EurekAlert reports on the work of Prolla and Kujoth, who find "that accumulation of mitochondrial mutations that promote apoptosis, or programmed cell death, may be a central mechanism driving aging and may be unrelated to the release of free radicals, previously thought to cause aging. ... By breeding mice with the inability to detect and repair mistakes in the DNA replication process, researchers discovered there was no increase in oxidative stress despite an increased mutational load. However, there was a significant increase in apoptosis. ... The finding disproves the previously believed mitochondrial 'vicious cycle' theory of aging, which states that increases in mitochondrial mutations increase oxidative damage, which is one cause of aging."
Link: http://www.eurekalert.org/pub_releases/2005-07/uof-usf071405.php