aspects of aging
major theories of aging.
We are all familiar with the outward signs of aging, such as
wrinkled skin, gray hair, and reduced vision. A number of cellular structures
or events appear to be involved in causing these effects. The major hypotheses
that attempt to explain how aging occurs concentrate on molecules within the
cell, such as lipids, proteins, and nucleic acids. It is estimated that at
least 35% of the factors affecting aging are genetic.
1.Cellular clock. One hypothesis of aging
suggests the existence of a cellular clock that, after a certain passage of
time or a certain number of cell divisions, results in the death of a given
2.Death genes. Another hypothesis suggests
that there are “death genes,” which turn on late in life, or sometimes
prematurely, causing cells to deteriorate and die.
3.DNA damage. Other hypotheses suggest
that, through time, DNA is damaged, resulting in cell degeneration and death.
4.Free radicals. DNA is also susceptible
to direct damage, resulting in mutations that may result in cellular
dysfunctionand, ultimately, cell death. One of the major sources of DNA damage
is apparently free radicals, which
are atoms or molecules with an unpaired electron.
5.Mitochondrial damage. Mitochondrial DNA
may be more sensitive to free-radical damage than is nuclear DNA. Mitochondrial
DNA damage may result in loss of proteins critical to mitochondrial function.
Because the mitochondria are the primary source of ATP, loss ofmitochondrial
function could lead to the loss of energy critical to cell function and,
ultimately, to cell death. One proposal suggests that reduced caloric intake
may reduce free-radical damage to mitochondria.