Comprehensive Notes on Biotechnology: Principles and Processes

Introduction to Biotechnology: Definitions and core Principles

Biotechnology is defined as the discipline that deals with the techniques of using living organisms or enzymes from these organisms to produce products and processes useful to human beings. A modern and comprehensive definition is provided by the European Federation of Biotechnology (EFB), which integrates both traditional and molecular perspectives. The EFB defines biotechnology as "the integration of natural science and organisms, cells, parts thereof, and molecular analogues for products and services." This definition encompasses the use of whole organisms as well as cellular components and artificial molecular structures to achieve practical ends.

The birth of modern biotechnology is attributed to two core principles or techniques: Genetic Engineering and the Maintenance of Microbial Contamination-Free (Sterile) Conditions. Genetic engineering is the technique used to alter the chemistry of genetic material, such as RNA or DNA, for introduction into a host organism. This process changes the phenotype of the host organism by incorporating new genetic information. The second principle involves chemical engineering processes that ensure a sterile environment. Maintaining such conditions enables the growth of only the desired microbe or eukaryotic cell in large quantities. This is essential for the manufacture of biotechnological products including antibiotics, vaccines, and enzymes.

Recombinant DNA (rDNA) and the Technology of Genetic Modification

Recombinant DNA (rDNA) is DNA derived from two different sources that has been combined in vitro, meaning outside of a living organism. There are three primary reasons for creating rDNA: to create protein products, to create multiple copies of specific genes, and to insert foreign genes into other organisms to grant them new traits. Recombinant DNA technology is the specific technique that permits the isolation of a desired gene from any organism and its subsequent transfer and expression into an organism of choice. Often, the term genetic engineering is used interchangeably with rDNA technology.

Applications of this technology allow for the production of transgenic organisms. A prominent example is the pharmaceutical production of human insulin using a transgenic strain of the bacterium Escherichia coli (E. coli). This bacteria contains and expresses the human insulin gene. Through rDNA technology, large quantities of recombinant proteins with significant medicinal value can be produced on a commercial scale.

Historical Foundations: The Success of Cohen and Boyer

In 1972, Stanley Cohen and Herbert Boyer achieved a major breakthrough in genetic engineering by inventing the technique of DNA cloning, which allowed genes to be transplanted between different biological species. They constructed the first artificial rDNA molecule by isolating an antibiotic-resistant gene from a plasmid of the bacterium Salmonella typhi. They used molecular scissors, known as restriction enzymes, to cut the DNA at restricted or specific locations.

To deliver this alien gene into a host (E. coli), they utilized a plasmid from Salmonella typhi as a vector. The antibiotic-resistant gene was linked with the E. coli plasmid using the enzyme DNA ligase. This created a new combination of autonomously replicating circular DNA in vitro, known as recombinant DNA. When this rDNA was transferred into E. coli, the gene was not only stable but also replicated alongside the E. coli plasmid. Furthermore, the gene was transcribed and translated, meaning the host E. coli exhibited the new trait of antibiotic resistance. This ability to multiply copies of recombinant DNA in E. coli is referred to as cloning.

Fundamentals of Gene Cloning and Organism Modification

Gene cloning is the process of synthesizing multiple copies of particular DNA sequences using a bacterial cell or another organism as a host. In this process, the gene of interest is integrated into a rDNA vector—often a self-replicating DNA molecule like a plasmid—and then amplified within an appropriate host cell. The basic steps required to genetically modify an organism include: (i) the identification of DNA with the desirable gene, (ii) the introduction of the identified DNA into the host, and (iii) the maintenance of the introduced DNA in the host and its transfer to the progeny.

Major Tools of Recombinant DNA Technology: Restriction Enzymes

There are three major tools required for rDNA technology: restriction enzymes, cloning vectors, and a competent host organism. Restriction enzymes (RE) were first isolated by Hamilton in 1970, with the first identified enzyme being Hind II. These enzymes belong to a larger group of proteins called nucleases and are often referred to as "molecular scissors" because they cut DNA strands at characteristic base sequences. Their discovery was fundamental to the development of modern rDNA technology.

In nature, these enzymes exist in bacteria as part of a defense mechanism known as the restriction-modification system. This system consists of two components. The first is a restriction endonuclease that selectively recognizes a specific DNA sequence and degrades any foreign DNA containing that sequence. The second is a modification enzyme that adds a methyl group to one or two bases within the sequence recognized by the RE. Once the DNA is methylated, the restriction enzyme fails to recognize and cleave it. This allows the bacteria to protect its own DNA from its own restriction enzymes.

Classification and Naming of Restriction Enzymes

Every restriction enzyme recognizes a specific host sequence called a restriction sequence or recognition site. For example, the functioning of Hind II from Haemophilus influenzae depends on a specific DNA nucleotide sequence of six base pairs. To date, more than $900$ restriction enzymes have been isolated from over $230$ strains of bacteria, each with its own unique recognition sequence.

The convention for naming these enzymes follows a specific pattern: the first letter comes from the genus of the prokaryotic cell, and the next two letters come from the species. For instance, in EcoRI, "E" comes from Escherichia and "co" comes from coli. The letter "R" is derived from the name of the strain (in this case, RY13). Roman numerals following the name indicate the order in which the enzymes were isolated from that particular strain of bacteria.

