- [Lecturer] Because my wife never smiled
when she was pregnant.
It's just not.
(audience laughing)
I don't know how much the pay
to her to smile like that,
but you're not that happy
when you're that far along.
At least my wife wasn't.
Okay, so Sexual Reproduction.
Obviously, we're not going
to spend most of the time
talking about the birds and the bees.
Hopefully you already know
everything regarding that.
But what we are going to talk about is
when any organism, not just humans,
but when any organism that
undergo sexual reproduction,
it creates the gametes,
the sperm in the egg.
What is it that is so special
that makes sexual reproduction
actually more advantageous
for evolutionary survival,
than asexual reproduction,
which is the cloning process.
We're gonna talk about why it is
that sexually reproducing species
do a better job of surviving as a whole,
because ultimately, sexual reproduction
is not a cloning process.
It's a way of taking two sets of genetics
and scrambling them up
so that the offspring
is always unique.
Okay, so that's why sex.
Okay, not beyond the written,
you know, normal reasons.
But that's why organisms that
are predominant in this world
are sexually reproducing
because of the amount of genetic variation
that is propagated from
one generation to the next.
So here's why.
This is why sexually reproducing species
have a greater advantage.
When you have a clone, it
takes a long time to generate
any type of genetic diversity,
because the only way in which you can get
typically any type of new diversity
is through random mutations
and I mean random,
meaning it's just, it's not directed
they're just, they happen
due to the environment
of various things.
You're not gonna get a lot of diversity.
Now, this is good for us because
when we get an infection,
say of a bacteria, this
ecological disaster
can be just the taking of an antibiotic.
We're assuming that because
all of these bacteria
are all clones of one another,
that a single shot of this
one type of antibiotic
is gonna kill them all off
and we're usually right.
We're usually able to
kill most all of them off.
But when you have an organism
that the population has a
great amount of diversity,
then one single change in the environment
is not enough to kill
the whole species off.
And the reason for that is because usually
there is enough variation
within the species
to adapt and survive.
And this is why sexually
reproducing species
have a greater advantage.
So we'll talk more about this
when we get to evolution,
when we get to ecology,
but this is, we've seen this all the time
that when environmental
factors change food sources,
change temperature or other
types of parameters change.
If the species can't
adapt, then they die off.
And 99.9% of the species that
have lived on this planet
are dead according to our fossil record.
So ultimately, it's
common for many species
to go the way of the dodo, so to speak.
However, this is advantageous primarily,
because even what we might
consider problematic genes
like cystic fibrosis, sickle
cell anemia, Tay-Sachs disease.
These diseases we've been talking about,
even though they're problematic
in the right conditions, they
can actually save your life.
We'll show how individuals
who have or carry
some of these diseases are
protected against things like
malaria and typhoid fever and cholera
and even tuberculosis
and things of that sort.
So there's a reason why
some of these things
persist in the human population,
even though we're like,
why are these diseases not eradicated?
Well, sometimes nature says,
hey, this will do you good.
It does a body good, right.
So that's why sex because ultimately,
any species that reproduces sexually
produces massive amounts
of genetic diversity
and that genetic diversity
is what gives them
an evolutionary advantage for survival
and reproductive success.
So we call this process of
gamete formation, meiosis.
And you're gonna see that
it's very similar to mitosis.
It's got some of the same phases,
prophase, metaphase,
anaphase, telophase okay.
But what happens during those
phases is very different,
especially in the first set
of processes in meiosis.
Now, in sexually reproducing species,
unlike mitosis, when
you produce the gametes,
you have to split the
genetic information in half.
Why do you think that's the case?
Why can't you just, you know,
let's say you have two
parents with two chromosomes.
Why can't you just fuse those together?
What will happen?
Yeah, it goes from there
and then it'll go, I don't know,
184 and so on and so forth.
You can't, in order to
maintain the constant nature,
which is very tightly
controlled by our cells,
the same number of chromosomes,
the same number of genetic information.
We have to split our
46 chromosomes in half
and ultimately pass on
one of all of the sets
of chromosomes that we have.
Now, we don't split them in half,
in that we cut them in half.
Remember this is carrier
type of the human genome.
We have two of every type of gene.
Well, during meiosis, what happens is
we only pass on one of those two
and it's random which one we pass on.
And remember that these
are the chromosomes
you inherited from your parents.
So if you inherited this
one for your mother,
you inherited that one from your father.
If your inherited this
one from your father,
you inherited that one from your mother.
And when you pass your genes on,
you might pass on your
mother's chromosome, number 17,
but your father's chromosome number 18.
And this is how your offspring
becomes a combination
of your parents and their
parents and so on and so forth.
This is where the genetic
mixing and matching
comes into play.
Now, most cells in your
body do not undergo meiosis.
Only a few select cells actually do this.
So like we've mentioned,
those somatic cells,
which are the body cells,
these undergo mitosis.
If they divide which
your adult stem cells do
to regenerate your tissues, it's mitosis,
but in the sex organs, the
testes and the ovaries.
This is where you have what
are called the germline cells.
These are special stem
cells that have the ability
to undergo meiosis.
So what ends up happening
is the parent cell,
remember we call it diploid
because it's got two sets
of all of our chromosomes.
So technically speaking,
you can look at it like this
23 times two, we have two sets
of all of these chromosomes.
When it goes through
the process of meiosis,
it essentially only takes
one of each of those sets.
And when you create the gametes,
the cells are now haploid at the end,
they only have one of
each of the chromosomes.
