Eukaryotic Theory

Decent Essays
Introns are nucleotide sequences universal amongst the Eukarya domain. Reconstructive models of maximum parsimony and likelihood suggest introns were present in the last eukaryotic common ancestor (LECA) (Csuros, 2005) and have since been gained, lost, and adapted to a multitude of functions by separate lineages; any degree of intron conservation in modern eukaryotes is suggestive of function, despite appearing to exist solely for removal. Although they possess many valuable roles this does not explain the ubiquity of introns within eukaryotic genomes. To understand their widespread presence despite their apparent deleterious effects it is necessary to discern the evolutionary origins and conditions which impelled their prevalence. Structural…show more content…
Per this theory, GpII invaded the eukaryotic lineage in its genesis via the mitochondrial endosymbiosis and have been inserting themselves into coding genes continuously since; a mechanism consistent with the high levels of GpII in some α-proteobacteria (Robart & Zimmerly, 2005) and the ability of yeast mitochondrial GpII to insert themselves into double stranded genomic DNA (Zimmerly et al., 1996). Symbiosis-related selection pressure upon the α-proteobacteria coupled with its bacterial nature of jettisoning unwanted genes caused the exchange of GpII and cognate maturases (Schmitz-Linneweber et al., 2015) between the proto-mitochondrion and the host cell which lacked the necessary control mechanisms to restrict their spread. Hence, the proportion of introns in the LECA genome is considerably greater than in contemporary eukaryotic lineages, excluding some vertebrates (Figure.1). Comparison of nuclear-encoded cytoplasmic ribosomal proteins and mitochondrial ribosomal proteins showed that intron positions were conserved (Yoshihama et al., 2006), suggestive of common descent between all eukaryotes. However, the presence of introns presents significant problems to the host. Principally, random and aggressive insertion of foreign gene elements is likely to disrupt essential coding sequences within the host genome; hence, only neutral intron-insertions could persist. Additionally, splicing reactions require the energetically expensive and complex machinery of the spliceosome, comprised of over 150 proteins and 5 snRNA units (Wahl et al., 2009), any fault in which would cause broad harm to the cell (Chorev & Carmel,
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