Biotechnology is the discipline that applies biological systems and living organisms to develop or make products. At its core for NEET is recombinant DNA (rDNA) technology — the deliberate combination of DNA from two different sources and its introduction into a host cell for expression and propagation.

Restriction Enzymes — Molecular Scissors

Restriction endonucleases are the foundational tools of rDNA technology. They were discovered in bacteria, where they protect the cell by cleaving foreign (bacteriophage) DNA at specific recognition sequences. Over 900 restriction enzymes have been isolated from more than 230 bacterial strains.

EcoRI, isolated from Escherichia coli strain RY13, is the gold-standard NEET example. It recognizes the palindromic hexanucleotide sequence 5'-GAATTC-3' and cuts between G and A on both strands in a staggered manner, generating sticky ends — short (4-nucleotide) single-stranded 5' overhangs (AATT) that are complementary and capable of base-pairing with matching fragments. A palindromic sequence reads identically on both strands in the 5'→3' direction. In contrast, enzymes such as SmaI make straight (blunt) cuts. DNA ligase seals the nick by forming phosphodiester bonds between sticky or blunt ends, joining the foreign insert to the vector.

Vectors — Molecular Vehicles

A vector is a DNA molecule capable of autonomous replication that carries foreign DNA into a host cell. Vectors must possess: (1) an origin of replication (ori) for self-replication in the host, (2) selectable markers to identify transformed cells, and (3) cloning sites recognized by restriction enzymes.

pBR322 is the prototype cloning plasmid in E. coli. Its four essential features: ori, ampicillin resistance (ampR), tetracycline resistance (tetR), and multiple restriction sites. The strategy of insertional inactivation exploits these markers: when foreign DNA is ligated into the tetR gene's restriction site, tetR function is lost. Transformants are screened on ampicillin (all survive) and tetracycline (only non-recombinants survive), identifying recombinant colonies as those growing on ampicillin but not tetracycline.

The Ti plasmid from Agrobacterium tumefaciens naturally delivers its T-DNA (transfer DNA) into plant nuclear genomes. T-DNA integration causes crown gall disease; for biotechnology use, the tumor-inducing genes are removed ("disarmed"), making it a safe vector for creating transgenic plants.

The Cloning Process — Step by Step

1. DNA isolation: Cells lysed with lysozyme (bacteria) or cellulase (plants); contaminating RNA removed by RNase; proteins digested by protease; pure DNA precipitated by chilled ethanol and collected as threads.
2. Cutting: Both vector and insert cut with the same restriction enzyme.
3. Ligation: DNA ligase joins insert into vector creating recombinant DNA.
4. Transformation: Recombinant DNA introduced into competent host cells. Competency is induced by soaking E. coli cells in $CaCl_{2}$ and then applying heat shock at 42 °C for 60–90 seconds, causing uptake of recombinant DNA.
5. Selection: Recombinants identified via insertional inactivation or other markers.

Alternative physical transformation methods include microinjection (direct DNA delivery into the nucleus), biolistics/gene gun (DNA-coated gold or tungsten particles fired at high velocity using helium pressure), and electroporation (brief electrical pulses create temporary pores in the membrane).

PCR — Amplifying DNA In Vitro

Polymerase Chain Reaction (PCR), invented by Kary Mullis (Nobel Prize, Chemistry 1993), exponentially amplifies a specific DNA sequence without a living cell. Each thermal cycle has three stages:

Taq DNA polymerase (from the thermophile Thermus aquaticus) is thermostable — it is not destroyed at 94–98 °C, enabling automated cycling. After n cycles, the target DNA is amplified to 2ⁿ copies — 30 cycles yield ~10^{9} copies from a single molecule.

Gel Electrophoresis — Separating and Visualizing DNA

Agarose gel electrophoresis separates DNA fragments by size. DNA carries a net negative charge (phosphate backbone), so it migrates toward the anode (positive electrode). Smaller fragments experience less resistance and travel farther. After separation, gels are stained with ethidium bromide, which intercalates into DNA and emits bright orange fluorescence under UV light, revealing bands. Fragment sizes are determined by comparison with a DNA ladder (molecular weight marker).

Bioreactors — Industrial Scale-Up

A stirred-tank bioreactor allows large-scale (hundreds to thousands of litres) production of recombinant proteins. Key components: an agitator system (mixing), a sparger (sterile air/oxygen supply), and monitoring systems for temperature, pH, and dissolved $O_{2}$. After fermentation, downstream processing — separation, purification, formulation, and quality control — converts crude broth into a usable product.