User:M bartolo/sandbox

Extrachromosomal rDNA circles (aka ERC) are self replicating sequences of ribosomal DNA (rDNA) found in a strain of yeast, Saccharomyces cerevisiae, and are suggested to contribute to their aging and found in their aged cells. By intra-molecular homologous recombination of the chromosome, extrachromosomal circular DNA (eccDNA) are formed as well as ERCs.

ERCs in yeast strains
The Sgs1 gene mutations of yeast were shown to have accelerated aging on their mother cells, suggesting they additional function to cellular senescence. Additionally, ERCs accumulate in old cells and mutations of Sgs1 were found to increase this accumulation, leading to the idea that ERCs lead to shorter lifespan of cells. The ERC accumulate in the mother cell during the budding process. Sinclair et al. mentioned a suggested common mechanism between the Sgs1 and WRN genes since they both had age related effects on yeast and human aging respectively.

Borghouts et al, determined that the generation ERCs negatively influence the life spans of grande and petite yeast strains.

ERCs in Xenopus Laevis
Circular extrachromosomal DNA are not only found in yeast but other eukaryotic organisms. Cohen et al. developed a regulated formation of eccDNA in preblastua Xenopus embryos. They determined from 2D electrophoresis analysis that eccDNA decreased with the degradation of amplified rDNA. Additionally, with an excess of rDNA sequences, they are not prone to generating circular extrachromosomes from random events of breakage and ligation.

ERCs in retrograde response
Poole et al. provided a model that resolves the role of retrograde response in lifespan. They depict a proces in which ERC production occurs and shortens lifespan in the TAR1 gene.