Biotechnology: Principles and Processes

Historical Context and Foundations of Modern Biotechnology

• Anthropocentric Evolution of Science

  • Since the era of René Descartes (seventeenth-century French philosopher, mathematician, and biologist), natural sciences have been directed toward developing technologies that enhance human comfort and add value to human life.

  • This anthropocentric approach led physics and chemistry to establish engineering, technology, and industrial sectors focused on human welfare.

  • The biological world served primarily as a source of food until the twentieth-century development of biotechnology revolutionized daily life, health, and agriculture.

• Pioneering Work of Herbert Boyer and Stanley Cohen

  • Herbert Boyer:

  • Born in $1936$ in western Pennsylvania.

  • Completed graduate studies at the University of Pittsburgh in $1963$, followed by three years of post-graduate work at Yale University.

  • Appointed Assistant Professor at the University of California, San Francisco (UCSF) in $1966$

  • In $1969$, discovered that restriction enzymes from Escherichia coli cut DNA strands at specific sites, leaving overhanging single-stranded regions termed sticky ends.

  • Stanley Cohen:

  • Studied plasmids—small, circular, extrachromosomal DNA ringlets that reside in the cytoplasm of bacterial cells and replicate independently of the chromosomal DNA.

  • Developed methods to isolate plasmids from bacterial cells and reintroduce them into other bacterial cells.

  • Creation of Recombinant DNA ($1972$):

  • In $1972$, Boyer and Cohen combined their techniques in Hawaii.

  • They isolated a gene conferring antibiotic resistance from a native plasmid of Salmonella typhimurium using restriction enzymes ("molecular scissors").

  • Using the enzyme DNA ligase, they joined the antibiotic resistance gene to a plasmid vector in vitro.

  • The resulting recombinant DNA was transferred into Escherichia coli, where it replicated autonomously using host DNA polymerase, producing multiple copies—a process termed gene cloning.

Definitions and Core Principles of Biotechnology

• Definition of Biotechnology

  • Traditional View: Techniques utilizing live organisms or their enzymes to produce goods and services useful to humans (e.g., microbe-mediated processes like curdling milk, bread-making, or wine fermentation).

  • Modern Molecular Biotechnology: Large-scale processes utilizing genetically modified organisms (GMOs) to achieve specific chemical or biological yields.

  • Broad Technical Scope: Includes in vitro fertilization ("test-tube baby"), artificial gene synthesis, DNA vaccine development, and gene therapy (correcting defective genes).

  • European Federation of Biotechnology (EFB) Definition: "The integration of natural science and organisms, cells, parts thereof, and molecular analogues for products and services."

• Core Techniques Enabling Modern Biotechnology

  • Genetic Engineering: Techniques that alter the chemistry of genetic material (DNA and RNA) and introduce it into host organisms to modify the host's phenotype.

  • Asexual reproduction preserves genetic identity, whereas sexual reproduction introduces variations.

  • Traditional hybridization introduces undesirable genes along with desired genes; genetic engineering overcomes this by isolating and transferring only targeted gene sequences.

  • Bioprocess Engineering: Maintaining sterile (microbial contamination-free) conditions during chemical engineering processes to cultivate desired microbial or eukaryotic cells in large quantities for producing antibiotics, vaccines, enzymes, and other biologicals.

• Requirements for Alien DNA Propagation

  • Alien DNA introduced into a host cell cannot multiply unless integrated into the host's genome.

  • Replication requires linking the alien DNA to a specific chromosomal sequence termed the origin of replication ($\text{ori}$).

  • Linking foreign DNA to $\text{ori}$ allows it to replicate alongside host DNA and be inherited by progeny cells.

• Three Basic Steps in Genetically Modifying an Organism

  1. Identification of DNA containing desirable genes.

  2. Introduction of the identified DNA into a suitable host.

  3. Maintenance of the introduced DNA within the host and its transfer to progeny cells.

Tools of Recombinant DNA Technology: Restriction Enzymes

• Discovery and Function

  • In $1963$, two enzymes restricting bacteriophage growth in Escherichia coli were isolated:

  • One enzyme added methyl groups to bacterial DNA.

  • The other cleaved viral DNA and was named restriction endonuclease.

  • In $1968$ (five years later), the first restriction endonuclease, Hind II, was isolated and characterized.

  • Hind II cuts DNA molecules at specific points by recognizing a distinct sequence of $6\text{ base pairs}$ (its recognition sequence).

  • Over $900$ restriction enzymes have been isolated from more than $230$ bacterial strains, each recognizing unique sequence motifs.

