And when we go into prophase I, the chromosomes have
already begun to wind up. So the chromatin has been
synthesized, duplicated and we are wrapping it up
into chromosomes. Those chromosomes are going to
continue being wound into prophase I as they pair
with each other. So as they pair and begin this
crossing over process, the homologous pairs are
becoming much more condensed. They are forming
their chiasmata. They are switching pieces or maybe
many pieces. And you can see in this figure
that the sister chromatids are sort of switching
pieces. So let's talk about for a moment
sister chromatids, right. We have one homolog and
the other homolog. This is a set of sister chromatids
and this is a set of sister chromatids. The
chiasmata, the crossing over actually happens between
non-sister chromatids. So again, a little bit
more of language. The non-sister chromatids,
so the paternal, one of the paternal chromatids and
one of the maternal chromatids are the ones that are
exchanging information. So we'll have two that are
the original parental type, and then we'll have
crossing over. And this will become particularly
important when you start to explore genetics.
So, here is the crossing over image. And chromosomes
are condensing as they are crossing over.
So it's not like they are really tightly wound and
crossing over whole chunks of chromosome.
They are actually slightly unwound while they are
exchanging pieces. And so after they have exchanged
they continue their coiling and super coiling and
super super coiling in order to get these
condensed chromosomes. And we are ready to move
forward. So, spindle fibers begin to form.
You are going to see very many familiarities
between mitosis and meiosis. Same thing goes on.
Those spindle fibers form. The asters microtubules
are projecting out. And they are starting
to move towards the poles of the cell.