We are now gonna shift gears a little bit and instead of looking
at the development of a specific body system,
we're gonna look at how the germ cells of the reproductive system
come into existence in the first place,
and to do so we need to investigate mitosis,
the process by which daughter cells are made normally and meiosis,
the process by which genetically dissimilar cells
are made from the parent's own cells.
So the cells of our body proliferate constantly
to replace dead cells and to keep us healthy
and constantly able to adapt to our environment.
The normal sort of division produces exact genetic copies of the cells
that the daughter cells come from, this is mitosis.
However, meiosis is the process by which we produce germ cells
and if our germ cells were exact copies of our own regular cells,
our children would have very little genetic variability from us,
but the process of meiosis has several means
by which genetic variability are introduced
into the process so that when one germ cell combines
with the germ cell from another person,
we get a new and very genetically distinct individual as a result.
So during early development,
we've already seen how the primordial or primitive germ cells
migrated from the epiblast into the yolk sac,
and then during the fourth week
they migrate back along the Allantois into the dorsal mesentery
to reach the developing gonads in the genital ridge.
By the end of the fifth week, they've entered the general ridge
and have associated with either the testes or ovary
which will be developing there.
During migration, these germ cells are undergoing mitosis
to create more copies of themselves
and also beginning to produce true germ cells via meiosis.
Mitosis is the 'normal' method of DNA replication
and creates two daughter cells
which are essentially genetically identical to their progenitor cell.
Aside from germ cells which come into existence via meiosis,
mitosis creates identical cells
and every cell in the human body is made of 23 pairs of chromosomes,
that means we have a diploid number.
23 pairs equals 46 total.
22 of those are matched pairs,
meaning chromosome 2 is matched with chromosome 2,
and we have one pair of unmatched chromosomes called sex chromosomes,
which are either XX in the case of a genetic female
or XY in the case of a genetic male.
Each chromosome is made of two subunits called a chromatid.
One that comes from the mother and one that comes from the father.
So we're gonna have chromosome 2 with a maternal and paternal chromatid
and we'll have it paired, chromosome 2,
with another maternal and paternal chromatid.
Between divisions, we have a process called interface
during which the DNA is replicated
which makes that have that diploid number of chromosomes
allowing further development to occur
and further division to occur.
The first step in cell division is called prophase.
At this time, the chromosomes have finished replicating.
We've got the diploid number of chromosomes
and they condense down and pair up.
At this point, we have two identical copies of each chromosome,
therefore, four chromatids total: two paternal and two maternal in each pair.
They are joined at their center by a protein called a centromere.
During the next step in cell division, we enter prometaphase.
Chromosomes are now tightly bundled together and can actually be visible
if viewed with enough magnification.
Very small organelles outside of the nucleus called centrioles
migrate to opposite poles of the cell
and they're thereafter gonna help us divide.
And that's because these centrioles extend spindle fibers
which is an array of microtubules
that attach to the centrosomes at the center of each one of these chromosomes.
They connect to the centrosomes and then during anaphase
they pull each chromosome one pair to one side of the cell or the other,
so the centrosomes are going to be split the chromosomes,
the other side of the nucleus during anaphase.
we have a new nuclear envelope formed around each one of the daughter nuclei
and the cleavage furrow or a little divot appears in the parent cell
and it's gonna get tighter and tighter
until eventually it splits and we undergo cytokinesis,
splitting of the cell to form two new daughter cells
from the single parent cell.
At this point, we are going to enter interphase
where DNA replication can occur
taking these cells from their now haploids state
to their diploid state of 46 chromosomes
at which point we could begin division, yet again, if we just so desire.
Meiosis is very similar to mitosis
but it's almost as though two rounds of mitosis take place,
and instead of separating the cells in such a way
that we have a maternal and paternal chromosome 1
in one cell and a maternal and paternal chromosome 1 in a different cell,
we're gonna split yet again,
so we have four cells with either a maternal chromatid 1,
a paternal chromatid 1 or a maternal chromatid 1 or a paternal chromatid 1.
Essentially, we're going to have four cells result
from the process of meiosis instead of two
and this is gonna create a haploid cell of 23 chromosomes
that can then combine with another haploid cell to create a new individual.
Initially, meiosis is very much the same process as mitosis.
The chromosomes condense and their centrosomes form
and the centrioles move to opposite side of the cell
to prepare for the splitting of the cell in the nuclei.
However, chromosome 5 will line up right next to other chromosome 5
and chromosome 18 right next to the other chromosome 18.
They're going to associate so that they can actually trade genetic material.
In this process, on screen we see a pair of representative chromosomes.
They're gonna line up and form a tetrad
so that they've got their arms stretched out
next to the similar region of its adjacent chromosome.
What happens next is that the genetic material of one chromosome
will cross over with the same region of its neighboring chromosome,
this forms an x called a chiasma
and at that point the genetic material from one chromosome
or the other switches and we wind up with the chromosome on one side
with the portion of its neighbor and vice versa.
This process happens 20 to 30 times during meiosis one
and it's a way that we've shaken up the genetic information in the germ cells,
whereas one of these chromosomes
had a strictly paternal or maternal chromosome before,
we now have chromatids made up of both maternal and paternal material.
So we've already introduced that massive amount of variation
into these germ cells.
Thereafter metaphase, anaphase, telophase, cytokinesis occur,
splitting these cells into daughter cells,
very much the same as mitosis and that brings us to the end of meiosis one.
So splitting movement and two daughter cells are the result,
but remember, these are no longer genetically identical to the progenitor cell.
They are different and very much different from other nearby germ cells.
The second phase of meiosis is gonna create cells
instead of with one chromosome of two chromatids each,
one chromosome of one chromatid each and the mechanics are very much the same.
Centrosomes in the center of those chromosomes,
centrioles on the opposite side,
they line up and undergo metaphase, anaphase, telophase and split
and end result are four daughter cells from one progenitor germ cell.
And these are very genetically dissimilar to the chromosomes of the parent
and when they combine with the germ cell from another parent,
we get a new and unique individual beginning to develop.
Thank you very much for your attention and I do appreciate it.