• Nomenclature Rules

  • First Letter: Derived from the genus name of the prokaryotic cell (capitalized).

  • Second and Third Letters: Derived from the species name (lowercase).

  • Fourth Letter: Represents the strain designation (e.g., 'R' in EcoRI comes from E. coli strain RY 13).

  • Roman Numerals: Indicate the chronological order in which the enzyme was isolated from that bacterial strain.

• Classification and Mechanism

  • Restriction enzymes belong to a broad class called nucleases, divided into two types:

  • Exonucleases: Remove nucleotides sequentially from the terminal ends of DNA molecules.

  • Endonucleases: Make internal cleavage cuts at specific palindromic sequences within the DNA double helix.

  • Palindromic Sequences in DNA:

  • Sequence of base pairs that reads identical on both complementary strands when orientation of reading is maintained in the same direction (e.g., $5' \rightarrow 3'$ or $3' \rightarrow 5'$).

  • Example (EcoRI sequence):

    • $5' \text{--- GAATTC --- } 3'$

    • $3' \text{--- CTTAAG --- } 5'$

  • Creation of Sticky Ends:

  • Restriction endonucleases cleave the strands slightly away from the center of the palindromic site, between the same two bases on opposite strands.

  • Leaves single-stranded overhanging terminals called sticky ends.

  • Sticky ends readily form hydrogen bonds with complementary cut fragments, facilitating the action of DNA ligase.

Separation and Isolation of DNA Fragments

• Agarose Gel Electrophoresis

  • Cleavage of DNA by restriction enzymes produces fragments separated via gel electrophoresis.

  • Principle: DNA molecules are negatively charged due to phosphate backbones and migrate toward the positive electrode (anode) under an electric field.

  • Matrix: Agarose, a natural polymer extracted from seaweeds.

  • Sieving Effect: Fragments separate based on size; smaller fragments move faster and travel farther through the matrix pores than larger fragments.

• Visualization and Isolation

  • Unstained DNA is invisible under normal light.

  • DNA bands are visualized by staining with ethidium bromide and exposing the gel to ultraviolet (UV) radiation, revealing bright orange-colored bands.

  • Elution: The process of cutting out separated DNA bands from the agarose gel and extracting the DNA from the gel matrix.

  • Purified DNA fragments are subsequently ligated with cloning vectors.

Tools of Recombinant DNA Technology: Cloning Vectors

• Properties of Vectors

  • Plasmids and bacteriophages replicate independently of bacterial chromosomal control.

  • Bacteriophages exhibit high copy numbers per host cell.

  • Plasmids maintain varying copy numbers ranging from $1\text{--}2$ copies to $15\text{--}100$ copies (or higher) per cell.

• Essential Features of Cloning Vectors

  1. Origin of Replication ($\text{ori}$):

  • Sequence where DNA replication initiates.

  • Controls the copy number of the linked foreign DNA. To yield high copy numbers, cloning must occur in vectors supporting high copy replication.

  1. Selectable Marker:

  • Enables identification and elimination of non-transformants while selectively permitting growth of transformants.

  • Transformation: Procedure by which exogenous DNA is introduced into a recipient host bacterium.

  • Common selectable markers in E. coli: Genes conferring resistance to antibiotics like ampicillin ($\text{amp}^\text{R}$), chloramphenicol, tetracycline ($\text{tet}^\text{R}$), or kanamycin. Wild-type E. coli cells naturally lack these antibiotic resistance markers.

  1. Cloning (Recognition) Sites:

  • Vectors require single or minimal recognition sites for restriction enzymes. Multiple sites cause vector fragmentation, complicating cloning.

  • Insertional Inactivation (Antibiotic Resistance Selection):

    • Ligation of foreign DNA at the BamHI site of the $\text{tet}^\text{R}$ gene in plasmid pBR322 destroys tetracycline resistance.

    • Recombinants grow on ampicillin-containing medium but fail to grow on tetracycline-containing medium.

    • Non-recombinants grow on both ampicillin and tetracycline media.

  • Alternative Selection (Chromogenic Substrates):

    • Insertion of foreign DNA into the coding sequence of $\beta\text{-galactosidase}$ causes insertional inactivation of the enzyme.

    • Non-recombinant bacterial colonies hydrolyze chromogenic substrates, turning blue.

    • Recombinant colonies with an inactivated gene produce white (colorless) colonies.

  1. Vectors for Plants and Animals:

  • Agrobacterium tumefaciens: Plant pathogen that transfers a specific segment called T-DNA into dicot plant cells, inducing tumor development. Modified into a non-pathogenic Ti plasmid vector to deliver genes into plant cells.

