Aging and Sex, DNA Repair in

Abstract

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. 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.