And that includes the sex chromosomes.
Each sperm and egg is
essentially going to have
one sex chromosome to it.
And then when the sperm
and egg come together,
you reconstitute those
two sex chromosomes.
Now for women, they
have two X chromosomes.
So they're gonna pass on
one of their X chromosomes.
For men, that's where it
becomes a 50, 50 shot.
Half of the sperm have the Y chromosome,
half of the sperm have the X chromosome.
And so if the sperm with the Y chromosome
fertilize the egg you're gonna have a boy.
If the sperm with the X
chromosome fertilizes the egg
you're gonna have a girl.
So it really is the guy's
fault on the sex of the child.
I shouldn't say fault. (chuckles)
It's the guy's sperm that
ultimately determines
what the sex of the child is going to be.
So the life cycle of us,
primarily from the
moment of fertilization,
when that zygote forms and
you form that diploid cell,
it's all mitosis.
It's all mitosis from the
formation of the fetus,
to the baby, to the adult.
But when you reach certain stages
that's when meiosis occurs.
Now here's, I'm gonna test
you on this next concept.
A lot of times people don't understand
the difference in sex
cycles in males and females.
Now we're primarily
going to focus on humans.
'Cause when you look at
other organisms like plants
and frogs, they can be
similar to us like frogs,
or they can be really
different like plants.
So even though plants
undergo sexual reproduction,
the stages that they go through
they're just kind of all over the place.
So we're just gonna
focus on human sex cycles
and things of that sort.
I mean, you can go further into it.
And I think your book covers
a number of different things,
but I choose just to
primarily focus on humans.
Now, here are some facts
that I wanna test you on
because a lot of people don't know this.
When women are in the
development stage as a fetus,
before they're born the germline cells,
these stem cells that produce the eggs
will actually begin meiosis
before they're even born, okay.
So before they leave the
mother's uterus and whatnot,
they are creating all of the
oocytes, which are the eggs.
And they're beginning meiosis
before they're even born.
So when they will create all of the eggs
that they'll ever have for their lifetime
before they're born
and then those eggs will begin
meiosis before they're born.
Now let's say they start
off with like two million.
It might be as many as four million,
but let's say two million or whatnot.
By the time that women
reach sexual maturity
and they start obsoleting the eggs
they'll have lost about 3/4 of them,
and they'll only have about
500,000 left or so, okay.
So most of those are just
going to die off essentially
during the infant stages, before
they reach sexual maturity.
Now, once they reach sexual
maturity, cells will keep dying,
but some will mature
and essentially go through
the ovulation process.
Once a woman reaches her 40s or whatnot,
mid to late 40s, when she runs out,
that's when menopause hits.
And so there's only a certain window
in which women are fertile
to be able to have a baby.
Now, men on the other hand,
they don't even start producing sperm
until they reach sexual maturity.
They have the cells in place,
they're ready to go in the testes,
but they will not start producing sperm
until they reach sexual maturity.
And then they'll just produce sperm
for the rest of their lives,
essentially until they die.
So, I mean, there may come
a point where it shuts off
because of hormonal levels or whatnot,
but pretty much they produce sperm
throughout the rest of their life.
So that's the fundamentals
of the different reproductive cycles.
Women have this window in which they can
ultimately have offspring,
men have a much bigger window,
but that's the differences when
meiosis essentially occurs.
Here's another fascinating thing.
I'm gonna show it later.
But when a woman ovulates an oocyte
and it starts traveling
down to be fertilized,
it still hasn't finished meiosis.
It's still kind of right in the middle
of the second stages of meiosis
and it won't finish it
until fertilization occurs.
If fertilization does not occur,
then it will just go straight through
and that's when you get
the menses and whatnot.
But ultimately, meiosis is almost,
a huge lifelong process in women.
In men, they are just pumping out
millions of sperm every day.
So big differences between
when meiosis occurs in males and females.
Now, let's talk a little bit about
some of the mechanics of meiosis
and why the cells are so different
when they go through
this process of meiosis
where they're not genetically identical,
like you would get in
cells undergoing mitosis.
Now here's the first difference.
Meiosis has two rounds of cell
division, not one but two.
So we usually distinguish them by saying
meiosis I and meiosis II.
Meiosis I is obviously the first round.
They got it right this time,
instead of photosystem II
and I and all kind of stuff.
So meiosis I is the first round,
meiosis II is the second round.
We're not gonna talk too
much about meiosis II.
The reason for that is because
this process is almost
identical to mitosis
and it's merely just kind
of a finishing up process.
All of the genetic recombination
that we're gonna focus on
happens during the first round of meiosis.
So that's where we're gonna
spend most of our time.
And that's where we're gonna
have most of our questions.
Meiosis II, there is no
genetic recombination
going on during that process.
So that's what we're
gonna focus on meiosis I.
As for all the shuffling occurs,
all of the mixing and matching
of what your parents gave you
to pass on to the next generation.
Now, when you look at the chromosomes,
we have two divisions of chromosomes.
We have what are called autosomes.
These are the non-sex chromosomes, okay.
We have 23 pairs of chromosomes.
So we have 22 pairs of autosomes
and then one pair of sex chromosomes.
So you're gonna learn in the next lecture,
why this is important because
we have autosomal disorders
and then we have sex-linked disorders.
There's a difference between
where the genes are found
on which chromosomes, and
what effect that they have,
depending upon your sex
or whether it doesn't
matter what your sex is,
in terms of the autosomes.
