Before we move in to examining the specifics of
meiosis I and meiosis II, let's take a closer
look at these homologous pairs or the tetrad
formation. We're going to see that these
homologous pairs not only come together but they
actually stick to each other in this synaptic pairing.
So, we form a synaptonemal complex between the two
homologous chromosomes that have been replicated.
And we're going to see that they actually
cross over and exchange pieces of information.
Now that we have this synaptonemal complex,
sometimes pieces of information might change.
And this is one of the places that we acquire
genetic variation in the process of meiosis.
There is one other place that we'll explore later on
in this lecture. So, we have a paternal chromosome
and a maternal chromosome. And they have paired with
the synaptonemal complex. The synaptonemal complex
sometimes the non-sister chromatids will cross over
and much like if you were hooking arms with someone,
and you swapped forearms. It would be kind of strange
but this is what happens in the homologous pairing
during prophase of meiosis I. So the paternal and
the maternal chromosome with their replicated
sister chromatid, we have a tetrad here, are
crossing over and literally switching pieces.
The point of crossing over is called the chiasmata.
The whole piece where they're stuck together is
the synaptonemal complex. And so the resulting
chromosomes are distinct from one another.
So this crossing over happens during prophase
and here we see the distinct chromosomes.
There is genetic recombination where the maternal
chromosome has actually switched pieces with the
paternal chromosome. This could happen at
multiple locations or loci along the chromosome.
So they could actually cross over twice. Again,
this whole thing is called genetic recombination.
It's one of the places that we see we get genetic
variation in offspring, because the sperm and the egg
are distinctly different from any of the parent cells.
So, moving forward into meiosis I. Prior to meiosis I
recall we were in the cell cycle. We had
germ line cells that are going to become gametes
after meiosis. The germ line cells have 46
chromosomes and they are going to go through S phase.
And they are going to replicate those. And so at
the end of S phase, before we go into meiosis I,
we have replicated homologous chromosomes.