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Figure 14.10 Derivation of senescent mutant from a heterokaryotic wild isolate of Neurospora using microconidia.

Heterokarotic "oarent" strain

Heterokarotic "oarent" strain

Time (hours)

Figure 14.11 Growth rate of senescent mutant. The subcultures numbers (in parenthesis) were made in race tubes.

Time (hours)

Figure 14.11 Growth rate of senescent mutant. The subcultures numbers (in parenthesis) were made in race tubes.

14.6 AEROBIC RESPIRATION, AGING AND SENESCENCE

Aging and ultimately death occur in all aerobic organisms, though most fungi are seemingly an exception. One widely accepted theory attributes aging and death to cellular damage caused by free radicals generated by normal metabolism. A free radical is a molecular species that contains an unpaired electron. Free radicals are highly reactive and destructive because of their tendency to pair up by donating or take up an electron. Free radicals are generated when electrons are incorrectly passed between different molecules in the electron transport chain in mitochondria. When radicals damage enough molecules in the cells, the organisms die (http://www.accessexcellence.org/LC/ST/bgfreerad.html). This is known as the free radical theory of aging.

Senescence is correlated with a switch from cyanide-sensitive, cytochromes-mediated to cyanide-insensitive alternative respiratory pathway—the electron flow at the level of ubiquinone (coenzyme Q) is bypassed and the enzyme alternative oxidase transfers the electron directly to molecular oxygen,

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