So it doesn't matter whether
you're male or female,
all males and females, all humans,
have the same groupings of genes
on the same chromosomes from one to 22.
For example, if the insulin
gene is on chromosome 17
is found there in both males and females.
If you have a hemoglobin here on 13,
is found in males and females
and so on and so forth.
The genes are pretty
much in the same place,
no matter where you look.
So the main difference
between males and females
comes down to whether we
have two X chromosomes
or an X and Y.
Now this is not the same for all species.
In some species,
it's the females that
have different chromosomes
and it's the males that
have the same chromosomes.
So don't think that
this applies to all life
in that females always have
the same types of chromosomes
and males have different types.
It's different for
different organisms, okay.
So notice the Y chromosome
is so much smaller.
There's only about 100 genes
on this little chromosome,
but that makes all the difference
in terms of sexual maturity
and identity and things of that sort.
The X chromosome, there are
over a thousand genes here.
Now, not all of these have anything to do
with sex development.
So just cause you say X chromosome,
for example, there are genes here
that encode the cones,
the proteins for your retina in your eye,
They have nothing to do,
whether you're a male or female.
So there's even a blood clotting
one on this one as well.
So not all of the genes
on the X chromosome
have to do with sex development, okay.
But all of the genes
on the Y chromosome do.
So in this case,
the Y chromosome makes all
the difference between it.
Now, just in case you think it's unfair,
we'll talk a little
bit later on how women,
they do have two X chromosomes,
but at a certain point in the development,
one becomes inactivated.
So women in all of yourselves,
you only have one active X chromosome.
Men, you have the X chromosome
and the Y chromosome active.
So even women essentially have only
one of their X chromosomes
is really gonna be active
in their cells for their entire life,
you don't need two.
You only need two in the
initial stages of development
and then one of them
just shuts down randomly.
Okay, now let's talk about
the difference between sister chromatids
and homologous chromosomes.
This is a part that if you
don't get this concept,
you're gonna have a difficult
time a little bit later on.
Now we've talked about
sister chromatids before.
When we were talking about mitosis,
we talked about how, when
chromosomes duplicate
during semi-conservative DNA replication,
each of the chromosomes
ends up being copied.
Well, that's the same thing
for these chromosomes.
It's no different in mitosis or meiosis.
So whether the cell is
getting ready for mitosis
or whether the cell is
getting ready for meiosis
doesn't matter, all of the
genetic information gets copied.
Okay, so you could say for mitosis,
you have 46 chromosomes each
one of them gets copied.
And when the cell splits through mitosis
then you essentially split
the sister chromatids,
each cell gets one of the copies.
Now in meiosis the same thing starts off.
We have 46 chromosomes and
all of those chromosomes
get copied and so each
one has sister chromatids.
Now, they've done a good
job of color coding,
these four pink for your mother
and blue for your father,
or whoever came up with
those colors, I don't know,
(chuckles)
but those stuck or whatnot.
Ultimately, because we have
two of every chromosome
one from each parent,
we call them homologous chromosomes, okay.
So there's a big difference
between sister chromatids
and homologous chromosomes.
Sister chromatids are
carbon copies of each other.
For example, here's
chromosome from your mother.
And when your cell gets ready for
either mitosis or meiosis, doesn't matter,
then it's gonna copy that one.
And then they're gonna be
held together by a centromere,
so that your mother's chromosome
for that chromosome pair gets copied.
Well, here's your father's
chromosome that he gave you.
That one also gets copied.
So each one respectably has
their own sister chromatids.
These two are identical to each other,
and these two are identical to each other.
But what about the pairs?
Well, they're not necessarily
identical to each other.
You've received these
chromosomes from separate parents
and those parents have had their parents
and their parents and so on and so forth.
And since mutations are always occurring,
which are changes in the genetics,
even though your pairs of chromosomes
have the same types of genes.
For example, let's say that
this B gene is for hair color.
And let's say, this is for brown hair
and let's say this is for blonde hair.
They're both a gene that
encodes the same thing,
they're found in the same location
on your homologous chromosomes.
They both encode the same
or supposed to encode
the same type of gene, the
protein for hair color,
or maybe the protein for eye color.
Maybe that's this one right here.
Well, here's the difference.
Maybe your father, his father
or his great grandfather
had a mutation that changed
the nucleic acid sequence of that gene.
So ultimately, what happens is
though homologous chromosomes
have the same types of genes on them,
the exact nucleic acid
sequences might not be the same.
What do we call those?
We call them alleles, okay.
In fact, Gregor Mendel is the
one that coined this phrase
back in the 1800s before they
even knew anything about genes
and things like that.
But alleles, alleles are
the same type of gene,
but different versions due
to genetic mutations, okay.
That's why there's so much differences.
That's why there's so
many different hair colors
and eye colors and skin
colors and different heights
and different intelligence,
there's different blood types.
You know, for example,
blood type is one gene,
we call it the ABO gene.
But there are multiple
versions of that gene.
Some of us have the A version,
some of us have the B version.
Some of us have one of each,
and you might be blood type AB,
or you might have the O version.
And that's where blood
type comes into play.
So one gene can actually
have multiple alleles to it.
All right, so what are
homologous chromosomes?
They have the same size, so
look at the karyotype here.
These two are homologous.
They're the same size.
They have the same types of genes on them.
They have the exact same
amount of genetic information,
which is pretty much saying the same thing
as they're the same size.
