II.8 A significant part of the information determining cell characteristics and, indirectly, also the characteristics of multi-cell organisms, is present in the form of epigenetic information.
Genetic information is defined as information entered in the primary structure of a nucleic acid, i.e. in the order of the nucleotides in the DNA molecule or (in some viruses) in the RNA. However, organisms also contain a large amount of further information in the structures of their cells and multi-cellular bodies, which also co-determine the course of all the molecular, biochemical and physiological processes, including ontogenesis, and thus co-determine both the characteristics and the behaviour of the organisms. This is called epigenetic information.
The entire molecular apparatus of the cell determines which DNA sections will be transcribed to the RNA at a given moment and which RNA molecules will be further translated to proteins. If an important regulation molecule were missing in the zygote, the course and result of the ontogenesis of the given organism could be seriously disturbed and altered, even if the genetic information in the zygote gene pool is not disturbed. According to some authors, the importance of the molecular apparatus interpreting the genetic information for the progress of ontogenesis is similarly important as the genetic information located in the genome (Jablonka & Lamb 1995; Markoš 2000).
It cannot be denied that, for setting into motion and directing the individual development of an individual, the presence and functioning of both components is absolutely essential. Interventions into any of them can lead to similarly important influencing of the progress and results of ontogenesis. Nonetheless, from an evolutionary standpoint, genetic information encoded in the primary structure of the nucleic acid plays a quite primary and incomparably important role. A random change in the molecular apparatus of a cell can, of course, affect the result of the relevant ontogenetic processes, i.e. the characteristics of the particular individual. However, it is very probable that the changes caused in the characteristics of the organism would lead to repetition of the same changes in the molecular apparatus of the cells in the offspring and that the given change would be transferred to further generations. In certain, quite exceptional cases, the particular change can cause the occurrence of the same changes and thus be passed on from one generation to the next. An example could consist in prions, protein molecules that can adopt two different conformations, where the presence of the less common of them can induce the transition of other molecules to this conformation. However, only a negligible percentage of molecules or other cellular structures have this characteristic and thus it cannot be expected that heredity based on this principle could occur to a greater degree in evolution. In contrast, if mutation occurs in the DNA, the relevant change is automatically transferred, because of universal copying of the primary DNA structure during the replication, to subsequent generations, and causes the same changes in ontogenesis in progeny organisms as it caused in the parent organisms. While primary those genetic changes that are in some way advantageous for their carriers have a chance to be fixed evolutionarily, only epigenetic changes that are not only advantageous, but also cause their own occurrence and can thus be transferred from one generation to the next, have a chance of evolutionary fixation. Put simply, all DNA changes have high heritability, but epigenetic changes in the molecular apparatus interpreting genetic information have much lower heritability, generally approaching zero, in the vast majority of cases. As the heritability of changes is an essential condition for the functioning of biological evolution, especially the formation of adaptive characteristics of organisms, it is almost certain that genetic and not epigenetic information acts as the main medium for the evolutionary memory of contemporary organisms.