Recombinant DNA technology combines DNA molecules from different sources to create new genetic combinations that do not occur naturally. It is the foundation of modern molecular biology and has enabled the production of therapeutic proteins, genetically modified organisms, and the sequencing of entire genomes.
Restriction Enzymes
Restriction endonucleases are bacterial enzymes that cleave DNA at specific recognition sequences, typically 4 to 8 base pairs in length. Type II restriction enzymes recognize palindromic sequences and cut within the recognition site, producing either blunt ends or staggered cuts that create sticky ends with short single-stranded overhangs. EcoRI recognizes GAATTC and produces sticky ends with an AATT overhang, while SmaI recognizes CCCGGG and produces blunt ends. The specificity of restriction enzymes allows defined DNA fragments to be generated and combined, while PCR provides an alternative method for amplifying specific DNA sequences.
DNA Ligase
DNA ligase seals nicks in the DNA backbone by catalyzing the formation of a phosphodiester bond between a 3-prime hydroxyl and a 5-prime phosphate. T4 DNA ligase, the most commonly used enzyme for cloning, can ligate both sticky and blunt ends. Sticky-end ligation is highly efficient because the complementary overhangs align the fragments correctly. Blunt-end ligation is less efficient but does not require compatible overhangs. The enzyme uses ATP as an energy source, forming an AMP-enzyme intermediate that activates the 5-prime phosphate.
Cloning Vectors
Plasmids are small, circular DNA molecules that replicate independently of the bacterial chromosome. They are the most common vectors for DNA cloning, carrying an origin of replication, a selectable marker such as an antibiotic resistance gene, and a multiple cloning site containing unique restriction sites. The pUC series of plasmids use the lacZ gene for blue-white screening, where insertional inactivation of lacZ allows recombinants to be distinguished from non-recombinants by their white color on X-gal plates.
Bacterial artificial chromosomes are based on the E. coli F factor and can carry inserts of up to 300 kilobases. Yeast artificial chromosomes contain centromeric, telomeric, and replication sequences for propagation in yeast and can carry inserts exceeding one megabase. These large-insert vectors are essential for genome mapping and sequencing projects.
Expression Vectors
Expression vectors contain regulatory sequences that drive transcription and translation of the cloned gene in a host organism. Bacterial expression vectors use strong promoters such as the T7 or tac promoter, often with an inducible system for controlled expression. The lac operator allows induction with IPTG, and the T7 system uses T7 RNA polymerase under lac control. Fusion tags such as 6xHis, GST, or MBP are often added to facilitate purification and detect ion.
Eukaryotic expression vectors use promoters from cytomegalovirus or simian virus 40 for strong expression in mammalian cells. They include a polyadenylation signal for proper mRNA processing and often a selectable marker such as neomycin resistance for stable cell line generation. Viral vectors derived from AAV, lentivirus, or retrovirus are used when efficient delivery and integration are required.
Host Organisms
E. coli is the most common host for DNA cloning, offering rapid growth, high transformation efficiency, and well-characterized genetics. Specialized strains are available for specific purposes: DH5-alpha for cloning, BL21 for protein expression, and CJ236 for generating uracil-containing templates. Yeast, particularly Saccharomyces cerevisiae, is used for cloning large DNA fragments and for expressing eukaryotic proteins requiring post-translational modifications.
Mammalian cell lines such as HEK293 and CHO cells are used for producing therapeutic proteins with human-compatible glycosylation patterns. Insect cells infected with baculovirus vectors provide high protein yields with eukaryotic processing. Cell-free systems using extracts from E. coli, wheat germ, or rabbit reticulocytes allow rapid protein production without living cells.
Applications
Recombinant DNA technology has produced many therapeutic proteins. Human insulin was the first recombinant therapeutic, produced in E. coli by Genentech and approved in 1982. Other recombinant proteins include growth hormone, erythropoietin, factor VIII for hemophilia, and monoclonal antibodies. The technology enables the production of human proteins in large quantities without relying on animal sources.
Genetically modified organisms are created by introducing recombinant DNA into plants, animals, or microorganisms. Bt crops express insecticidal proteins from Bacillus thuringiensis. Recombinant microbes produce industrial enzymes, biofuels, and biochemicals. The technology is also used for gene function analysis, creating knockout and transgenic organisms, and producing vaccines such as the hepatitis B surface antigen produced in yeast.