Restriction enzymes are classified into two kinds of nucleases: Exonucleases and Endonucleases. Exonucleases digest DNA or RNA by removing nucleotides starting from the ends of the strands; they require a free end to function. Endonucleases, however, catalyze the hydrolytic cleavage of DNA in the middle of a strand or double helix. These endonucleases are the critical tools used in rDNA technology.

Functioning of Restriction Enzymes and Sticky Ends

Restriction enzymes function by inspecting the length of a DNA sequence until they find a specific recognition sequence. Once found, the enzyme binds to the DNA and cuts each of the two strands of the double helix at specific points in their sugar-phosphate backbones. These enzymes recognize palindromic sequences of nucleotides—sequences where the base sequence of one strand reads the same as its complement when read in the opposite direction. For example: $5'\text{ GAATTC }3'$$3'\text{ CTTAAG }5'$

Restriction enzymes typically cut the DNA strands a little away from the center of the palindromic site, but between the same two base pairs on both strands. This leaves single-stranded portions at the ends known as "sticky ends." These ends are named so because they form hydrogen bonds with their complementary cut counterparts. This adhesiveness facilitates the action of the enzyme DNA ligase, which connects the sugar-phosphate backbone by forming covalent bonds. To ensure proper joining, both the vector DNA and the source DNA must be cut by the same restriction enzyme.

Separation of DNA Fragments via Gel Electrophoresis

After DNA is cut by restriction endonucleases, the resulting fragments are separated using gel electrophoresis. This technique separates macromolecules based on their size. It exploits the fact that DNA molecules are negatively charged due to the presence of phosphate groups. When placed in an electric field, DNA fragments are forced to move towards the anode ($+\text{ve}$ terminal) through a matrix.

The most common matrix used today is agarose gel, a natural linear polymer of D-galactose and $3,6\text{-anhydro-L-galactose}$ extracted from seaweeds. The gel acts as a sieve; smaller fragments move farther and faster than larger ones. Because DNA is invisible to the naked eye, the separated fragments are visualized by staining the gel with ethidium bromide followed by exposure to UV radiation. Under these conditions, the DNA fragments appear as bright orange-colored bands. These bands are then cut out from the agarose gel and the DNA is extracted from the gel piece in a process called elution. The purified DNA fragments are then ready to be joined with cloning vectors to form rDNA.

Cloning Vectors and the Origin of Replication

Vectors are DNA molecules used to carry a foreign DNA segment into a host cell for cloning. Common vectors include plasmids (autonomously replicating circular extrachromosomal DNA found in prokaryotes like E. coli) and bacteriophages (viruses that infect bacteria). To facilitate cloning, a vector must possess several key features: an Origin of Replication (ori), a selectable marker, and cloning sites.

The Origin of Replication (ori) is the specific DNA sequence where replication is initiated. Any foreign DNA linked to this sequence will replicate within the host cell. The ori also controls the "copy number," which is the number of copies of the vector present in a single cell. Depending on the vector, the copy number can range from $1$ to $100$ copies per cell.

Selectable Markers and Cloning Sites

A selectable marker is essential for identifying and eliminating non-transformants while permitting the growth of transformants. Transformation is the procedure by which a piece of foreign DNA is introduced into a bacterial host. In E. coli, genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline, or kanomycin serve as useful selectable markers, as normal E. coli do not naturally possess resistance to these antibiotics.

Cloning sites are unique restriction endonuclease recognition sites where the foreign DNA is inserted. A vector should ideally have only one recognition site for a particular enzyme; multiple sites would generate several fragments, complicating the cloning process. Ligation of alien DNA is often performed at a restriction site located within one of the antibiotic resistance genes. Furthermore, vectors should be small in size, as large molecules are prone to breaking down during purification and manipulation.

Selection of Recombinants and Insertional Inactivation

Distinguishing recombinants from non-recombinants can be done through the inactivation of antibiotic resistance genes or insertional inactivation. For example, in the vector pBR322, if a foreign DNA is ligated at the BamHI site within the tetracycline resistance ($\text{tet}^R$) gene, the plasmid loses its resistance to tetracycline. These recombinants can still be selected because they retain ampicillin resistance ($\text{amp}^R$). When plated on ampicillin, all transformants grow. When sub-cultured onto tetracycline plates, the recombinants will fail to grow, whereas non-recombinants will grow on both. This simultaneous plating method is considered cumbersome.

An alternative is insertional inactivation using a chromogenic substrate. In this method, the recombinant DNA is inserted within the coding sequence of the enzyme $\beta\text{-galactosidase}$. This insertion inactivates the enzyme's production. Bacterial colonies that do not have the insert will produce $\beta\text{-galactosidase}$ and turn blue in the presence of a chromogenic substrate. Recombinant colonies, having inactivated the gene, do not produce color (appearing white), allowing for easy visual identification.

