Aging and Sex, DNA Repair in
83
Overall, the evidence reviewed in this
section lends strong support to the idea
that the primary general function of re-
combination enzymes, whether in somatic
cells or during meiosis, is to remove DNA
damage, either by repair or apoptosis.
3.9
Other Expectations of the DNA Repair and
Complementation Theory of Sex
If DNA damage and its repair are impor-
tant in maintaining sexual reproduction,
then certain expectations follow. If re-
combination during meiosis and sexual
reproduction reflect recombinational re-
pair of germ line DNA, then there should
be other evidence of avoidance of DNA
damage in the germ line as well.
As indicated in Table 1, the largest
known source of DNA damage is oxida-
tive damage. Such damage occurs due to
endogenous cellular metabolism. Presum-
ably, to avoid DNA damage, germ line cells
should have evolved ways to avoid high
levels of metabolism. Eggs would seem,
at Frst sight, to be poor candidates for
avoiding metabolism. They are, in gen-
eral, much larger than somatic cells of
the organism (e.g. egg cells have about
a 1,000-fold greater mass than somatic
cells in humans). It requires considerable
metabolism to produce large egg cells.
However, much of the cytoplasmic ma-
terial within an egg cell is generated by the
activity of other cells. Some insects have
nurse cells around each egg cell. These
nursecellsareconnectedtotheeggcellby
cytoplasmic bridges and provide most of
the ribosomes, mRNA, and proteins of the
egg cell. The nurse cells themselves con-
tain hundreds to thousands of copies of
their genomes, presumably to protect the
nurse cells themselves from losing func-
tion from the oxidative damage they may
suffer while providing large amounts of
metabolic products for the egg cell. Verte-
brate egg cells are surrounded by follicle
cells rather than nurse cells. The follicle
cells do not have cytoplasmic bridges to the
egg cell, but rather have small gap junc-
tions connecting them to the egg. While
these gap junctions are not large enough
to transmit bulky macromolecules, they do
transmit precursor molecules to the egg.
In addition, for chickens, amphibians, and
insects, the yolk proteins accumulated by
theeggaremadeinliverorliver-typecells.
These mechanisms allow eggs to be pro-
tected from oxidative damage while they
store up material to sustain the zygote in
its initial growth.
Sperm or pollen cells, in contrast to
egg cells, are usually the smallest cells
of an animal or plant. This allows a
different strategy for effective protection
against oxidative damage to their DNA.
Because of their very small size, minimal
metabolism would have been used in their
formation. Thus, both egg and sperm
production appears to have been adapted
to circumvent the production of DNA
damage in their especially important germ
line DNA.
Another way to avoid production of
sperm with damaged DNA is for spermato-
genic cells with DNA damage to undergo
cell cycle arrest to allow more time for
meiotic repair, and then, if this fails, to
undergo apoptosis. The p53 protein plays
a key role in mediating cell cycle arrest
in response to DNA damage. Low-level
irradiation was found to activate a p53-
dependent premeiotic delay, allowing time
for increased DNA repair leading to in-
creased motile spermatozoa. Higher levels
of
γ
-irradiation induced p53-independent
apoptosis during meiosis.
If complementation of mutations is im-
portant in maintaining the outcrossing
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