RNA Takes Over
LITTLE HAS BEEN SAID so far about information, the reason being that whatever led to the appearance of the first RNA molecules, it was not an anticipation of their informational role. Initially, RNA was a product of chemical determinism. Information arose as an emergent property. A few words concerning this property will be helpful before we proceed.
All living organisms are constructed according to a blueprint that is transmitted from generation to generation. Plague bacilli beget plague bacilli, orchids beget orchids, mites beget mites, humans beget humans. For this reason, the blueprint is called genetic (the root gen, as in genesis, comes from the Greek verb meaning "to be born"). The genetic blueprint consists of units, or genes, which together make up the genome, or genotype, of the organism. Genes have two cardinal properties: (1) they can be copied, the condition of hereditary transmission; and (2) they can be expressed into properties of the organism, which, together, constitute its phenotype. The underlying phenomena are entirely chemical.
In all extant forms of life, the genetic blueprint is written into molecules of deoxyribonucleic acid (DNA), a material closely related to RNA and similarly constructed from four different kinds of nucleotides. The sequence of nucleotides determines the informational content of the molecules--just as the sequence of letters determines the informational content of words.
DNA is the stuff of our genes and, for this reason, deserves its preeminent position in the symbolism of life. Its function, however, is strictly limited to the storage of genetic information (and the replication of that information when a cell divides, so that each daughter cell has a copy). When it comes to expression of the information, DNA is invariably transcribed first into RNA. Transcription is not very differ