Specialized Vectors for Plants and Animals

Specific vectors are used for different types of organisms. For plants, the Ti (tumor-inducing) plasmid of Agrobacterium tumifaciens is used. Agrobacterium tumifaciens is a rod-shaped, gram-negative soil bacterium and a pathogen that causes Crown Gall disease in several dicot plants. It delivers a piece of DNA known as "T-DNA" to transform normal plant cells into tumor cells. This Ti plasmid is modified (disarmed) to remove its pathogenic qualities while remaining a functional vector for delivering genes. Similarly, in animals, viruses such as retrovirus, adenovirus, and papillomavirus are disarmed and used as vectors to deliver desirable genes into animal cells.

Creating a Competent Host for Transformation

Because DNA is a hydrophilic molecule, it cannot pass through cell membranes easily. Therefore, the host bacteria must be made "competent" to take up the DNA. Several methods are used for this purpose:

1. Chemical Treatment: Cells are treated with a specific concentration of a divalent cation, such as Calcium ($Ca^{2+}$), which increases the efficiency of DNA entry through pores in the cell wall.
2. Heat Shock: The rDNA is incubated with the cells on ice, then placed at $42^\circ\text{C}$, and then returned to ice.
3. Microinjection: Recombinant DNA is directly injected into the nucleus of an animal cell using a glass micropipette.
4. Biolistics or Gene Gun: In plants, cells are bombarded with high-velocity micro-particles of gold or tungsten coated with the gene of interest.
5. Electroporation: An electric current creates transient microscopic pores in the host cell membrane through which DNA enters.
6. Disarmed Pathogens: Vectors like Agrobacterium tumifaciens or retroviruses are allowed to infect the cell, transferring the rDNA naturally.

The Process of DNA Isolation and Cutting

Recombinant DNA technology follows a specific sequence of steps. The first is the isolation of the genetic material (DNA). This involves breaking the cell membrane or cell wall using enzymes like lysozyme (for bacteria), cellulase (for plants), or chitinase (for fungi). RNA is removed using ribonuclease, and proteins are removed using protease. Purified DNA is ultimately precipitated out by adding chilled ethanol.

The second step is the cutting of DNA at specific locations. The purified DNA is incubated with restriction enzymes under optimum conditions. Agarose gel electrophoresis is used to monitor the progress of the digestion. This process is repeated for the vector DNA. Finally, the gene of interest and the vector DNA are ligated using DNA ligase to form the recombinant DNA.

DNA Amplification via Polymerase Chain Reaction (PCR)

Polymerase Chain Reaction (PCR) is a process of repetitive bidirectional synthesis of DNA in vitro. It allows for the production of multiple copies (billions) of a gene of interest using two sets of primers and the enzyme DNA polymerase. Specifically, Taq polymerase, a heat-resistant enzyme extracted from the bacterium Thermus aquaticus, is used. This bacterium lives in hot springs and its polymerase can withstand the high temperatures required for the reaction.

The three steps of PCR are:

1. Denaturation: The double-stranded DNA is heated to $95^\circ\text{C}$ for $15$ seconds to separate into two template strands.
2. Annealing: Two sets of primers are added and allowed to bind to the $3'$ ends of the separated strands.
3. Extension: DNA polymerase (Taq) extends the primers by adding nucleotides complementary to the template.

This cycle is repeated many times (usually $30$ cycles) to amplify the DNA up to a billion times. Taq polymerase is preferred over standard E. coli DNA polymerase because it remains stable during the high-temperature denaturation phase.

Ligation, Insertion, and Scale-up Production

Once amplified, the DNA fragments and vector (cut with the same enzyme to ensure matching sticky ends) are ligated by mixing them with DNA ligase. The resulting rDNA is then inserted into recipient cells that have been made competent. For example, if rDNA carrying an ampicillin resistance gene is transferred into E. coli, those cells become ampicillin-resistant. These transformants can be selected by growing them on agar plates containing ampicillin, where only the transformed cells will survive.

To produce the desired product, the transformed host cells are grown in nutrient medium under optimal conditions. When a protein-encoding gene is expressed in a heterologous host, the resulting protein is called a recombinant protein. This can be done on a small scale in laboratories or on a large scale. On a large scale, cells are grown in a continuous culture system, where fresh medium is added from one side and the desired protein/cells are collected from the other to maintain the cells in their exponential growth phase.

Bioreactors and Downstream Processing

Large-scale production utilizes bioreactors, which are vessels of large volume ($100$ to $1000\, \text{litres}$). These provide optimal growth conditions (temperature, pH, substrate, salts, vitamins, and oxygen) to achieve high yields of the desired product. The most common type is the stirred-tank bioreactor. These are usually cylindrical with a curved base to facilitate mixing. Components include an agitator system, an oxygen delivery system, foam control, temperature control, pH control, and sampling ports for periodic withdrawal of the culture. A variation is the sparged stirred-tank bioreactor, where sterile air is bubbled through the tank to dramatically increase the oxygen transfer area.

After production in the bioreactor, the product undergoes downstream processing before it is marketed. This includes the separation of products from the reactor, purification, formulation with suitable preservatives, and strict quality control testing. In the case of drugs, clinical trials are also mandatory.