Bruder speculates that such variation is a natural occurrence
that accumulates with age in everyone. "I believe that the
genome that you're born with is not the genome that you
die with—at least not for all the cells in your body," he says.
Charles Lee, a geneticist at Brigham and Women’s Hospital
in Boston, agrees. Genetic variations can arise after a
double strand of DNA breaks when exposed to ionizing
radiation or carcinogens. "It reminds us to be careful
about our environment because our environment can help
to change our genome," he says.
Plus, these variations may predict age-related diseases.
Lee adds: "As you age … your chances for having a genomic
rearrangement that causes a certain disease increases all the time."
The differences between identical twins increase as they age,
because environmentally triggered changes accumulate. But twins
can also begin their lives with differences, according
to Bruder's study, and that calls into question their very name.
this is an interesting bit;
Bruder suggests that variations
pile up over time, -after birth-,
and this can also suggest that such variations
may begin to pile up, directly from -conception-,
and these twins were all adults when compared.
the notion of variations piling up -after birth-
and possibly as soon as -after conception- does
not bode well for establishing a "mutation rate"
this calls in to question the entire scheme,
as variations of this sort may resemble things
like getting plastic surgery after birth, and
not passing these variations on to anyone.
Spermatogenesis, which begins at puberty, includes all
of the events by which spermatogonia are transformed
At birth, germ cells in the male can be recognized in the
sex cords of the testis as large, pale cells surrounded
by supporting cells (Fig. 1.21A). Supporting cells, which
are derived from the surface epithelium of the gland in
the same manner as follicular cells, become sustentacular cells,
or Sertoli cells (Fig. 1.21C ). Shortly before puberty, the
sex cords acquire a lumen and become the seminiferous tubules.
At about the same time, primordial germ cells give rise
to spermatogonial stem cells. At regular intervals, cells
emerge from this stem cell population to form type A spermatogonia,
and their production marks the initiation of spermatogenesis.
Type A cells undergo a limited number of mitotic divisions
to form a clone of cells. The last cell division produces
type B spermatogonia, which then divide to form primary spermatocytes
(Figs. 1.21 and 1.22). Primary spermatocytes then enter
a prolongedprophase (22 days) followed by rapid completion
of meiosis I and formation of secondary spermatocytes. During
the second meiotic division, these cells immediately begin
to form haploid spermatids (Figs. 1.21–1.23). Throughout this
series of events, from the time type A cells leave the stem
cell population to formation of spermatids, cytokinesis is incomplete,
so that successive cell generations are joined by cytoplasmic bridges.
Thus, the progeny of a single type A spermatogonium form a clone
of germ cells that maintain contact throughout differentiation
the germ cells are very well insulated from the
envirnomental 'hazzard' sort of thing which will
tend to cause disruptions in the dna -after- birth
and pile up incorrect copies in =adult= samplings.
environmental disruptions, which constitute
the larger portion of total disruptions,
are taking place -after- birth,
and are not passed on.
we have no set predetermined 'mutation rate'
being passed along in the germ cells.
dubious consequences for 'gene clock'
but, it's really quite beautiful, in that
the germ cells are so very well insulated
from envirnmental exposure,
the germ cells are sheathed and encased in layers
of cells that =filter= water and nutrients that
would be passed on as nutrients for
the primordial germ cells.
wheras, liver and kidney and heart and blood cells, etc.
may take a direct 'hit' from any passing environmental
'bullet' and see disruptions in their sequencing,
because they have water and nutrients passing
directly to and through them
these germ cells are fed on filtered and purified
water and nutrients that -first- pass through the
surrounding 'sertoli' and/or 'sustenacular' cells.
like as if, -these- cells are little nannies
and an elaborate police force
for the germ cells.
it's layers of protection.
the germ cells are not directly exposed
to the environment in the ways all other
cells are exposed.
and much disruption occurs after birth
in the cells which are so exposed,
heart liver blood etc...
of course, things like X-rays can still
penetrate this protective screen,
but, remember to wear your little
lead sheath when you go to the
dentist for X-rays.
if the dentist forgets, you remember.
"ay, put the lead sheath on"
because, in real life,
this sort of 'hazzard' is deleterious.
persons who stood in front of an X-ray machine
for too long never passed on 'super-human' powers
to their offspring.
in all likelihood, they became sterile.
as it stands, "mutation theory" is rather dubious