Genetic programs [in biology]
Genetic programs are encoded as sequences of four possible nucleotide bases on strands of DNA or RNA. The simplest known viruses have programs that are a few thousand elements in length; bacteria typically have programs that are a few million elements; fruit flies a few hundred million; and humans around four billion. There is not a uniform correspondence between apparent sophistication of organisms and lengths of genetic programs: different species of amphibians, for example, have programs that can differ in length by a factor of a hundred, and can be as many as tens of billions of elements long. Genetic programs are normally broken into sections, many of which are genes that provide templates for making particular proteins. In humans, there are perhaps around 40,000 genes, specifying proteins for about 200 distinct cell types. Many of the low-level details of how proteins are produced is now known, but higher-level issues about organization into different cell types remain somewhat mysterious. Note that although most of the information necessary to construct an organism is encoded in its genetic program, other material in the original egg cell or the environment before birth can probably also sometimes be relevant.