'Epigenetic inheritance,' is the transmission of information from a cell or multicellular organism to its descendants without that information being encoded in the nucleotide sequence of the person's genes. Epigenetic inheritance happens in the development of multicellular organisms; dividing fibroblasts for example gives rise to new fibroblasts, even though their genome is exactly the same as that of all other cells. Epigenetic transmission of traits also happens from one generation to the next in some people, although is is fairly rare. It was first observed in maize. The study of epigenetic inheritance is known as, 'epigenetics.'
Epigenetic Inheritance - Defined as an unconventional finding. It goes against the idea that inheritance happens only through the DNA code that passes from parent to offspring. It means that a parent's experiences, in the form of epigenetic tags, can be passed down to future generations.
Epigenetics - Defined as the study, in the field of genetics, of cellular and physiological phenotypic trait variations that are caused by external or environmental factors that switch genes on and off and affect how cells read genes instead of being caused by changes in the DNA sequence.
'Epigenetic Inheritance Systems (EIS's),' permit cells of different phenotype, yet identical genotype, to transmit their phenotype to their offspring, even when the phenotype-inducing stimuli are not present, as is often what happens. There are three types of EIS's that might have a role in what has become known as, 'cell memory.'
Some metabolic patterns are self-perpetuating. At times a gene, after being turned on, transcribes a product, either directly or indirectly, that maintains the activity of that gene. Descendants of the cell in which the gene was turned on will inherit this activity, even if the original stimulus for gene-activation is no longer there. Diffusion of the gene's product to other cells might make the heritable characteristic spread.
Structural Inheritance Systems:
In ciliates such as Paramecium and Tetrahymena, genetically identical cells present with heritable differences in the patterns of ciliary rows on their cell surface. Experimentally changed patterns may be transmitted to daughter cells. It appears that existing structures act as templates for new ones. The mechanisms of this inheritance are not clear, although reasons exist to assume that multicellular organisms also use existing cell structures to assemble new ones.
Proteins or chemical groups that are attached to DNA and modify its activity are called, 'chromatin marks.' The marks are coped with the DNA; for example, several cytosines in eukaryotic DNA are methylated. The numbers and pattern of these methylated cytosines influence the functional state of the gene. Low levels of methylation correspond to high potential activity, while high levels correspond to low activity. Even though there are random changes in the methylation pattern, there are also very specific ones induced by environmental factors. Following DNA replication, maintenance methylases ensure the methylation pattern of the parental DNA is copied to the daughter strand.
Epigenetic variants show spontaneous emergence and reversion. They may; however, be induced by the presence of other genetic factors and some alleles of a gene have been demonstrated to convert the epigenetic status of the same locus on the homologous chromosome. Environmental factors are also known to influence the emergence and reversion of epigenetic factors. The result is the potential that epigenetic variations may be produced at several loci and in a number of cells or organisms. If these systems would affect biological evolution, adaptive variation would happen, which is a Lamarckian form of evolution. The question then becomes, 'to what extent does epigenetic inheritance play a direct role in evolution?'
Orthodox theories on biological evolution hold that the only role the environment has is in the phase of selection. The environment determines on what grounds selection occurs and what characteristics are needed for better reproduction opportunities. For selection to be possible, people or other species have to differ to some degree so that good characteristics may amplify and bad ones can be deleted from the gene pool. The differences between people are usually thought to arise from random mutations.
The source of the variation that is needed in Darwin's theory of evolution is the random variation in the sequence of the DNA bases that constitute the genes. The environment may influence these variations somewhat; for example, radioactivity is known to influence the structure of DNA, yet only in a manner that is random. More recently; however, scientists are realizing the role of the environment might well have been underrated. Some forms of epigenetic inheritance may be maintained, even through the production of germ cells.
Several experimental studies appear to indicate that epigenetic inheritance has a part in the evolution of complex organisms. Tremblay and associates; for example, have shown that methylation differences between maternally and paternally inherited alleles of the mouse H19 are preserved. There are also several reports of heritable epigenetic marks in plants.
The epigenetic heredity appears to exist transgenerationally in complex organisms and may be explained by permitting for minor epigenetic changes not affecting totipotency. What this does is place some constraints on the extent to which epigenetic changes may be brought upon DNA, while allowing for EIS's to play direct evolutionary roles.
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