Principle of Genetic Engineering

The principle of genetic engineering rests on a small number of biological facts that, together, make it possible to move a gene from one organism into a completely different one and have it still function correctly.

The first fact is that DNA is a chemically uniform molecule across all forms of life. Whether it comes from a bacterium, a plant, an animal, or a human being, DNA is built from the same four nucleotide bases -- adenine, thymine, guanine, and cytosine -- linked in the same double-helical backbone. This chemical uniformity means that a stretch of DNA cut out of a human chromosome is, physically and chemically, indistinguishable from a stretch of DNA cut out of a bacterial chromosome; both are simply sequences of the same four bases. It is this uniformity that allows a gene from one species to be physically joined to DNA from an entirely different species without any chemical incompatibility.

The second fact is that the genetic code -- the rule by which a sequence of three DNA bases (a codon) specifies a particular amino acid -- is, with only very minor exceptions, the same in every known living organism. Because the code is shared, a bacterial cell's ribosomes can correctly translate a human gene into the corresponding human protein, exactly as they would translate one of their own genes. Without this shared code, moving a gene into a foreign host would be pointless, because the host's translation machinery would not read it correctly.

The third fact is that many bacteria naturally carry small, circular, self-replicating DNA molecules called plasmids, separate from their main chromosome, and that some viruses (bacteriophages) similarly package and replicate their own DNA inside a bacterial host. Both plasmids and phage genomes can be modified in the laboratory to carry a foreign gene of interest and, crucially, they retain the ability to replicate themselves -- and therefore the foreign gene riding along with them -- every time the host cell divides. A molecule engineered this way, built from DNA of more than one source and capable of self-replication inside a host, is called a recombinant DNA molecule, and the modified plasmid or phage genome that carries it is called a cloning vector. …