And you get one from each parent.
If you've got this one from your mother,
you got that one from your father,
but the alleles might
not be exactly the same.
Your mother may have passed on
the brown hair allele to you
and your father may have
passed on a mutated version,
the blonde hair allele of that gene
and so on and so forth,
all the way down the line.
Your mother may have passed on
the O allele for blood type,
but your father passed on
the A allele for blood type.
And so you're gonna be the
combination of those two alleles.
Okay, so homologous chromosomes
have the same size, shape construction,
but the alleles may be different.
Now some alleles might be the same.
Maybe both parents pass
on blue eyes, okay.
Maybe both parents pass on
blood type A, that's possible.
So the alleles can be same,
but they don't have to be,
they're not carbon copies of each other.
So that's the difference
between sister chromatids
and homologous chromosomes, all right.
So what ends up happening,
now I can explain meiosis in a nutshell.
What ends up happening is in
the first round of meiosis,
homologous chromosomes
separate from one another
while the sister
chromatids remain attached.
So what ends up happening
is on each of these chromosome pairs,
one of your mother's chromosome
will go into one cell with
its sister chromatids.
So the pink one right over there,
will go into cell
and the blue one will
go into the other cell
with it's sister chromatid.
In the second round of meiosis.
That's when sister chromatids
pull apart from one another,
that's meiosis,
and that's why it has to
go through two rounds.
So in the first round, all
of these pairs split up.
Now let's look at the sex
chromosomes for males,
the X chromosome, which you
inherited from your mother, guys
because you're a guy
because your father gave you
his Y chromosome.
The X chromosome with its
copy, it's sister chromatids.
It goes into one cell,
while the Y chromosome
goes into the other cell.
And then during the second round,
the sister chromatids of
the Y chromosomes split up
and the sister chromatids of
the X chromosomes split up.
So it's very easy to visualize it
when you're dealing with
male sex chromosomes,
because these are the only
non-homologous pair of chromosomes.
Why are these non-homologous?
What did we say about
homologous chromosomes?
- [Students] Same size.
- [Lecturer] Same size, have
the same types of genes.
These do not have the same
types of genes on them.
The X and the Y chromosome
have totally different
genes from one another.
Now women do have
homologous sex chromosomes
because the X chromosomes
are the same size.
They do have the same types of genes
all the way down the line.
They've got the same types of genes
in the exact same locations.
The alleles might be different though.
Just like all of the other chromosomes,
you got one X chromosome from your mother
and the other X chromosome
from your father
and the alleles might be different, okay.
So we'll finish off on
this concept right here.
They start out diploid.
After the first round of
meiosis, they become haploid.
We haven't even gotten into
the little nitty gritties
of this, we'll cover that next week.
After the first round of meiosis,
the homologous chromosomes
are separated from one another
to where each cell is now haploid.
Then through meiosis II,
that's when the sister chromatids
separate from one another
and they're still haploids.
Remember, sister chromatids
are exact copies of that
original chromosome.
So when you split them
up, you still only have,
those two chromosomes in this scenario.
Now the phases are the same.
You've got prophase, metaphase,
anaphase and telophase.
And since our two stages of meiosis,
then we also give the
phases a one or a two,
depending upon which phase they're in.
So if it's part of meiosis I,
then we just say prophase I,
and you'll see this on some of
your quiz questions as well,
or it'll ask you what would
be going on during each stage.
If it says prophase I,
that's prophase of meiosis I,
if it says prophase II, then
is prophase of meiosis II.
So I mentioned most of
what we're gonna go over
is in meiosis I,
but there will be some
things that we'll talk about,
especially with anaphase
two, that become important
in one of the concepts
we're gonna discuss.
So you will need to just know that,
that I'm not gonna say
prophase of meiosis I,
we'll just say prophase I, okay.
Now, like mitosis, you got some
of the same things going on
in the early stages of prophase
had to do with meiosis.
So what were those things that happen
in prophase of mitosis
that we already know
that we've already learned?
What were some of the things?
What happens to the chromatids?
(group chattering)
It becomes what?
(indistinct)
Remember, chromatin what?
- [Student] Condenses.
- Condenses, okay.
So chromatin condenses.
Again, you have to be able to manage
and move the chromosomes around.
So they have to super coil
down to a thousandth of
their original length.
What about the nucleus, what happens?
What happens to the nuclear envelope?
- [Student] It unforms.
- [Lecturer] What's that?
- [Student] Unforms.
- [Lecturer] It dissolves,
essentially it breaks apart.
And the same thing happens
here in this prophase.
And what's the third thing
that typically happens?
Let me see here, what forms?
- [Students] The mitotic spindle.
- [Lecturer] The mitotic spindle, okay.
Same three things happen
here in prophase, okay.
But that's not what we're
concerned about with this chapter.
What we mainly wanna look
at is what's different?
What makes this unique?
Well, they're actually
two different things
that occur in prophase I
that are unique to meiosis
that you will not find in
mitosis, oh, here they are.
The first one is,
remember when we talked
about homologous chromosomes,
those are the pairs of chromosomes
you get from your parents
when you were formed
from the fertilization
of the sperm and the egg.
So remember homologous
chromosomes are the same size.
They have the same shape,
same types of genes,
but the alleles might be different
because you inherited them
from two separate parents.
And so therefore
mutations down their line,
ultimately, arrive at you.
Sometimes you have the same alleles,
both your parents pass on the blood type O
and sometimes you have different alleles.
