Passage III

As human life expectancy increases, people are becoming more familiar with the complex signs of aging; however, scientists still do not completely understand what causes aging at the molecular level.

Three scientists propose models to explain the human aging process.

Scientist 1

Aging is caused by the shortening of telomeres, specialized DNA sequences at the ends of chromosomes. Telomeres play an important role in protecting and replicating chromosome ends. Each time a cell replicates its DNA and divides, its telomeres become shorter. Eventually, the telomeres become too short to perform their necessary roles, and the cell is no longer able to divide, ultimately causing defects in cell structure and function that result in aging.

Scientist 2

Aging is caused by random DNA damage that accumulates over the lifetime of somatic (nonreproductive) cells. As humans age, the DNA in their somatic cells accumulates damage that is not repaired. This results in mutations that cause defects in proteins and in gene expression, leading to aging. Most DNA damage is caused by free oxygen radicals (oxygen species containing oxygen atoms with unpaired electrons) formed during metabolic processes. Thus, the accumulation of DNA damage can be slowed by decreasing the rate of metabolism. Although telomeres do become shorter each time a cell divides, this occurs too slowly to affect the aging process.

Scientist 3

Aging is caused by mutations present in each human at birth that usually produce defects after an individual is too old to reproduce. Because individuals with these mutations reproduce, the mutations can be passed on to the next generation and maintained in the population. Thus, aging is caused by the mutations that humans inherit from their parents. This is why closely related individuals tend to have similar life expectancies. Telomere shortening occurs too slowly to affect the aging process, and DNA damage from free oxygen radicals is easily repaired.