First, let's begin by gaining an understanding of
the language of chromosomes.
Without this language, it's really hard to fully understand
what is happening during mitosis.
So, here we look at some chromosomes. First of all,
on the one side we have single chromosomes.
You have one chromosome coming from each parent,
the dark one and the light one.
Dark one from mum, light one from dad.
During S phase, we see that these chromosomes replicate.
These chromosomes are replicated to become chromosomes again,
and this is where the language becomes quite confusing.
Because now, each of these chromosomes are
replicated homologous chromosomes.
So we had homologous chromosomes, one that came to you from mum,
and one that came to you from dad.
But in order for cell division to occur,
we need to make an exact copy of each of those chromosomes.
So we go through S phase, or replication,
and now we have two replicated homologous chromosomes.
That's why the language is important.
We either have homologous chromosomes
or we have replicated homologous chromosomes,
each with the carbon copy or xerox copy of themselves.
So as we look at these chromosomes,
there are a couple more pieces of anatomy.
How could we also describe these as differently?
The replicated chromosomes are made up of
two sister chromatids, right.
So they are exactly the same. They are sister chromatids.
We have two sister chromatids.
One sister chromatid is the copy of the mother's chromosome.
And the other sister chromatid is the copy of the father's chromosome.
So we have two sister chromatids from father,
two sister chromatids from mother,
in which case, we now have what we turned a tetrad, so four.
In meiosis and mitosis, it's important for the spindle apparatus,
the microtubules that we spoke off earlier
to be able to grab onto each of the chromosomes
in order to separate them.
Each chromosome, at the centromere, has a connector core.
And the connector cores are the little green pieces,
they are sort of like handles.
So when we want to separate the sister chromatids,
the copies from each other to put into new cells during mitosis,
we yank on those handles and pull them apart.
Well, the cells yank on those handles and pulls them apart.
We'll look at those detail shortly,
but it's important to understand
that the connector cores are right at the
centromere of the chromosome.
We also have these cohesin proteins
that hold sister chromatids very tightly together
so that they don't just fall apart before it's their time.