One parent passes on
their allele for blood
or for blonde hair,
the other passes on the
allele for brown hair.
So homologous chromosomes
pair up during prophase one,
that is a key aspect of prophase one.
So what that means is let's
look at the karyotype here.
Each of these homologous pairs
will find each other, okay.
So chromosome 17, will find each other,
chromosomes, 18, 21, 22,
including the sex
chromosomes find one another.
Now here's the only
exception to this rule,
the male sex chromosomes,
though they are homologous,
still find one another.
They still pair up.
So that's the only
exception to what goes on,
but all of the pairs,
the 23 pairs of chromosomes that you have,
the homologous pairs,
all of them find their respective pair.
That's the first thing.
So the question becomes, why?
Why do they pair up with one another?
Because that's crucial to this.
This is what we call the
first recombination event
of three, that you're gonna be tested on.
That you're gonna have to know.
So the first one is called crossing over.
Now has nothing to do with dying
and going to the other
side or anything like that.
But it has everything to do
with genetic recombination of alleles.
So here's what crossing over habits.
So let's look at this scenario.
Here's your father's chromosome
that he gave you in blue.
Here's your mother's chromosome
that she gave you in pink.
Notice that they have
the same types of genes,
A, B, C, D, E, and F.
And over here on the mother's
A, B, C, D, E, and F.
So that's how we represent
the same type of gene
by the same letter.
But when there's a mutation
that ultimately makes
the nucleic acid sequence
a little different,
so you get brown hair here
and blonde hair there.
Remember those are different alleles.
So we illustrate this by
capitalizing one allele
and lowercase in the other allele
to show you that alleles are different.
Well, notice in all of these cases,
all of the alleles are different,
where you have capitals all on the fathers
and lowercase on all the mothers, okay.
So that's part of the ease
of explaining crossing over.
So what ends up happening,
and this is a random process.
We don't understand exactly why
it undergoes this at
different places every time,
it does not occur in the
same place every time,
and it doesn't have to occur,
or it can occur multiple times
between homologous chromosomes.
So it's totally random as
far as we understand it.
And when we say something it's random,
it's usually because we don't understand
what ultimately is doing the selecting.
So they may cross over
at this point one time.
Next time they might cross
over here and there and there,
I mean, they can do it multiple times
where they can do it not at all.
It's just totally random.
So what is crossing over?
It's an equal exchange of genetic material
between homologous chromosomes.
Let me say that again.
Equal exchange of a genetic material
between homologous chromosomes.
So what ends up happening
what's crossing over is done
is the chromosomes that result,
will all still have the
same types of genes.
Notice you don't lose A or B.
You have A, B, C, D, E,
F on every chromosome,
but now look, what's happened.
On this chromosome,
you still have your
father's original alleles
that he gave you.
Nothing's been recombined in that one,
but on this one, you get two alleles
from your mother's chromosome
mixed with four alleles
from your father's chromosome.
That's a new combination of
alleles that gets passed on
Over here are two alleles of your father's
that got traded with
your mother's chromosome.
And then here are the rest
of your mother's alleles.
That's a new recombination.
And here, this is the original combination
that your mother gave you
when she donated her half of the genetics.
Each one of these chromatids
will end up in a different
gamete by the end.
And so, depending upon
which one of these gametes
is involved in the fertilization process
will ultimately determine what
you pass on that instance,
in that recombination
event to your offspring.
If your child gets this,
they're gonna have more
of your father's features.
If they get this,
they're gonna have more
of your mother's features.
If they get these,
they're gonna get some
combination of both.
And this happens across all 23 pairs.
The only exception to this again
is the male non-homologous
sex chromosomes.
Why do you think that crossing over
doesn't occur between those?
Because they're not homologous
and any crossing over
would result in loss of genetic material.
Now with the females, the
X chromosomes do cross over
because they are homologous
and therefore you're not
losing any genetic information.
Now, sometimes mistakes happen
and you do get crossing over
but that ends up having
some certain results
where you could actually have an XX,
but it develops into a male.
We'll talk more about
that in the next lecture.
And it's not hermaphroditism.
So we'll talk about the ultimately,
some of the things that can
go wrong and what happens.
So you don't want crossing
over to occur in here.
It only happens if it's a mistake,
but then you pair,
because you do need to still separate them
during meiosis at a later stage.
Okay, that's the first
recombination event,
which is crossing over.
So I'll ask you when that occurs.
And I'll ask you to define crossing over.
Again, here's just kind
of the summary of it all.
Homologous chromosomes exchange,
equal amounts of genetic information,
and they create new allele combinations.
And sometimes it can make mistakes
and delete some of these things out.
But again, those are mistakes.
When everything's running fine,
you don't lose any genetic information.
You don't lose the genes for hair color
or for eye color or
anything like that, okay.
So what ends up happening,
so let's say the B was for hair color
and the E was for eye color.
Let's say your dad was
or brown hair, blue eyed.
Well, in this scenario,
if you pass this gene on
or this chromosome on,
your child would inherit
the brown hair, blue eye alleles.
Let's say over here,
your mother was blonde,
it's not this simple
hair color and eye color
are more complex than
what I'm making them.
But for the sake of example,
just simplifying it.
So your mother was blonde
hair and blue eyed.
Well, here is the blonde hair allele,
but you still have your
father's brown eye allele.
So if the child inherits this,
they'll inherit the allele for
blonde hair, but brown eyes,
over here, the child if
they inherit this chromosome
would have the brown hair
allele and the blue eyes allele.
