Mitosis, Meiosis and Sexual Reproduction
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Mitosis, Meiosis and Sexual Reproduction

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I touched on this a little bit in the video on how variation
can be introduced into a population, but I think it's
fairly common knowledge that all of us-- when I talk about
us I'm talking about human beings, and frankly, most
eukaryotic organisms-- we're the product of sexual
reproduction.
So if this is the first cell that had the potential to
become Sal, we know that this first cell-- let me say this
is the nucleus of that first cell so I can draw the whole
cell and all that, but let's just focus on the nucleus.
It has 23 chromosomes.
Well, let me put it this way.
It has 46 chromosomes, 23 from my father and 23 from my
mother, so that's 1, 2 3, 4, 3 5, 6, 7, 8, 9, 10, 11, 12, 13,
14, 15, 16, 17, 18, 19, 20, 21, 22, 23 from my father.
And then let's say that last one actually helps to
determine my gender, or it fully determines my gender.
That's my Y chromosome.
And let's say I had 23 homologous chromosomes, or one
chromosome that kind of was the homologue for each of
these, but I have 23 of them from my mother, so 1, 2, 3, 4,
5-- oh, you get the idea.
I can just draw a bunch of them, and then have the X
chromosome that is essentially one of the gender-determining
chromosomes from my mother.
And we learned before that each of these pairs are
homologous chromosomes, that they essentially code for the
same gene, one from my father and one from my mother.
Now, that first cell that had the potential to become me, it
was a product of fertilization, of an egg from
my mother-- so an egg from my mother.
I'll just draw the whole egg like that.
I'll just focus on the DNA from now, so my mother's DNA,
it had 23 chromosomes.
So it didn't have pairs, and this is key.
So there's 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20, 21, 22, and then 23 was the X
chromosome.
And so it's a combination from my mother, so this is from my
mother, and a sperm from my father.
Let me do that here.
And I'll draw the sperm much larger than it is normally
relatively to the egg.
This is kind of the nucleus of the egg, but let's say that
this is the sperm, and it has a tail that helps it swim, and
it has 23 chromosomes.
So 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
17, 18, 19, 20, 21, 22, and then it has that Y chromosome.
Let me do that Y chromosome in a separate color.
Just as an aside, this unification, this
fertilization that occurred from this sperm cell to this
egg cell, so it essentially penetrates into this egg cell
and it creates this zygote, which is a fertilized egg cell
from my mother, and this contains all DNA from both my
father and my mother.
So this very first cell that was created from this
fertilized egg, this is called a zygote.
It's a product of
fertilization between two gametes.
So that's a gamete and this is a gamete.
Both a sperm cell or an egg cell, they're
both examples of gametes.
Now, the whole reason why I'm doing this is I want to
introduce you-- and I already introduced this notion to you
when we talked about the variation of population, that,
look, this has my full chromosome complement.
It has 23 pairs, and each pair is a pair of homologous
chromosomes.
They essentially code for the same things, one from my
mother, one from my father, and that is 46 individual
chromosomes, 23 from my mother, 23 from my father.
These gametes, they each have only 23 chromosomes, or half
the number of a full complement.
Now, everything that I'm talking about here, the number
46, or 23 pairs, or 23 individual chromosomes, this
is unique to human beings.
If I talked about other species, they might have 10
chromosomes or they might have 5 chromosomes.
But one thing that is universal for all sexually
reproducing organisms is that gametes have half the number
of chromosomes as the zygote, or you can kind of view it as
the organism itself, the way we
conventionally think about it.
So when people talk about half the number of chromosomees,
they say it has a haploid number.
And that literally just means half the number of
chromosomes.
It's very easy to memorize, because haploid starts with
the same two letters as half.
Haploid number for humans is 23 chromosomes.
And so, you say, oh, if you say this is a haploid number,
what do you call it when you have the full complement of
chromosomes?
Well, that's called the diploid number.
And I remember that because di- often is a prefix
associated with having two of something, and so you have
twice the number of chromosomes.
So this is haploid, this is diploid number, and this is
for humans, right?
For an organism where the diploid number is N, and
you'll sometimes see this notation, so I want to make
sure you're comfortable with it, there's some organism, or
actually any organism.
If the diploid number is 2N, then the haploid number is
going to be half of that, or just N.
Now, in the case of humans, the diploid number is 46, so N
is equal to 23.
So a fertilized egg or even just a regular somatic cell or
a body cell will have a diploid number of chromosome,
while a sex cell, and I'll be a little bit clearer about
that in a second, will have a haploid number of chromosomes.
