Let's take a look at the production of
sperm versus the production of eggs.
The production of sperm happens in the testis,
specifically in the Sertoli cells in the
seminiferous tubules. But that's for anatomy and
physiology. Either way, you can see that we have our
Sertoli cells at the top. These are going to contain
germ line cells. Recall, germ line cells are diploid.
They have all 46 chromosomes. 23 pairs. And then, they
are going to be determined to become sperm
at a certain point. And we call that the primary
spermatocyte. It's diploid. It undergoes meiosis I
and becomes haploid. So now we have haploid spermatocytes
or secondary spermatocytes. After meiosis II,
the sister chromatids separate and we have
spermatids. Now they are not quite ready to go.
There are a lot of modifications that have to happen
to the spermatids to become actual spermatozoa
and be mobilized and able to swim and go find the
egg. Let's contrast this to what happens in female
gametogenesis and the production of eggs. In females,
this happens in the ovaries clearly. And the egg is
ovulated, floats down the fallopian tube. One of the
interesting things here is that in females,
we have our germ line cell. And prior to birth, it
begins, once they are determined to become eggs,
that germ line cell begins meiosis I. But then it stalled
till ovulation. So for 15, 30, I don't know 45 years maybe,
you have those eggs stalled during meiosis I. Then, at
the time of ovulation, meiosis I is completed
and it's not until that egg becomes fertilized that it
actually undergoes meiosis II. This is one of the issues
with women reproducing later in life.
The machinery has been in place since prior to birth.
What if those spindle fibers don't produce themselves
quite properly and don't grab on to the kinetochores
properly and pulls the chromosomes apart properly. So
there's opportunity during both meiosis I and meiosis II
for things to go awry and chromosomes not to separate
properly. We could have the homologous pairs
not separating or we could have sister chromatids not
separating properly. And that is where we result in
something like Down's syndrome. Where we have not
separated chromosome 21 usually in meiosis II I think.
If we don't separate one of those sets of sister
chromatids, we might end up with three copies
in one cell giving us Down's syndrome and one copy in the
other cell. So, reproducing later in life for women,
is a little bit more of an issue than it is for men.
Because men make fresh sperm every day.
All the way through life. So, that's the
explanation for that difference.