  • Retroviruses: Disarmed viral vectors capable of delivering therapeutic or desirable genes into animal host cells.

Competent Hosts and Gene Transfer Methods

• Bacterial Transformation Techniques

  • DNA is a hydrophilic molecule and cannot naturally pass through hydrophobic cell membranes.

  • Chemical Competence: Cells are treated with specific concentrations of divalent cations (such as calcium, $\text{Ca}^{2+}$), increasing cell wall permeability.

  • Heat Shock Protocol:

  1. Incubate competent cells with recombinant DNA on ice.

  2. Subject cells to heat shock at $42^\circ\text{C}$ for a brief duration.

  3. Return cells immediately to ice.

  • This temperature differential forces DNA uptake through cell wall pores.

• Alternative Direct Vectorless Methods

  • Micro-injection: Recombinant DNA is directly injected into the nucleus of an animal cell using a micro-needle.

  • Biolistics / Gene Gun: Plant cells are bombarded at high velocity with tungsten or gold micro-particles coated with DNA.

  • Disarmed Pathogens: Host cells are infected with modified, non-virulent viruses or bacteria carrying recombinant DNA.

Processes of Recombinant DNA Technology

• Sequential Overview

  1. Isolation of genetic material (pure DNA).

  2. Fragmentation of DNA using restriction endonucleases.

  3. Isolation of targeted DNA fragments.

  4. Ligation of DNA fragments into cloning vectors.

  5. Transfer of recombinant DNA into a host organism.

  6. Culturing host cells at large scale in growth media.

  7. Downstream extraction and purification of desired proteins.

• Isolation of Genetic Material (DNA)

  • Cell walls and membranes must be lysed using cell-wall digesting enzymes:

  • Lysozyme: Breaks bacterial cell walls.

  • Cellulase: Breaks plant cell walls.

  • Chitinase: Breaks fungal cell walls.

  • Contaminants are removed using degradation enzymes:

  • Ribonuclease (RNase): Digests RNA.

  • Protease: Removes proteins (including histone proteins).

  • Precipitation: Addition of chilled ethanol precipitates purified DNA out of solution, visible as fine collection threads removed by spooling.

• Cutting and Ligation of DNA

  • Purified DNA and vector DNA are incubated separately with the same restriction enzyme under optimal temperature and pH conditions.

  • Digestion progress is monitored via agarose gel electrophoresis.

  • Cleaved gene fragments and opened vectors are mixed in the presence of DNA ligase to construct recombinant DNA.

• Amplification of Gene using PCR (Polymerase Chain Reaction)

  • Synthesizes multiple copies of a targeted DNA segment in vitro using two sets of oligonucleotide primers and DNA polymerase.

  • Cycle Steps:

  1. Denaturation: High temperatures melt double-stranded target DNA into single strands.

  2. Primer Annealing: Synthetic primers complementary to specific regions bind to target single strands.

  3. Extension: Thermostable Taq polymerase (isolated from the thermophilic bacterium Thermus aquaticus) extends primers using provided deoxynucleotides (dNTPs).

  • Subjecting DNA to $30\text{ cycles}$ amplifies the template target region approximately $1\text{ billion}$ times.

• Obtaining Foreign Gene Products

  • Expression of foreign genes in heterologous host organisms produces recombinant proteins.

  • Culturing methods for optimal expression:

  • Laboratory Culture: Small-scale culture systems used to optimize expression settings and perform preliminary purifications.

  • Continuous Culture System: Used medium is continuously drained from one side while fresh medium is added from the other, maintaining cells in their exponential/logarithmic growth phase for higher biomass yields.

• Bioreactor Design and Processing

  • Bioreactors are large-capacity vessels ($100\text{--}1000\text{ litres}$) that process raw biological materials into products using living cells or enzymes.

  • Maintains controlled growth conditions: temperature, pH, substrate availability, salts, vitamins, and oxygen levels.

  • Stirred-Tank Bioreactor Features:

  • Cylindrical body or curved base facilitating smooth mixing.

  • Equipped with an agitator system, oxygen delivery system, foam control system, temperature control system, pH control system, and periodic sampling ports.

  • Sparged Stirred-Tank Bioreactor: Sterile air bubbles are sparged through the liquid medium to maximize oxygen transfer surface area.

• Downstream Processing

  • Post-biosynthetic processing required before marketing finished products.

  • Core Steps:

  1. Separation of product from biomass.

  2. Purification of active biomolecules.

  3. Formulation with preservatives.

  • Medical products require strict clinical trials and quality control testing, which vary according to the specific product.