Two weeks ago I wrote about the central dogma and how the analogy of computer technology relates. This week, I would like to further that analogy and talk about some of the regulation mechanisms, and also point out some areas where this analogy could be misleading.
Selective protein production is important in cell differentiation, so it is also important to look at how protein production is regulated. One mechanism called enzyme induction was intensively studied in E. Coli, a type of bacteria. E. Coli metabolizes glucose as an energy source, but also has that ability to break down more complex sugars like lactose using an enzyme called β-galactosidase. Scientists discovered that when E. Coli was grown on a medium including glucose, little of the enzyme B-galactosidase was produced, but when there was no glucose present, only lactose, this enzyme was produced in large quantities. Carroll deems this "gene logic," meaning that enzymes are only produced when they are needed. When lactose is present it binds to the repressor (a molecule blocking the transcription of the gene when it is not needed), and makes the repressor fall off which allows the gene to be transcribed and translated to produce the B-galactosidase enzyme to break down lactose. The repressor of the B-galactosidase gene is an example of one of many DNA-binding proteins that bind to genetic switches in bacteria and yeast. Very similar to the "Boolean logic" used in computer technology where essentially all "decisions" made by the computer are either 0 or 1, these systems are deemed either "on" or "off."
A gene may have one of these switches, or may have various switches. It is also possible that one protein may control multiple switches. In order to coordinate development of an organism, the switches of different genes may share multiple inputs or similar sequences. For specific cell types, it is often found that the genes for certain proteins are activated by switches with common sequences and utilize the same "tool kit" protein. The developmental steps performed by these individual switches and proteins are connected to the switches and proteins of other genes, in essence creating a large "cascade," which can be diagramed in a similar way to electrical circuits and networks. Each switch is a decision point, like one node in an electrical circuit; the activators and repressors act on the switches to turn them on/off. There are multiple "tiers" of a circuit that define the cascade. Each one of these "circuits" can represent one structure with-in the organism. It takes multiple circuits connected to form a network in order to produce a whole and complete, complex organism.
Although modern computer technology provides a good analogy for the cascade of regulatory mechanism within complex organisms, no metaphor is perfect. Modern computer technology and computer programming is strict, if one code is entered improperly the program simply doesn't work. However, with biological systems there are mechanisms for repair and essentially "room for error," in most cases. If a wrong base is added to a sequence the cell has multiple mechanisms to excise this base, or minimize the effects the error will cause. There are lethal mutations, that are more similar to the mis-coding of a computer, but again there are many mechanisms in place to avoid these mutations. Another area that this metaphor fails to address is reproduction and evolution. Obviously a computer system is absolute and does not pass on its hard-disk information to future progeny, but it is crucial to understand this process when talking about mutations, natural selection, evolution, divergence of species and how this relates to the central dogma. Overall, comparing the central dogma to the computer technology analogy may be helpful in understanding the mechanical operations of producing and regulating different proteins, but it lacks the "big picture" view when looking at the effects of mutations and the divergence of species.