II.

The Nature of Genes

A gene is a long segment of the molecule deoxyribonucleic acid (DNA). This segment, composed of minute subunits called nucleotide bases, serves as the blueprint for manufacturing a single protein or enzyme needed for the structure or function of cells. In humans, genes are compressed and bundled into a set of 23 pairs of chromosomes, which stabilize and protect the DNA. Even a tiny error in the arrangement of a gene's nucleotide bases can lead to the production of a protein or enzyme that works improperly, disrupting cellular biology like a broken piston would wreak havoc in an automobile engine. Or the needed compounds might not be produced at all.

Biologists have known about the importance of genes since the pioneering work of Austrian monk Gregor Mendel. In a series of experiments beginning in the 1850s Mendel showed that the traits of plants are inherited in a precise, predictable manner. But little was known about the physical nature of genes until the 1950s when American biochemist James Watson and British biophysicist Francis Crick developed their revolutionary model of DNA. Watson and Crick showed that DNA is composed of two strands of nucleotides, linked to form a chain and arranged in a structure resembling a twisted ladder, called a double helix. Another key breakthrough came in the early 1970s when researchers discovered a series of enzymes that made it possible to snip apart genes at predetermined sites along a molecule of DNA, then glue them back together in a reproducible manner. These genetic advances set the stage for the emergence of the genetic engineering industry, which has produced new drugs, antibodies, and a host of other naturally occurring chemicals. The discoveries also enabled scientists to contemplate gene therapy.