimer's, and for toxicity testing of vaccines and drugs.

Concerns include potential allergenicity of GM proteins, ecological risks of transgene escape to wild relatives, and unpredictable non-target effects. In India, the Genetic Engineering Appraisal Committee (GEAC) under the Ministry of Environment, Forest and Climate Change regulates the manufacture, use, import, export, and storage of GMOs.

The first clinical gene therapy was performed in 1990 on a four-year-old girl suffering from Severe Combined Immunodeficiency (SCID). SCID is caused by a mutation in the gene encoding adenosine deaminase (ADA), leading to a lack of T and B lymphocytes. Functional ADA genes were inserted into a retroviral vector, transfected into the patient's isolated lymphocytes in vitro, and reinfused into her bloodstream. Because lymphocytes have a finite lifespan, the treatment required periodic reinfusions.

• Gene Replacement / Addition: Delivering a functional copy of a gene to synthesize a missing protein (e.g., replacing mutated CFTR in cystic fibrosis or introducing wild-type p53 to inhibit tumor growth).
• Gene Inhibition: Introducing a gene construct that blocks the expression of a dominant-acting mutated gene or infectious agent (applicable in Tay-Sachs, phenylketonuria, and color-blindness).
• Gene Repair / Editing: Directly correcting mutations in situ using programmable nucleases like the CRISPR-Cas9 system.

• Merits: Offers a potential permanent cure for devastating single-gene disorders and cancers where conventional treatments fail.
• Demerits: Risk of insertional mutagenesis (e.g., triggering oncogenes if integrated randomly), transient expression requiring repeated treatments, host immune response against viral vectors, and high cost.
• Ethical Issues: Safety concerns regarding germ-line modifications, potential misuse for genetic enhancement (e.g., athletic ability, intelligence), and unpredictable long-term evolutionary consequences.

1. Live Genetically Modified Vaccines: Pathogens are attenuated by deleting or inactivating virulence genes. Can also act as vector-based vaccines (e.g., using vaccinia virus engineered with antigen genes from hepatitis B, herpes simplex, and influenza to create a polyvalent vaccine). Includes Edible Vaccines, where antigenic genes are expressed in edible plant tissues (e.g., expressing HBsAg in bananas).
2. Recombinant Subunit Vaccines: Contain only specific immunogenic proteins of the pathogen, eliminating the risk of infection. For example, the Hepatitis B Vaccine produced by cloning the gene for the surface antigen HBsAg into the yeast plasmid vector pMA56 and expressing it in Saccharomyces cerevisiae.
3. DNA Vaccines: Plasmid DNA encoding the target antigen under a strong eukaryotic promoter is injected directly into host muscle tissue (using a gene gun or nasal spray). Host cells take up the plasmid and express the antigen, inducing both humoral and cellular immunity.
4. RNA Vaccines: Deliver synthetic mRNA encoding the target antigen, encapsulated in protective lipid nanoparticles (LNPs). Host ribosomes translate the mRNA into antigenic proteins. Highly adaptable to emerging viral variants. Developed extensively during the COVID-19 pandemic.

Monoclonal antibodies are monospecific antibodies designed to bind to a single specific epitope on an antigen. They are produced using Hybridoma Technology (developed by Georges Köhler and César Milstein in 1975).

• Mechanism: Antibody-producing B cells isolated from an immunized animal are fused with immortal myeloma cells using polyethylene glycol (PEG).
• Selection: The fusion mixture is cultured in HAT Medium (Hypoxanthine, Aminopterin, Thymidine). Unfused myeloma cells die because they lack the salvage pathway enzyme HGPRT and cannot synthesize DNA in the presence of aminopterin. Unfused B cells die naturally due to their short lifespan. Only hybridoma cells survive and proliferate.
• Chimerization: Recombinant DNA technology is used to replace mouse antibody domains with human sequences to produce chimeric or humanised antibodies, reducing immunogenicity in patients.

Insulin is a peptide hormone secreted by pancreatic beta cells that regulates glucose homeostasis. Structurally, active insulin consists of two polypeptide chains: Chain A (21 amino acids) and Chain B (30 amino acids) linked by disulfide bonds.

Wild-Type Molecular Limitation: Direct expression of preproinsulin in bacteria is not feasible due to the lack of eukaryotic post-translational processing machinery to cleave the C-peptide. Purifying insulin from animal pancreas (pigs/cows) carries risks of allergic reactions due to minor amino acid differences.

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Engineering Methodology & Mechanism: Genes coding for Chain A (21 amino acids) and Chain B (30 amino acids) of human insulin were chemically synthesized and cloned separately into plasmid vectors of Escherichia coli. Each gene was inserted adjacent to a promoter and the eta-galactosidase gene to produce a fusion protein (eta-gal-insulin), protecting the insulin chains from bacterial degradation. The fusion proteins were extracted, purified, and treated with cyanogen bromide (CNBr) to cleave the methionine residue linking eta-galactosidase to the insulin chains. The isolated Chain A and Chain B were mixed and chemically joined by reestablishing disulfide bonds under oxidizing conditions to yield active, recombinant human insulin (Humulin).

Human Growth Hormone is a 191-amino-acid peptide secreted by the pituitary gland. Deficiency in childhood leads to Dwarfism.

Wild-Type Molecular Limitation: Natural HGH is synthesized as a precursor with a 26-amino-acid signal peptide. Bacteria lack the processing enzymes to cleave this signal peptide from the intermediate HGH molecule, resulting in an inactive protein. Extracting HGH from human cadaver pituitary glands is expensive and carries the risk of transmitting Creutzfeldt-Jakob Disease (CJD).

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Engineering Methodology & Mechanism: HGH-encoding mRNA was isolated and used as a template to synthesize complementary DNA (cDNA). To bypass the bacterial inability to process the signal peptide, the nucleotide sequence coding for the 26-amino-acid signal peptide was removed. Because no restriction enzyme could cut precisely at the junction, restriction enzyme EcoRI was used to cleave the cDNA, which removed the signal peptide sequence plus the first 24 codons of the mature HGH gene (totaling 50 codons removed). A synthetic DNA linker containing the missing 24 codons of mature HGH (without the signal peptide) was chemically synthesized and ligated back to the remaining 167-codon cDNA fragment. This reconstructed gene was cloned into an expression vector and transformed into E. coli, enabling direct expression of mature, active 191-amino-acid HGH (Humatrope / Protropin).