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Paramecium : Genetics and Epigenetics
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Paramecium: Genetics and Epigenetics by patonce - Issuu
Description The techniques used to decipher the genetic makeup of species as well as epigenetic mechanisms are essential for explaining life forms and studying their DNA. As a eukaryotic model, Paramecium is well suited for genetic analysis. Taking a rather unconventional view of genetics, Paramecium: Genetics and Epigenetics explores how to use this protozoan as a basis for studying complex cells. The book discusses various aspects of Paramecium, including the cortex, the cytoplasm, nuclei, asexual fission, conjugation, autogamy,macronuclear regeneration, cytogamy, life cycle phases, and behavior.
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It examines the assorted mating types of the genus and how these mating types are determined. It also elucidates some techniques that identify genetically defined genes with the DNA from a library that comprises those genes and details the genetic, epigenetic, chemical, and molecular facets of several different traits. With many powerful tools now available, Paramecium research is entering a new frontier in molecular biology.
A full account of Paramecium genetics, this book presents a wealth of time-consuming observations and remarkable phenomena that will lead to a better understanding of complex cells. As early as the s, scientists such as Tracy Sonneborn had noted that mating type was not transmitted to progeny in Mendelian fashion: a new type of trait transmission, not dependent on the chromosomes, had to be involved, but they did not succeed in elucidating it. To achieve this, they first showed that the difference between the E and O mating types was due to a transmembrane protein called mtA.
Although its encoding gene is present in both types, it is only expressed in E individuals.
The scientists then revealed the mechanism by which this gene is inactivated in type O individuals. Paramecia have two nuclei: a germinal micronucleus that is transmitted during sexual reproduction and a somatic macronucleus -- resulting from the latter -- where the cell's genes are expressed. The mechanism for the transmission of mating types is based on small RNA, called scnARN, which are produced during meiosis.
The original function of these RNA is to eliminate from the macronucleus a whole series of genetic sequences called transposable elements, which, like introns, have been introduced into the genes during evolution. As a first step, the scnARN scan the maternal macronucleus in order to identify the sequences that were deleted in the previous generation, and then make the same rearrangements in the new macronucleus. However, unexpectedly, this genome "cleaning" mechanism also allows the cell to silence functional genes. In type O individuals of Paramecium tetraurelia, scnARN eliminate the mtA gene promoter, thus deleting its expression.
Thus, it is through the scnARN inherited from the maternal cytoplasm, and not from a particular gene sequence, that the mating type of Paramecium is defined. This silencing process could in principle affect any gene. Thus in theory, Paramecia could transmit to their sexual progeny infinitely variable versions of the macronuclear genome from the same germline.
As with genetic heredity, this mechanism may cause errors that might occasionally endow progeny with a selective advantage. In other words, the somatic macronucleus genome of Paramecium may evolve continuously, and in certain cases allow a short-term adaptation to changes in environmental conditions. And this can occur without any genetic mutations being involved. This type of Lamarckian heredity may thus offer a hitherto unsuspected lever for natural selection.
Note: Content may be edited for style and length. Science News. Genome-defence small RNAs exapted for epigenetic mating-type inheritance.