Aging and Sex, DNA Repair in
85
chloroplasts) of germ cell lines. While
nuclear genes occur as single copies per
gamete, there are thousands of mitochon-
drial DNA (mtDNA) molecules in most
cells, and several hundred thousand may
occur in a mature oocyte. The many mtD-
NAs in oocytes appear to stem from a
vastly smaller pool of mtDNA molecules
that must have survived a process of
replicative segregation in earlier cytoki-
netic divisions of the germ line lineage,
since most heterogeneity of mtDNA is
distributed among, rather than within,
individuals. This implies that there are
mtDNA population bottlenecks in germ
lines. That is, the mtDNA, which is gener-
ally solely transmitted from one generation
to the next through the oocyte, is of
only one or a few genotypes. Avise con-
cluded that the mtDNA molecules that
survive and replicate to populate a mature
oocyte probably have been scrupulously
screened by natural selection for replica-
tive capacity and functional competence
in the germ cell lineages they inhabit.
This strategy, at the molecular level, is
equivalent to a vegetative cell replace-
ment strategy for cell lineages that do
not have a sexual cycle. While the mi-
tochondria and chloroplasts within cells
follow a vegetative replacement strategy,
they, like vegetatively replicating bacte-
ria and yeast, utilize homologous DNA
recombinational repair to remove DNA
damages due to oxidation or environmen-
tal stresses.
4.3
Dolly the Cloned Lamb, Cumulina the
Cloned Mouse, and Low Success of Cloned
Mammals
The evidence reviewed above indicates that
germ cells, having undergone meiosis,
are relatively free of DNA damage, and
thus should be able to give rise to viable
offsprings with high probability. At lower
probability, a given somatic cell nucleus
may also be sufFciently free of damage
to be able to produce viable progeny if
transferred to an enucleated egg cell.
Eight viable lambs were derived in
1997 – one (named Dolly) from donor
cells of a mature sheep, four from donor
embryo-derived cells, and
three from
donor fetal Fbroblast cells. ±or Dolly,
the lamb derived from a somatic cell
of a mature sheep, they Frst made 277
fused couplets (enucleated oocytes fused
to donor cells), using donor cells from
their 3rd to 6th passage, cultured from
the mammary gland of a six-year-old preg-
nant ewe. A morula or blastocyst was able
to form from only 11.7% of the cultured
fused couplets from the mature ewe. By
comparison, when couplets were derived
from embryo or fetal Fbroblast cells, a
morula/blastocyst was formed 27 to 39% of
the time. Some of the morula/blastocysts
implanted in recipient ewes formed fe-
tuses detectable by ultrasound at 50 to
60 days. Subsequently, 62% of these fe-
tuses were lost, a much greater proportion
than the estimated 6% after natural (meio-
sis based) mating in sheep.
Similar observations were made with
mice. Ten healthy mice were cloned, the
Frst of which was named Cumulina, from
donor nuclei of differentiated, nonrepli-
cating granulosa cells of mature mice
injected into enucleated recipient mouse
oocytes. Cumulina and the other nine
cloned healthy newborn mice were the
only successful progeny produced from
800 injected oocytes, which had formed
embryos and were transplanted into foster
mother recipient mice. In multiple cloning
experiments by these authors, the rate of
successful implantation of embryos was 57
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