Arquebio. Disclosure 34
Due to the common embryonic origin, all the cells of a pluricellular organism, have the same genetic information. However, not all the cells express the information at the same time and in the same way. Thus, hepatic, heart, intestinal, nerve or bone cells have the same genetic information but expressed, each cell, in a different way and allowing the realization of different specialized functions.
This phenomenon is possible because of the existence of two types of genetic material encoded in the genetic information and another epigenetic related to the environment. The first genetic material is encoder, of the protein synthesis and the second controller of the mechanisms on and off, known as genetic switches, and whose operation determines the mission that each cell plays in a complex organism.
We can see that exists common genetic material in species, apparently distant between them, in external appearance but so near, or genetically united, by a common joint evolution of encoded material genetic in the last 600 millions of years and by the controller material genetic in the last 400 millions of years.
As regard to the encoder genetic material, it is known that live beings, before fishes, like Saccharomyces or nematodes from 540 millions of years, have similar genetic design, like nowadays mammals, based on 23000 codifiers genes.
Saccharomyces cerevisiae is a unicellular yeast that contains 12 millions base pairs along 6000 genes, of which at least the 31% have equivalents in human beings.
Studies done by Sidney Brenner and Martin Chalfie lead to the conclusion that the nematode Caenorhabditis elegans has a genome about 99 millions nitrogenous base pairs along 19,100 genes of which approximately 40% coincide with other posterior organisms including humans.
Also is known, that Drosophila melanogaster, multicellular animal has 165 millions base pairs in the DNA along 13,600 genes, of which approximately the 50% has human equivalents and Mus musculus, pluricellular animal, has 40,000 genes, like humans, with 3 billions base pairs along 40,000 genes, so that almost all the human genes has a counterparty in the mouse and it seems impossible to distinguish among some DNA blocks sequenced in mouse and in humans.
It leads to the conclusion that exists a basic pattern of codifier DNA that has remained without changes along millions of evolution years and in consequence contains genetic information, basic, vital, in form of master genes. These master genes have a ancestral origin, unknown, because of the ignorance relative to the origin of nucleic acids in probionts and about their organization and how their sequencing resulted in proteins that build different organs.
In relation to the controller genetic material, it is a part of the genome, involved in the control of when and where are produced the proteins, it is a big control panel with millions of switches which regulate the activity of the codifiers genes and without them, these don't work well. These functional elements are overlapped and are the necessary information to build the differents types of cells and organs. Thus it is known that the arising of articulated limbs, into the pectoral fins of coelacanth, 416 millions of years ago, and the tiktaalik, fossil of sarcopterygian fish, of 380 millions of years, is joined to the existence of genetic switches called CsB, present in other fishes (zebrafish, mantaraya), in amphibious (frogs), reptilians, birds (chicken) or mammals (mice). The genetic material is so similar that, as Igor Schneider and Neil Shubin from Chicago University demonstrated, the CsB from mouse could activate the genetic expression in the outer edge of the fin of zebrafish in development, and the CsB of zebrafish, such as mantaraya , have the capacity to activate the genetic expression of the scaphoid, trapezium, trapezoid and phalanges in the mouse limbs. In conclusion, in spite of the evolutionary separation of 400 millions of years, the genetic switches that controls the gene expression in the fingers of mice, which are not only present in fishes, but also the versions of the fishes can activate the genetic expression in embryos of mouse.
Exposed the existence of genetic material (master genes and switches) it is important to know the epigenetic material that forms the activation and deactivation mechanisms of the genetic switches. These knowledge are the epigenetic bases that studies all the non-genetic factors involved in the organism development, from fertilized egg up to the death, and which are not consequential to change the DNA sequence. Therefore it is a question of studying the external stimulies, that by physicochemical changes of the environment turns on or off the genetic switches, and the biochemical messengers, which are the epigenetic information no encoded that interacts with them. In this way a determined environmental condition produces a biochemical messenger, which acts activating or deactivating a genetic switch, depending on the circumstances, and transmits the order to the coding gen of the protein. Thus the environment, including the nutrition, interacts with the basic patterns of the master genes. One of the most important mechanisms of deactivation is the methylation of the DNA, in which a chemical residue, known as methyl group, attacks to sections from DNA specially the cytokines. The methyl group interrupts the genetic activity avoiding to the ribosomes realize the reading and in consequence avoiding the activation of the production of the corresponding protein.
There are a lot of examples about these epigenetic mechanisms and their consequences in the changes of the genetic expression, it can have a regular seasonal life or irregular, and others are permanent. Among the epigenetic seasonal changes we can describe the adaptation of colour of the butterfly wings, of the fleece of the mammals to the temperature and to the day length differences between summer and winter. This genetic expression is regulated through endocrine hormones that acts like epigenetic factors.
Among the spontaneous epigenetic changes or irregular changes, we can explain the changes caused by the increase of metabolytes concentration in cases of overpopulation and the resulting migratory phenotype like in the case of the lobster Schistocerca gregaria.
Among the permanent epigenetic changes in the individuals we can show the influence of the alimentation and the temperature in the determination the sex during the embryonic development. Thus alimentation and the content of juvenile hormone in the royal jelly permits the silencing of Dnmt3 gene, delaying the metamorphosis, permitting the development of fertile ovaries, origining queen bees and in consequence controlling the social structure of the colony. In relation to the high temperatures of hatchability in fishes, turtles and cocodriles, it increases the percentage of females in the next generation, through the effect of the aromatase, altering the hormonal relationship between estrogen and testosterone.
These epigenetic mechanisms described are simple in the sense that a single environmental cause have a repercussion epigenetic. However there are more complicated cases. Thus Borrell, J. (Husbandry Files, 1975. Volume 24 number 93 pages 85-108) describes the deviation of sexual percentage in the next generations of Drosophila melanogaster by the difference between the radiation received by males and females. When the gender receives the same quantity of radiation, the percentages are equilibrated but when the difference of radiation recieved by males and females are above a determined level in the F1 exists an imbalance in favour of males and in the F2 (without radiation in F1) it produces an imbalance in favour of the females, in the F3, it gets to an equilibrated state again. These complex mechanisms involves that the epigenetic changes can have a complicated character and their results are different in the next generations in function of the fusion of the epigenetic changes in the previous generation. This can be attributed to the total silencing of chromosomes to obtain monoallelic expressions in extreme situations that can affect the group of master genes.
In conclusion from primitive RNA and DNA, of unknown origin up to now, it has organized a master genetic material with the capacity to produce proteins, through the ribosomes, and in consequence cells, organs and different species. This material interacts with the enviroment, epigenetic factors, through another genetic material known as control panel of switches. For this reason the apparent difference between the different shapes of yeasts, nematodes, fishes, reptilians, insects and mammals are caused by the quantity of switches genes, and this is why the most evolutionated animals are more complex because they can express more codifiers genes (protein producing) and in the same way their external design (phenotype) is more complex but their basic genome is similar.