And over here, it would
be blonde hair, blue eye.
So that just kind of shows you
how things can just get mixed and matched
and why, when you pass those on,
it's a random process
of what gets passed on.
And that's why siblings look differently,
even from the same parents
because of this genetic recombination.
Okay, now let's jump to me phase one.
Another huge thing that happens here.
This is the second form of
genetic recombination, yeah.
- [Student] Real quick, do you believe
that it's just random
or is it (indistinct)
- They've shown that some parts
are more likely to cross
over than other parts.
So there is something to why
they pick certain locations,
but we're not sure how
that selection occurs.
So it's like, we know that
twins are genetically,
you know, it runs in the family,
but we're not quite
sure exactly about that.
So that's why when we say,
it's a random process,
it's usually because we don't know
what ultimately is causing it.
So it's gonna remain
a mystery for a while,
but eventually we might figure out,
hey, 90% probability that we'll
be in these parts right here
because of this and this.
And at that point, we start
understanding it less as,
you know, an ambiguous process, but say,
hey, this is most likely
what's going to happen
and make those predictions.
So it's just about a lack
of knowledge, ultimately.
All right, okay, let's see metaphase.
So a metaphase one,
remember that in mitosis,
all the chromosomes neatly
line up along the middle,
but a little something different
happens in metaphase I,
they don't all line up individually,
they line up, still paired up
with their homologous pair.
So instead of lighting
up all 46 chromosomes
along the middle, they line
up as pairs along the middle.
Now you may think, well, how
has that genetic recombination?
Because ultimately what
happens in anaphase I,
when the homologous chromosomes
get separated from one another,
how they line up here has everything to do
with genetic recombination,
we call this independent assortment.
So here's what independent assortment is.
Here's the second mode
of genetic recombination.
And that occurs during metaphase I.
So here's an example.
We're ignoring, crossing over for this,
for the sake of this example as well.
So here are the blue chromosomes,
which are your father's
half that he gave you.
Here are your mother's chromosomes,
which are the hat that she gave you.
As these homologous pairs
line up along the middle.
They don't have to line up
the same way every time.
So on this scenario,
all the fathers are lined up on one side,
all the mothers are
line up on another side.
That's one possibility.
Another possibility is
these two chromosomes flip.
This one goes to this side,
this one goes to that side.
Now, when these homologous
chromosomes go into this side
and these go into that side,
you're gonna get these alleles
from your father's chromosomes.
And these alleles from
your mother's chromosomes.
So that's another form
of genetic recombination.
Now this is not crossing over.
This is just what chromosomes
from what parents do you inherit?
Do you inherit all of
what your father gave you?
I mean, do you pass on
what your father gave you?
Do you pass on what your mother gave you?
That's one scenario.
And notice there are all
sorts of different ways
in which these could flip
to one side or another.
Now, why is it called
independent assortment?
Is because when these two flip
it doesn't influence whether
any of the other homologous pairs flip.
So it doesn't matter which
side they line up on.
It is completely random,
again, of a which side,
the homologous pairs line up on.
So, in this scenario, this
cell that results from it
would have 2/3 of the father's genetics
and a 1/3 of the mothers
and so on and so forth.
So that's independent assortment
crossing over is able to assort alleles
between homologous chromosomes.
Independent assortment,
mixes up everything else.
Now, again, you don't lose
any genetic information.
Every person is going to,
or every gamete that's produced
is going to have all 23 chromosomes.
It just depends upon what combination
that your parents gave
you, do you pass on?
Do you pass on 2/3 of your fathers
and 1/3 of your mother's?
1/2 and 1/2 most of your fathers.
That's why you start
seeing children that say,
he looks more like your
side of the family,
or more like your side of the family.
So that's where independent assortment
is genetic recombination.
You're not mixing matching alleles
between homologous chromosomes,
you are literally scrambling
up all of the homologous pairs
that your parents gave you
and passing on a different
combination of those.
All right, now, anaphase I.
This is where homologous
chromosomes separate,
but notice the sister chromatids
still remain attached.
So during the first stage of meiosis,
you don't have any separation
of the sister chromatids,
remember they're attached
together by a centromere.
So the sister chromatids remained attached
during meiosis I.
All you do is separate
the homologous pairs
from one another.
So that's anaphase I,
homologous chromosomes separate.
Now remember this one
too, because later on,
when we talk about a process
called nondisjunction,
which is a problem that
occurs later on in life,
this is gonna come up again.
Now it's not a mode of recombination,
it just separates the homologous pairs,
but it is important for certain issues
that we deal with later on in life.
Now, do you have a phase
one in cytokinesis?
Pretty much.
Again, we get into some differences here,
depending upon whether
you're a male or female.
Now, if you're a male,
if you produce sperm,
cytokinesis will always occur, okay.
One cell will become four separate sperm,
but with females, that's not the case.
I'll talk a little bit more
later on about how that works,
but for now we have
telophase I in cytokinesis.
So same thing, once you
separate the homologous pairs,
the mitotic spindle breaks down
a nuclear envelope reforms.
However, the chromosomes
don't unwind at this point.
Why do you think?
- [Student] They're disapproved.
- [Lecturer] They're gonna go through
another round of meiosis,
there's no point in unwinding,
just to ravel back up again.
So yeah, that doesn't
happen during telophase I,
you don't get unraveling of the chromatin.
because there's still
one more round to go.
