Molecular medicine-Keywords

Aging
The progressive impairments of functions experienced by many organisms throughout
their life span.
Aging and Sex, DNA Repair in 55
DNA Damage
A DNA alteration that has an abnormal structure, which cannot itself be replicated
when the DNA is replicated, but which may be repaired.
DNA Repair
The process of removing damage from DNA and restoring the DNA structure.
Mutation
A change in the sequence of DNA base pairs, which may be replicated and
thus inherited.
Sex
The process by which genetic material (usually DNA) from two separate parents is
brought together in a common cytoplasm where recombination of the genetic material
ordinarily occurs, followed by the passage of the recombined genome(s) to progeny.
Complementation
The masking of the expression of mutant genes by corresponding wild-type genes
when two homologous chromosomes share a common cytoplasm.

A number of theories have been proposed to account for the biological phenomena
of aging and sexual reproduction (sex). An emerging unified theory that accounts
for a considerable amount of the data relating to both aging and sex is
presented here.
Aging appears to be a consequence of DNA damage, while sexual reproduction
(sex) appears to be an adaptation for coping with both DNA damage and mutation.
DNA, the genetic material of most organisms, is composed of molecular subunits
that are not endowed with any peculiar chemical stability. Thus, DNA is subject to
a wide variety of chemical reactions that might be expected of any such molecule
in a warm aqueous medium. DNA damages are known to occur very frequently,
and organisms have evolved enzyme-mediated repair processes to cope with them.
In any cell, however, some DNA damage may remain unrepaired despite repair
processes. Aging appears to be due to the accumulation of unrepaired DNA damage
in somatic cells, especially in nondividing cells such as those in mammalian brain
and muscle.
On the other hand, the primary function of sex appears to be the repair of damages
in germ cell DNA through efficient recombinational repair when chromosomes pair
during the sexual process. This allows a relatively undamaged genome to initiate the
next generation. In addition, in diploid organisms, sex allows chromosomes from
genetically unrelated individuals (parents) to come together in a common cytoplasm
(that of progeny). Since genetically unrelated parents ordinarily would not have
common mutations, the chromosomes present in the progeny should complement
each other, masking expression of any deleterious mutations that might be present.
56 Aging and Sex, DNA Repair in
Thus, aging and sex appear to be two sides of the same coin. Aging reflects the
accumulation of DNA damage and sex reflects the removal of DNA damage, and in
diploid organisms, the masking of mutations by complementation