So gametes, which are either a sperm or an egg, those are
both examples of gametes, they have half the number, they
merge, and then you get a zygote, which is that very
first cell that had the potential to turn into me,
that has a diploid number of chromosomes.
And I actually want to do a little bit of a side here,
because it's fascinating.
We talk about natural selection, and we even wonder
today to what degree is it occurring, because our
society, it's not as tough of an environment as the natural
world would be where we're being stalked by predators and
we have to live out in the wild and find
food and all of that.
But even this process of fertilization is an incredibly
competitive process, because this sperm that happened to be
the one that kind of won the race from my father to
fertilize my mother's egg, it was actually the first of
roughly 200 million other sperms. There were 200
million, roughly.
There could've been 200 to 300 million other
sperms in that race.
From the moment we're born, we're already the product of
an intense competition amongst these male-- I guess we could
call them male gametes, or amongst these sperm cells.
Some of them might have had weird mutations, that they
didn't know which direction to swim, they happened to go in
the wrong direction, maybe some of them had weird tails
that didn't allow them to swim as fast, so you're already on
some level selecting for fitness within this
environment.
So if you had some weird mutations from the get go in
some of these sperm cells, it would have been less likely,
especially if they affected their ability to kind of swim,
it would've been less likely that they would have been the
ones to win this race.
So already, you are the product of a race of 280
million organisms, if you consider each of these sperm
cells an organism, and you are the product of that winning
combination.
So, you know, sometimes we feel lost on this planet.
We're one of 6 billion people and all that, or just a
number, but we already are the product of a pretty intense
accomplishment.
But now with some of this vocabulary thrown out of the
way, let's talk a little bit about zygotes and how do
zygotes turn into people, and then how do those people
essentially produce gametes, which then can fertilize other
people's gametes to form more zygotes.
So the general idea: So that very first cell that was
essentially my mom's egg fertilized by a sperm cell
from my father, that was a zygote, and as soon as it's
successfully fertilized, it has 2N, or it has the diploid
number of chromosomes in the case of humans, which I
believe I am one of them.
I have 46 chromosomes.
And then this cell right here begins to split and divide
over and over and over again.
We'll do a whole series of videos on the actual mechanics
of that, but it splits by a mechanism called mitosis.
And mitosis literally is just a cell splitting to form
copies of itself.
So it just starts splitting into two more cells that are--
and actually, let me do it this way, just because the
actual way it works is right when a cell is split, the
cells that it splits into aren't that much larger than
the original one.
But now each of these have 2N chromosomes, or 46 in the case
of humans, and you keep splitting, and it happens over
and over and over again.
So eventually-- well, let me just do it this way.
This keeps splitting, and then you have-- and I'll go into
the words for some of these initial collections of cell,
but I won't go into that right now.
2N, all of these are original copies from a genetic point of
view of that original cell.
And then eventually, they start to really-- I start to
have tons of them.
There's just a gazillion of them that are all duplicates
of the cell, and they all contain the 2N number of
chromosomes, the diploid number of chromosomes.
They all contain all of my genetic material, but based on
how they relate to each other and what they see around them,
they start differentiating.
So all of these have 2N number,
so they're all diploid.
And mitosis-- this is the process the whole time-- is
these divide one cell into two cells and those two cells into
four cells and keep going.
And then these begin to differentiate.
Maybe these cells eventually differentiate into things
that'll turn to my brain.
These cells right here differentiate into things
that'll turn into my heart.
These cells here differentiate into things that will turn
into my lungs and so forth and so on.
And eventually, you get a human being.
But it doesn't have to be a human being.
It could be whatever species we happen to be talking about.
So let me draw the human being.
So I'll draw my best shot at an outline of a human being.
Now, we're talking about gazillions of cells.
You have your human being, and I'll just draw a very simple
diagram, outline of a human being.
When I was in high school, I was a class artist, so I don't
want to make this representative of my true
artistic ability.
I'm doing this here just to kind of give you an idea.
But anyway, eventually, you keep dividing these cells and
you end up with a human being, and this human being, you
know, you wouldn't even notice the cells on this scale.
Now, most of these cells of this human being, if this is
me or you, these are all the product of mitosis that
started off with that zygote, and it just kept dividing and
dividing and dividing into mitosis.
But it differentiated.
I said some of them will turn into brain cells.
Some of them will turn into heart cells.
