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 parents' own cells.
So, the cells of our body proliferate constantly to replace dead cells
and 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,
migrate back along the allantois into the dorsal mesenteric
to reach the developing gonads in the genital ridge.
By the end of the fifth week, they've entered the genital 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 two is matched with chromosome two,
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 two with a maternal and paternal chromatid
and will have it's paired chromosome two with another maternal and paternal chromatid.
Between divisions, we have a process called interface,
during which the DNA is replicated, which makes 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 are thereafter gonna help us divide.
And that's because the centrioles extend spindle fibers,
which is an array of microtubules that attach to the centromere.
At the center of each one of these chromosomes, they connect to the centromere,
and then, during anaphase, they pull each chromosome,
one pair to one side of the cell or the other.
So, the centromeres are going to split the chromosomes to either side of the nucleus during anaphase.
During telophase, we have a new nuclear envelope formed around each
one of the daughter nuclei, and a 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 interface where DNA replication can occur.
At which point, we could begin division yet again if we just so desired.
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 one -- in one cell, and maternal and paternal chromosome
one in a different cell, we're gonna split yet again.
So, we have four cells with either a maternal chromatid one, a paternal chromatid one,
or a maternal chromatid one, or a paternal chromatid one.
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, their centromere forms,
and the centrioles move to opposite sides of the cell to prepare for the splitting of the cell and the nuclei.
However, chromosome five will line up right next to the other chromosome five,
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 crossover 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 a portion of its neighbor, and vice versa.
This process happens 20 to 30 times during meiosis I,
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 a 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 I.
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 going to 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.
Centromeres in the center of those chromosomes, centrioles on the opposite side,
they line up and undergo metaphase, anaphase, telophase, and split,
and the 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 a 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.