Now you do get separation
of the cytoplasm,
which we call cytokinesis,
this is for males.
This doesn't happen in females, okay.
Now meiosis II is pretty
much the same as mitosis.
So I'm just gonna kind of breeze through
this real quickly.
You have almost like no
questions on meiosis II,
but there is some part
that you will have to know
for one of the concepts, like I said.
So let me just go through this.
So prophase II, remember the
chromatids already condensed.
So the only two things that occurred
are the nuclear envelope breakdown
and the mitotic spindle forms.
Now notice you've already,
you're starting with two cells here
for male sperm production, okay.
Now metaphase II chromosomes lined up,
but notice these cells only
have half of the genetics
because the homologous
pairs have been split up.
So instead of starting out
with 46 chromosomes here,
you have 23 chromosomes
in each one of these.
So the 23 chromosomes
line up along the middle.
Now during anaphase II,
this is when the sister
chromatids separate,
but notice too, that not
even the sister chromatids
are exactly identical to one another, why?
What has happened previously?
Not independent assortment.
- Crossing over.
- Crossing over.
Because of crossing over
you've had some genetic
recombination of the alleles,
so not even sister chromatids
are exactly the same anymore.
That means that when these
two cells separate here
and these two separate here,
each of these are going to
have a unique assortment
of alleles, no two gametes
are ever going to be the same.
Okay, so anaphase II,
sister chromatids
separate from one another
and telophase II, same thing.
Cytokinesis, myotonic,
spindle breaks down,
nuclear envelope reforms.
And now you have four
haploid, non identical,
meaning the allele combinations are unique
to each of these gametes.
I think on my quiz, I say
genetically dissimilar
is the phrasing that I use.
So they're genetically dissimilar, why?
Because of all of the crossing over
and independent assortment
that occurred back in meiosis I, okay.
Now, I told you that there were
three forms of genetic recombination.
Anybody wanna guess what the third is?
So we have crossing over, we
have independent assortment.
The last one is fertilization itself,
the sperm and the egg coming together.
How is that genetic recombination?
- [Student] Two people.
- [Lecturer] Two
different people's genetic
coming and fusing into one.
So that's the third,
it happens obviously
at the very, very end.
All of these combined in the human genome
can produce over 70 trillion
unique combinations.
So unlike what X files would tell you,
you don't have a twin over in China
or other planetary systems
that may have humans on them as well.
It's a very unlikely event.
Now, if you've watched "Jupiter Rising",
then it can happen over
enough, no, anyway,
I'm just kind of give a spoiler, but oh.
It's a good movie anyway, okay.
So those are the three forms
of genetic recombination
crossing over independent
assortment fertilization, okay.
So I'll ask you about these three,
as well as being able to describe them.
Now, let me give you an overview
of the difference between
meiosis in males and female,
not the timing, but the end result,
because like I said, there
are some unique differences
that don't occur in females
that do occur in males.
The overall process of
how the crossing over
independent assortment
and all of that happens,
it's the same between males and females.
The difference is what
happens to the actual cell
during meiosis I and II.
So for males, you get
cytokinesis every time.
So you get cytokinesis after meiosis I,
and you get cytokinesis after meiosis II.
In the end, for every one germline cell,
which we call a spermatocyte
that undergoes meiosis,
you're gonna get four sperm,
but in females, one oocyte,
that's the germline cell
for females or the eggs,
one oocyte remains one oocyte.
You're like, well, how does that happen,
when you have all of the same things
that happened in the males?
Well, here's the thing.
When these nuclei formed during meiosis I,
instead of undergoing cytokinesis,
the egg simply throws
out one of the nuclei.
So one of them will remain with the egg,
the other one gets ejected out of the egg,
so no cytokinesis occurs.
Now, whichever one gets left in there,
will go through the
second stage of meiosis.
And again, it'll choose one of the two
and the other one will be
thrown out at the very end.
So in the end, one egg
will always remain one egg
during meiosis, you get no cytokinesis,
but you get the same process
of what's happening with
the genetic recombination.
So women will produce four nuclei,
but there will only be one egg.
Now, the reason for this is because
there are proteins and other
things in the cytoplasm
that you just can't split up, okay.
So the egg needs to remain intact
with all of what we call
maternal components.
And a lot of those are
forgetting the metabolism started
and infusing the nuclei
and other things like that.
So that egg all of what starts in this egg
needs to remain in there.
The only thing the cell doesn't need
is the extra nuclei and
it just retains one.
Here's the other thing
I was telling you about
when a woman ovulates the egg, the oocyte,
is actually right about metaphase II,
and only when the sperm
fertilizes the egg,
will it finish meiosis II
and get rid of second
nuclei from meiosis II.
So you essentially have the
sperm coming in one end,
once it comes in, this will finish off
and throw out the egg or throw
up nucleus from the other,
and then the two nuclei
fuse and you have the zygote
or that stem cell that becomes you.
So that's the thing about meiosis.
As I mentioned, meiosis begins
before a woman's even born
continues on through their life
and is about meiosis II
when the egg ovulates
and or halfway through meiosis II,
and will finish it unless
fertilization occurs.
If it doesn't occur, then
it just stays at meiosis II,
The egg goes through and you
don't get pregnant, okay.
Now twins, monozygotic versus dizygotic.
All right, so monozygotic
or identical twins,
the reason why they're identical
is because they come from a
single fertilization event.
If you remember, I told you about how
after several cell
divisions you end up having
what we call a blastocyst,
where it has all of these
cells on the outside
that will become the placenta
and the amniotic sac.
