Silent clock mechanism long stem3/31/2023 ![]() Standard laboratory strains of mice, which are thought to have originated mainly from a European subspecies, Mus musculus domesticus, and partially from an Asian subspecies, M.m.musculus/molossinus, have rDNA clusters within the centromeric regions of chromosomes 12, 15, 16, 18 and 19. In humans and apes, rRNA genes are located between the short arm and the satellite body of acrocentric chromosomes 13, 14, 15, 21, and 22. rRNA gene number can also vary between individuals of the same species or even between the cells of an individual. The number of rRNA gene repeats varies enormously throughout phylogeny with humans and mice bearing ~200 rRNA genes per haploid genome, while amphibians and plants may have thousands of copies. Accordingly, tumor cells generally showed an increased size and/or number of nucleoli that is used by pathologists as a diagnostic marker for certain cancers. Elevated rRNA transcription and ribosome biogenesis is a common feature of many cancers. Dividing cells produced elevated amounts of rRNA and often possess large nucleoli whereas downregulation of rRNA gene transcription leads to a reduction in nucleolar size. The size of the nucleolus positively correlates with rRNA synthesis. ![]() Recent results revealed that these nucleolar subcompartments represent distinct, coexisting liquid phases and that phase separation induces the formation of multilayered liquids that may facilitate sequential RNA processing reactions in a variety of ribonucleoprotein bodies. Transcription of rRNA is thought to occur at the boundary between FC and DFC whereas processing of the pre-rRNA occurs in the DFC region and pre-ribosomal subunit assembly takes place in the GC region. The nucleolus is a membrane-less compartment that consists of the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC).
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