pBR322 is one of the most important and widely used cloning vectors in the history of molecular biology. Constructed by Francisco Bolivar and Raymond Rodriguez (BR = their initials, 322 = clone number) in 1977 at University of California, San Francisco. Properties: 4361 bp circular double-stranded DNA. Origin of replication (ori): allows autonomous replication in E. coli (derived from ColE1 plasmid). Ampicillin resistance gene (ampR): encodes beta-lactamase enzyme which inactivates ampicillin by hydrolysing the beta-lactam ring. Tetracycline resistance gene (tetR): encodes a membrane protein that exports tetracycline from the cell. Multiple restriction enzyme sites at known locations for inserting foreign DNA. Maintained in E. coli in ~15-20 copies per cell.

Key restriction enzyme sites and their locations: Within ampR gene: PstI site — insertion here disrupts ampR → bacterium becomes ampicillin-sensitive. Within tetR gene: BamHI site — insertion here disrupts tetR → bacterium becomes tetracycline-sensitive. SalI site — also within tetR → insertion → tetracycline-sensitive. EcoRI site: outside both resistance genes → insertion here does not disrupt either gene (both resistances retained). HindIII site: within tetR. ClaI site: near tetR. Selection strategy exploits these insertional inactivation events. Different cloning strategies use different restriction sites to allow screening for recombinants by differential antibiotic sensitivity.

Insertional inactivation: when foreign DNA is inserted into the middle of a gene, the gene sequence is disrupted → the gene product is non-functional → the phenotype conferred by that gene is lost. In pBR322: Normal bacteria (no recombinant): both ampR and tetR intact → grow on both ampicillin AND tetracycline media. Recombinant bacteria (insert at BamHI): ampR intact, tetR disrupted → grow on ampicillin but NOT on tetracycline. This differential growth pattern allows identification of recombinants. Steps: Transform E. coli with pBR322-insert ligation mixture. Plate on ampicillin medium → select for bacteria that took up any plasmid (all transformants survive). Replica plate on tetracycline medium → recombinants die (tetR disrupted). Colonies that grow on Amp but NOT Tet = recombinant colonies with insert.

pUC vectors (derived from pBR322) use blue-white screening instead of dual antibiotic selection. pUC19: contains lacZ gene (coding for N-terminal portion of beta-galactosidase). Multiple Cloning Site (MCS) is located within lacZ. Selection: X-gal (5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside) added to growth medium. IPTG (induces lacZ expression) added. Plates contain ampicillin. Non-recombinant (intact lacZ): cleaves X-gal → blue colour → BLUE colonies. Recombinant (insert disrupts lacZ): no active beta-galactosidase → X-gal not cleaved → WHITE colonies. Selection: grow on ampicillin (selects transformed bacteria). Look for white colonies (recombinants). Much easier than replica plating needed for pBR322. Modern expression vectors: pET (IPTG-inducible T7 promoter), pGEX (GST fusion), pMAL (MBP fusion) — all designed for specific purposes.

Essential features of a cloning vector: Origin of replication (ori): allows autonomous replication inside host cell without being integrated into host chromosome. Selection marker: usually antibiotic resistance gene — allows selection of cells that took up vector. If no marker: cannot distinguish transformed from non-transformed cells. Restriction enzyme sites: unique restriction sites for inserting foreign DNA. Site must be unique (cut only once) to insert DNA in one place. Multiple Cloning Site (MCS): region with many unique restriction sites clustered together — gives flexibility in choosing restriction enzyme. Small size: smaller plasmids replicate more efficiently and are easier to manipulate. Stability: plasmid must be stably maintained in host cells through many generations. Expression elements (if expression vector): promoter, ribosome binding site, terminator.

Plasmid vectors: pBR322, pUC19, pET — for moderate-sized inserts (up to ~10 kb). Bacteriophage vectors: Lambda phage (8-20 kb inserts), M13 (for single-stranded DNA). Cosmid vectors: plasmid + cos sites of lambda → 35-45 kb inserts. BAC (Bacterial Artificial Chromosome): based on F-factor → 100-300 kb inserts. Used in Human Genome Project. YAC (Yeast Artificial Chromosome): telomeres + centromere + ori → 200 kb-2 Mb inserts. Shuttle vectors: can replicate in two different host organisms (e.g., E. coli AND yeast). Expression vectors: have strong promoter + ribosome binding site → high level protein expression (pET series with T7 promoter). Binary vectors: for plant transformation via Agrobacterium (contain T-DNA borders).

Transformation: process of introducing foreign DNA (plasmid) into bacterial cells. Natural competence: some bacteria (Bacillus, Haemophilus, Streptococcus) naturally take up DNA. E. coli is not naturally competent — must be made artificially competent. Chemical competence (CaCl2 method): Bacteria treated with cold CaCl2 → cell membrane becomes permeable. Mix with DNA → heat shock (42°C for 90 seconds) → heat shock temporarily destabilises membrane → DNA enters. Ice → recovery in rich medium → plate on selective medium. Efficiency: ~10⁶ transformants per microgram of plasmid. Electroporation: high voltage electric pulse → creates transient pores in membrane → DNA enters. More efficient than chemical method (~10⁸ transformants/μg). Used for difficult-to-transform bacteria and yeast. Agrobacterium: naturally transforms plant cells (via Ti plasmid T-DNA). No artificial competence needed for plants with Agrobacterium.

Medical applications: Recombinant insulin (human insulin gene in E. coli/yeast → Humulin, 1982). Human growth hormone (prevents pituitary dwarfism). Factor VIII (haemophilia A treatment). Hepatitis B vaccine (HBsAg in yeast). Erythropoietin (anaemia in kidney failure). Interferon (antiviral). Monoclonal antibodies (Herceptin, Rituximab). Agricultural: Bt crops (insect resistance), herbicide tolerance, virus resistance, drought tolerance, Golden Rice (Vit A). Industrial: enzymes (amylase, lipase, protease for detergents), biofuels, bioplastics. Research: gene knockout mice, gene therapy experiments, CRISPR/Cas9 genome editing, sequencing. The entire biotechnology industry rests on the ability to clone, express, and manipulate genes — all made possible by restriction enzymes, cloning vectors, and host organisms like E. coli.