Mechanisms of Cellular Epigenetic Inheritance
Epigenetic inheritance is basically the observation that offspring may inherit altered traits due to its parents past experiences. So a parents experiences, in the form of epigenetic tags, can be passed down to future generations. Epigenetic inheritance systems are the processes and mechanisms that underlie cellular inheritance. There are four types of EISs recognized today and they are....
- EIS based on self sustaining regulatory loops
- EIS based on three-dimensional templating
- Chromatin marking EIS
- RNA mediated EIS
All of these can contribute to between-generation epigenetic inheritance.
SELF SUSTAINING LOOPS: These are metabolic circuits through which different patterns of activity can be maintained resulting in alternative heritable phenotypes. The early studies of the loops involved the bistability of the lac operon of Escherchia coli and this system has been analyzed at molecular and theoretical levels. The studies showed that when inducer concentrations are low, genetically identical cells can generate two alternative, true breeding, stable phenotypes. These loops have been described in bacteria and other taxa.
Three-Dimensional Templating: Structural inheritance refers to the inheritance of alternative three dimensional structures through spatial templating: a variant 3-D structure in a mother cell guides the formation of a similar structure for a daughter cell, leading to the transmission of the architectural variant. - This all started from the investigation of cortical variations in ciliates. A modified organization of the cilia on Paramecium can be inherited through many asexual and sexual generations. Another form of this structural inheritance is the propagation of prions. A single protein can misfold into several different conformations that have specific growth dynamics, stabilities, pathologies, and cross species infectivity. Many prions can interact which could lead to the formation of different transmissible phenotypes. SO unicellular organisms which have the same genotype and live in the same enviroment can exhibit heritably different morphologies and physiologies.
CHROMATIN MARKING EIS: Chromatin is what is inside chromosomes and includes DNA and everything in it. The organization and location of chromatin and chromosomes determine everything concerning how genes are transcribed, how DNA repair works, how different chromosomal domains are organized, and how chromosomes behave during the cell cycle. There is evidence that chromatin variations can be transmitted through generations of people. Therefor the study of the chromatin marking EIS is crucial for the understanding of development and heredity.
RNA MEDIATED EIS: RNA is central to the regulation of cellular dynamics in the eukaryotes and is also involved in cell and organism heredity. RNA interference has been located in all eukaryote phyla from yeast to man. In RNA pathways, double stranded RNA molecules are chopped into shorter dsRNAs by the enzyme dicer. The resulting siRNA is loaded onto a complex of proteins, one strand of the duplex is removed and the other directs silencing. RNA can affect cell and organism heredity in several different ways, one example being the result of replication of siRNA through RNA dependent RNA polymerase.