Biotechnology: Principles & Processes

Biotechnology encompasses the tools and processes of recombinant DNA (rDNA) technology — the ability to combine DNA from two different organisms and introduce it into a host cell for expression. NEET tests this topic heavily through factual recall of enzyme functions, vector features, PCR steps, and electrophoresis principles.

Restriction enzymes are molecular scissors discovered in bacteria, where they restrict bacteriophage growth by cleaving foreign DNA. EcoRI, isolated from Escherichia coli strain RY₁₃, recognizes the palindromic sequence 5'-GAATTC-3' and makes a staggered cut, producing sticky ends — short single-stranded overhangs that are complementary and can base-pair with matching fragments. This is distinct from blunt ends, produced by enzymes that cut both strands at the same position. A palindromic sequence reads identically on both strands in the 5' to 3' direction. Over 900 restriction enzymes have been identified from more than 230 bacterial strains. DNA ligase acts as molecular glue, joining DNA fragments by sealing phosphodiester bonds between sticky or blunt ends.

Vectors carry foreign DNA into host cells. The plasmid pBR₃₂₂ is the classic cloning vector with four essential features: an origin of replication (ori) for autonomous replication, two selectable markers (ampicillin resistance — ampR, and tetracycline resistance — tetR), multiple restriction sites, and small size for easy manipulation. Recombinant selection uses insertional inactivation: when foreign DNA inserts into the tetR gene, tetracycline resistance is lost, distinguishing recombinants from non-recombinants. The Ti plasmid from Agrobacterium tumefaciens naturally transfers its T-DNA into plant genomes, making it the primary vector for plant genetic engineering — it is disarmed by removing tumor-inducing genes.

The cloning process follows a defined sequence. DNA isolation begins with cell lysis using lysozyme (bacterial walls) or cellulase (plant walls), followed by RNA removal (RNase), protein removal (protease), and DNA purification by ethanol precipitation, where DNA spools out as fine threads. Both insert and vector are cut with the same restriction enzyme, then joined by DNA ligase to create recombinant DNA. This is introduced into competent host cells prepared by CaCl₂ treatment followed by heat shock at 42 degrees C for 60-90 seconds. Alternative transfer methods include microinjection (directly into nucleus), biolistics or gene gun (DNA-coated gold or tungsten particles fired at high velocity), and electroporation (brief electrical pulses create transient pores in the membrane).

PCR (Polymerase Chain Reaction), developed by Kary Mullis, amplifies specific DNA segments in vitro through repeated thermal cycling. Each cycle has three steps: denaturation at 94-98 degrees C (DNA strands separate), annealing at 50-65 degrees C (primers bind to complementary sequences), and extension at 72 degrees C (Taq DNA polymerase synthesizes new strands). Taq polymerase, isolated from the thermophilic bacterium Thermus aquaticus, is thermostable and withstands repeated heating. After n cycles, 2^n copies are produced.

Gel electrophoresis separates DNA fragments through an agarose gel matrix. DNA, being negatively charged due to its phosphate backbone, migrates toward the anode (positive electrode). Smaller fragments travel faster and farther. Fragments are stained with ethidium bromide and visualized as bright orange bands under UV light.

Bioreactors enable large-scale production. The stirred-tank bioreactor features an agitator for mixing, a sparger for air supply, and systems for controlling temperature, pH, and dissolved oxygen. After production, downstream processing involves separation, purification, formulation, and quality control of the desired product.

The key testable concept is the PCR thermal cycling sequence (denaturation-annealing-extension) with exact temperatures, and the reason DNA migrates toward the anode in gel electrophoresis (negative phosphate charge).