phenomena, as The Alchemist
mentioned, are not related directly to the genome
. Outside influences and DNA methyl
ation can cause epigenetic effects, but there are much more vivid
examples of epigenesis
found in the cytoplasm
of the cell.
For example, most RNA viruses reside entirely in the cytoplasm, never requiring entry into the nucleus during their life cycle. Therefore, they do not get separated with the genetic material during mitosis. Thus, during cell division in a cell carrying only one or a few copies of a given virus, it is entirely possible that one daughter cell will receive all of the copies of the virus, while the other will receive none.
Another example of cytoplasmic inheritance that manifests itself epigenetically is the prion. Prions are protein particles that are capable of and cause infection; examples include scrapie, Creutzfeldt Jacob Disease (CJD), Bovine Spongiform Encephalopathy, and some yeast prions. Briefly, prions are thought to have the same coding as normal cellular proteins, but are susceptible to changes in folding that make them infectious. Once a prion particle enters a cell, that particle is thought to recruit normal cellular foldings and subject them to the same refoldings to make more prion protein. Thus, the generation of more prion protein requires the presence of the protein in the cytoplasm without requiring the alteration of a gene in the cell.
A somewhat recently discovered example of epigenesis in yeast is a phenomenon called [KIL-d]*, a phenomenon in which the yeast killer virus exhibits one of a number of bizarre phenotypes in a haploid colony. When mated to a wild-type cell that does not carry the virus, the virus manifests itself normally in the diploid parent, but when sporulated, any one of the bizarre phenotypes may show through in the progeny. Thus, the original haploid strain could be ONE of K+R+, K-R-, or anywhere in between; the diploid parent would be K+R+, as per normal; and the haploid offspring would vary through the range mentioned before, EVEN within the tetrad formed through a single sporulation event.**
Epigenetic phenomena also occur due to mRNA processing. For example, during intron excision, certain introns have specific codes that allow for exon exclusivity; that is, only one exon or another could be included in the final message, but not both. Or, there could be a code allowing for any arrangement of a series of exons to be included in the final message. In this example, the limitations in the number of variations seen lie in the number of different copies of the message generated at any one time, and the speed at which the final message gets translated. However, since the variation is not induced on the level of the gene, multiple variations can result from the same gene in the same cell.
* [KIL-d] stands for "defect in killer virus". Yeast geneticists are really uninteresting with their naming.
** [KIL-d] is thought to be a prion at this point; unfortunately, we're having trouble proving OR disproving that in my lab. Anyone with any ideas, I'd be thrilled if you'd /msg me.