The whole process of differentiation is actually
fascinating, and we'll talk a lot more about that when we
talk about stem cells, embryonic stem cells, and
maybe we'll even talk about the debate of it.
But the question is, well, how do I then
produce those gametes?
How do I produce those things that eventually, if I'm going
to reproduce, turn into these kind of
haploid number of cells?
And that's what happens in your sexual organs.
So in a male, you have some germ cells, so some of these
cells turn into germ cells.
And the germ cells exist as part of your
reproductive organs.
So let's say those are the germ cells.
In a male, they would be part of the gonads, so
they would be there.
In a female, they would be involved in the ovaries.
And these germ cells, they're the product of mitosis.
So let me draw a germ cell.
So a germ cell is the product of mitosis, so it still has 2N
number of chromosomes, so it still is a diploid cell or has
a diploid number.
But what's special about a germ cell is it has the
potential, one, it can either continue to do mitosis and
produce more germ cells that are identical to it, so it
could produce two germ cells that are identical to it, or
it can undergo meiosis.
And meiosis is essentially what a germ cell undergoes to
produce gametes.
And so if this germ cell undergoes meiosis, and I'll do
a whole video on the mechanics of it, instead of two cells,
it'll actually produce four cells that each have half the
number of chromosomes in them, so these cells are haploid.
In the case of a male, these would be sperm cells.
This would be sperm.
In the case of a female, these would be ova, sperm or ova,
and these are the gametes.
So it's an interesting thing to talk about, because in the
last several videos, I talked a lot about mutations and what
does that do to a species, but think about what happens.
If I have a mutation in some cell here, some somatic cell,
some body cell, somatic cell, will that mutation or can that
mutation in any way affect what's going to
happen to my kids?
Will that mutation be carried on to my kids?
Well, no.
Because in no way will what goes on in this cell affect
what I actually pass on eventually in the sperm cells.
It'll just be a random mutation.
It could affect my ability to reproduce.
For example, it could be-- God forbid, it could be some type
of cancer or something that, especially if you contract it
at a young age, it might be some type of terminal form so
that might affect your ability to reproduce, but it will not
affect the actual DNA that you pass on to your offspring.
So if you have some really bad mutation here, it could affect
how you live or it could turn cancerous and start
reproducing, but it will not affect what you pass on to
your children.
The traits that will be passed on or the changes that will be
passed on are those that occur in the germ cells.
So if you have mutations in your germ cells, or during the
process of meiosis, you have essentially recombination of
DNA because of crossovers, and we saw that in the variation
video, then that will introduce new forms or new
variants inside that could be passed on to your children.
And I really want to make that point there, because we talk
about mutations, but there's different types of mutations.
There's some mutations that won't be passed on to your
children, and those are the ones that occur in your
somatic cells.
Maybe some of them do nothing so then it really doesn't
affect your overall function, but in the mutations that
either occur in your germ cells or the recombination or
the variation that is introduced during meiosis,
that will be passed on to your children.
But even there I want to be careful.
Because remember, this is a severe competition.
So out of all of the-- let's say there's 280 million sperm
cells that at one time are being competitive for an egg,
it's possible that some of them have mutations.
In order for one of those mutations-- let me do the
mutations in different colors.
That's a purple mutation.
That's a blue mutation.
But in order for that mutation to truly be passed on to my
offspring, the sperm containing the mutation is the
one that has to win the race.
So already you have a selection going on at kind of
this sexual reproduction level where you're selecting for
things that are at least good enough-- I mean, to some
degree, the sperm has to be good enough to win this is
incredibly, incredibly competitive race.
So that mutation that somehow made the sperm deformed or
didn't allow it to swim or made it behave in some weird
way, it's very unlikely that that mutation would go on to
be the one or that cell would go on to be the one that would
successfully fertilize an egg.
So anyway, I wanted to introduce you to these ideas.
The main idea is really some of the
vocabulary: haploid, diploid.
It's very confusing when you first learn it, but it
literally just means half the normal group of chromosomes.
And in the case of humans, that would be 23.
And the cells that have a haploid number of chromosomes
are our gametes, which are sperm cells for men, and ova,
or egg cells for women.
But everything else in our body, all of our somatic
cells, are diploid, which means that the full complement
of chromosomes, they all have a copy of our DNA.
And that's why DNA testing is so interesting because you can
get any cell from someone anywhere, and you have their
full complement of DNA.
You have all of the information that describes
them genetically.
Anyway, see you in the next video.

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