The Central Dogma of Molecular Biology

Having established that DNA is the genetic material, that it is structured as a double helix, and that it replicates semiconservatively to be faithfully passed from one generation of cells to the next, the chapter now turns to the equally fundamental question of how the genetic information stored in DNA is actually USED by the cell — how a gene's coded sequence of nucleotides is converted into a functional protein that carries out the cell's biological activities. Francis Crick, in 1958, proposed a general framework describing the normal direction of flow of genetic information within a cell, which he called the Central Dogma of molecular biology.

The Central Dogma states that genetic information normally flows in one direction: from DNA to RNA (a process called transcription), and then from RNA to protein (a process called translation). In other words, DNA acts as the permanent, master template from which working copies (RNA) are made as and when needed; these RNA copies are then, in turn, used as the template to direct the actual assembly of protein molecules, the functional workhorses that carry out most of a cell's biological activities. Crically, the Central Dogma also specifies what does NOT normally happen: information does not normally flow backward from protein to nucleic acid (a protein's amino-acid sequence cannot be "read back" to reconstruct the DNA or RNA sequence that produced it), reflecting the fact that a protein is the functional END PRODUCT of gene expression, not itself a template for further copying.

The original, simplest statement of the Central Dogma (DNA to RNA to protein, one-way only) has since been refined and extended as subsequent discoveries expanded our understanding of how genetic information can move within a cell — most notably with the discovery of the enzyme reverse transcriptase in certain RNA viruses (retroviruses, such as HIV), which is able to synthesise a DNA copy from an RNA template, an information flow from RNA to DNA that the original Central Dogma's simplest one-way statement did not anticipate. This reverse-transcription pathway is nonetheless still considered fully consistent with the deeper principle the Central Dogma expresses — namely, that a protein's own amino-acid sequence is never itself used as a template to reconstruct a nucleic-acid sequence; information can flow between the various forms of nucleic acid (DNA to RNA, RNA to DNA) but never backward out of protein. …