Previously, we looked at cell division and considered
mitosis. But now, let's take a look at cell division
and meiosis. Meiosis is for sexual reproduction
in eukaryotes. By the end of this lecture,
you should be able to recall the language of
chromosomes. It's particularly important when we
consider meiosis and comparing it to mitosis. We will
also be able to discuss the distinguishing features of
meiosis. What makes it specifically different from
mitosis? In addition to summarizing two means
of acquiring genetic variation. One of the important
things of sexual reproduction is precisely that.
That we can obtain some genetic variation
upon which evolutionary mechanisms can act.
Next we'll be able to relate meiosis to gamete
formation. Understand how it relates to the production
of sperm and eggs. And in addition, you will be able
to distinguish between mitosis, meiosis I and meiosis II.
And define some of those distinguishing factors. So,
let's begin by thinking about why it is that we need
meiosis at all. We have 46 chromosomes as humans right.
23 pairs and each of those pairs of chromosomes
need to separate in order to form haploid gametes
and sperms. So we are diploid. Our gametes,
sperm and eggs are haploid. So each of them has
to have 23 chromosomes. One of each copy.
So, diploid organisms have a pair of each chromosomes.
Haploid organisms have a haploid, one set of each
of those chromosomes. So, germ line cells are the
cells that are going to become sperm and egg.
So germ line cells themselves are actually diploid.
And sperm and eggs become haploid after the process
of meiosis. And then sexual reproduction leads the
sperm to the egg and they fertilize each other.
And once again restore the diploid condition in
the zygote. So once we have a diploid zygote,
it will develop and cell division will occur. And this
is when we see many of those cell cycle controls
coming into place. So now let's look at the language
of chromosomes. It's particularly important for us
to review this before we move into understanding what
goes on in meiosis. More important than knowing
what happens during prophase, metaphase, anaphase,
so on and so forth, is that we understand
what's happening with the chromosomes. So,
important to be able to diagram it rather than
just to be able to describe the events of each phase.
So, let's step into that for just a quick moment.
First of all recall that we each have two copies of
each chromosome. During S phase, those chromosomes
are replicated or synthesis of a new chromosome
happens. So now each chromosome has an exact
copy of itself. One of them originally came from
your father and one of them originally came from
your mother. And then during the process of meiosis,
we're going to separate the homologous chromosomes
that are replicated and we are going to separate
the sister chromatids in meiosis II.
So, reviewing the language homologous chromosomes are
two of the same kind. Once they undergo replication
during S phase, these homologous chromosomes can be
called replicated homologous chromosomes
cause they each have an identical copy. Now, recall
that there is a small indentation or tightly packed
DNA region which we call the centromere. And the
centromere has some kinetochore handles on them.
Those kinetochore handles are again what we're going
to use or what the cell uses in order to pull
the sister chromatids or homologous pairs apart.
So, moving on from here we will look into how this
cell division happens. When do we actually reduce
chromosome number. That's a question to keep in
your mind. So meiosis, has two nuclear divisions.
and only one round of DNA replication. So, the
germ line cells are undergoing the cell cycle
producing more and more germ line cells. They know
they are going to eventually undergo meiosis and
become sperm and eggs. But they undergo mitosis
until they're ready. At some point in time,
there is going to be some sort of cell signalling
that tells these germ line cells that are diploid,
that it's time for them to divide into four gamete
cells. And that's the point that we are going to
look into with more depth right now.
So, meiosis I is where we are going to have the
reduction division. I'll repeat that a number of times
because it's really important to recognize that
that is when we go from diploid to haploid.
After meiosis I we have one copy of each of our
chromosomes in each resulting cell.
So either the mothers chromosome or your fathers
chromosome end up in one of the resulting cells
from meiosis I. During meiosis II, we take this
separated homologous chromosomes and split the
sister chromatids so that each of the resulting
cells has one copy of each chromosome.
So meiosis I, we had our homologous chromosomes. They
separate. And in meiosis II, sister chromatids separate.
So that's where the language
becomes especially important.