Principles of genetic engineering

Modern biotechnology rests on two core techniques. Genetic engineering changes the chemistry of the genetic material (DNA and RNA) and introduces it into host organisms, changing the host's phenotype.

Bioprocess engineering keeps chemical-engineering processes sterile (free of microbial contamination), so that only the desired microbe or eukaryotic cell grows, in large quantities, to make products such as antibiotics, vaccines and enzymes.

Traditional hybridisation in plant and animal breeding often carries undesirable genes along with the desired ones. Genetic engineering (recombinant DNA, gene cloning and gene transfer) lets us introduce only the one gene, or set of genes, that we want.

A piece of foreign DNA put into an organism usually cannot multiply in its progeny cells. It multiplies only if it becomes part of a chromosome carrying the origin of replication, the sequence where replication begins.

So foreign DNA is linked to an origin of replication, letting it replicate in the host. This making of many identical copies of a template DNA is cloning.

The first recombinant DNA was made in 1972 by Stanley Cohen and Herbert Boyer. They cut an antibiotic-resistance gene out of a plasmid and joined it to a native plasmid of Salmonella typhimurium. A plasmid is circular, extra-chromosomal DNA that replicates on its own.

Restriction enzymes act as molecular scissors that cut DNA at specific places. DNA ligase joins the cut ends. The plasmid is the vector that carries the foreign DNA, much as a mosquito carries the malarial parasite.

Transferred into Escherichia coli, a close relative of Salmonella, the recombinant plasmid replicated using the host's DNA polymerase. This was called cloning the antibiotic-resistance gene in E. coli.

Genetically modifying an organism takes three basic steps: identify the DNA carrying the desirable genes, introduce it into the host, and maintain it in the host and pass it on to the progeny.