And we skip the pro-metaphase piece and move
directly into metaphase. I'm not sure really why
but that's just some general convention. And we talk
about what's happening on the metaphase plate.
Now in meiosis it's particularly important to talk
about this metaphase plate as it is in mitosis.
It is a fantastic place for you to compare chromosome
number and such as you begin diagramming
these processes. So first of all, we still have
kinetochore microtubules. And you're still going to
see that we have the polar microtubules. Remember
the kinetochore microtubules have grabbed on
to the kinetochores of the chromosomes, but this time
they are grabbing on to kinetochores of non-sister
sets of chromatids and their jostling them around
and moving them to align on the metaphase plate.
The key language point here is homologous pairs or
replicated homologous pairs are lining up
on the metaphase plate. Think back to mitosis for a
moment. What was lining up on the metaphase plate
during mitosis? During mitosis, we had a single file
line so that each of the 46 chromosomes were lined up
single file down the center. We only had to undergo
one division and sister chromatids would separate.
In meiosis, the big distinction is we are separating
homologous chromosomes. So that's where the language
really comes into play. Note
that down before we move on.
So, here's a diagram to depict that. Metaphase, we've
got the synaptonemal complexes. We are going to
pull them apart in metaphase I. Metaphase of mitosis,
sister chromatids, kinetochores on each sister
chromatid are being pulled by the microtubules. Just
a quick note. When you're reading something like an
exam question and it just says metaphase, you don't
need to ask yourself "Well is it I or II or
mitosis or meiosis?" Metaphase alone without a I or a II,
that means mitosis. Otherwise we would say metaphase I
or metaphase II. So, just a little hint there. This
is the second point where we could acquire some
genetic variation. The way that chromosomes align on
the metaphase plate, whether it's the paternal one
on this side and the maternal one on this side
or vice versa, is completely random.
So it's the shuffling of chromosomes as they align
on the metaphase plate that is the second place
in meiosis that we will see genetic variation
resulting in the gametes that are formed.
So where was the first place?
Yeap. The first place was when we had crossing over
of the non-sister chromatids during prophase.
So the alignment of homologous chromosomes on the
metaphase plate during meiosis I is our second place
to acquire genetic variation in gametes. You could
potentially have all 23 of your paternal chromosomes,
and all 23 of your maternal chromosomes line up on
opposite sides. That would result in production of a
sperm or egg that had all of your grandmothers
information and all of your grandfathers information.
So this sort of explains; It's generally not
what happens but it sort of explains
how sometimes a child will look very much like one
side of the family and not so much like the other.
Generally, with that many chromosomes, we'll see a
little bit more shuffling and some features are there
from one parent and some features are there
from the other parents family.
But again, metaphase, genetic shuffling. Right?
Genetic recombination during prophase I.