ABSTRACT
To return to the third question (How do we age?) at the beginning of this book, it is now possible to address some long-standing questions in light of recent exper-
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imental evidence. No single theory of aging to date has been able to account for all observed phenomena in invertebrates and mammals. Additional complications arise if one attempts to construct a theory of aging that encompasses phenomena such as extrachromosomal ribosomal DNA circle formation, which is sufficient to cause aging in yeast (8) but has not been observed in other species. Indeed, not all of the cellular damage accumulating with aging may be a consequence of nucleic acid damage. For example, proteins are also subject to chemical modifications with aging, and the life span may be extended in Drosophila by overexpression of a protein repair methyltransferase. This effect is dependent upon the ambient temperature-it is observed at 29°C but is abolished at 25°C (9).
