Kurt W. Runge
Telomeres are the physical ends of linear eukaryotic chromosomes. In many organisms, including humans and yeasts, telomeres are composed of short repeated DNA sequences and their associated proteins. These repeats are lost gradually due to incomplete DNA synthesis of the chromosome end, or by degradation from nucleases, and can be resynthesized by the enzyme telomerase. Many human somatic cells do not express sufficient telomerase activity to prevent telomere repeat loss, resulting in cell senescence or death when telomeres shorten to a critical length. This telomere length checkpoint for aging and cell growth is also seen in yeast whose genes for telomerase components have been deleted, allowing yeast to serve as a model for telomere-linked senescence and aging in human cells. Telomeres also serve as a reservoir of bound proteins that are released as the telomere DNA shortens, and work in yeast has shown that these telomere-associated proteins play important roles in yeast replicative aging. The advantages of yeast as an aging model include rapid growth and aging, a small well-defined and well-annotated genome, powerful molecular genetics for modifying this genome, simple but robust assays for the activity and location of telomeric proteins, publicly available collections of mutants in every gene, and multiple assays for replicative and chronological aging. This chapter will describe how the budding yeast Saccharomyces cerevisiae has contributed to our knowledge of telomeres and aging, and the emerging role of the evolutionarily divergent fission yeast Schizosacchar-omyces pombe as another important model system. The powerful genetics of these two different yeast systems should identify central evolutionarily conserved processes that control cellular aging.
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