And they have these cells inside
called the inner cell mass.
That's what becomes you.
That group of cells right there.
Well, these cells in the earliest stages
have the potential to become
every cell in the human body.
And we're still trying to
figure out all the reasons why,
but on occasion, the
cell mass splits in two.
Now, because all of those cells
came about through the process of mitosis.
The genetics on all of the
cells are exactly the same,
which is why identical twins
have the same genetics, right?
So that's why we call it monozygotic
is 'cause it's from a
single fertilization event
where but then it splits
into two inner cell masses
and you ended up having identical twins.
Now dizygotic twins are two
separate fertilization events.
When a woman obviates
two ways instead of one,
And that happens quite
readily when you have hormones
or other drugs that will stimulate
that for someone who maybe
is going through a hard time,
getting pregnant or whatnot.
Ultimately, if you release two eggs,
there's more than enough
sperm in every ejaculation.
There's like 10 million sperm or whatnot
to fertilize both of the eggs.
So it's not a question of how much sperm
is a question on how many
eggs the woman ovulates.
So if two eggs ovulate,
each one of those have gone
through their own meiosis.
And the sperm obviously have gone through
in their own meiosis as well.
Which means that they're
gonna be no different
than any two siblings,
brother and brother,
or sister and sister
or brother and sister.
This is why fraternal twins
can be different sexes
because (indistinct) fertilize
it with the X chromosome
or the Y chromosome.
Here for identical twins
must be the same sex
because it's a single fertilization event.
You're not getting two different
sets of sex chromosomes.
Now I'm sure there's an
exception somewhere in the world
of a one in a trillion chance
that this happens and whatnot,
but I won't go into how that occurs, so.
Now problems do arise,
especially as you get older.
Now they occur more often in women.
Because if you remember,
I told you that women make all
of the oocytes they'll have
for their entire life
before they're even born.
Whereas men won't even
start to use their gametes,
their germline cells to create the gametes
until they reach sexual maturity.
So there's this offset
of at least a decade,
maybe a little bit more
in which women start,
using the eggs versus men producing them.
As such, we usually track
this problem in women
because it's more likely
that the man is about the
same age as the woman.
You're not having a guy in his 70s,
having kids with someone in
her 20s, although it happens.
So the reason why I say this
is because this occurs in men as well,
but it doesn't occur until
we're in our 60s or 70s
with as much frequency
as what it occurs in
women's in their 40s, okay.
It's called nondisjunction.
Now, before we get to nondisjunction,
let me explain one major issue
that sometimes happens, but
it doesn't come to term.
In fact, this is how almost
20% of miscarriages occur
because of this problem.
And it's called polyploidy.
Polyploidy is when more than one sperm
typically fertilizes the egg.
Now there are usually mechanisms in place
in the egg to prevent
what we call polyspermy,
because what ends up happening
is you ended up getting three
of every chromosome type instead of two.
And the cell needs to have
exactly 46 to really work,
any more or any less chromosomes in this,
and there usually are major problems,
if not fatal problems in the
development of the embryo,
there are always exceptions,
but the general rule is any deviation
from the 23 sets of chromosomes
that we have in ourselves
results in huge problems,
resulting in miscarriage.
So an extra set of all of your
chromosomes is a big problem.
Now, sometimes plants can
actually overcome this issue,
but for animals, it just doesn't happen.
Okay, so that's polyploidy.
However, what we're gonna focus on
is what's called nondisjunction.
Mainly because this is
where the exceptions
to the rule come into play.
So what is nondisjunction?
Well, it can occur in one of two phases.
It can occur in either
anaphase I or anaphase II.
And this is why I told you
that these two processes
will come into play.
So what is nondisjunction?
Well, remember anaphase
I or in anaphase II,
you get the separation
of homologous chromosomes
or the separation of
sister chromatids, okay,
respectively, anaphase I and anaphase II.
As the cells get older,
it becomes more likely that
when this process of anaphase
I occurs or anaphase II,
that the homologous pairs don't separate.
And so when that happens or in anaphase II
it would be the sister
chromatids don't separate,
and I'll show you a sample rafter that,
when that happens, the
resulting gametes at the end,
end up having an extra
or one less chromosome.
So instead of having 23 chromosomes,
they end up having 24 chromosomes,
or they have 22 chromosomes,
one more, one less.
So here's an example of what would happen
if it occurred in anaphase I.
Let's take now,
it's ignoring all of the
other chromosome pairs.
So it doesn't mean there's
nothing in this cell.
It just means with this chromosome pair,
none of them went over to this cell, okay.
But all of the others separated just fine.
So that's what this is illustrating.
So what is happening is
when these chromosomes
go into one cell and they
don't split as they should,
that's called nondisjunction.
Then when these split during anaphase II,
half of the gametes, like let's
say these are the oocytes,
because they're usually
what we track is in women.
Half of the gametes will
have one extra chromosome.
And over here, half of the gametes
will have one less chromosome.
Now in almost all cases, this is fatal.
If the embryo cannot survive,
let's look at another example.
Here in anaphase I, you
get separation just fine,
okay, no problem.
However, in this one right here,
the sister chromatids
don't separate properly
during anaphase II.
And so what you get is
this one has one less.
This one has one more.
These two, they're fine.
So you only have half of the
gametes affected by this,
if it occurs in the anaphase II.
Whenever it occurs,
anaphase I or anaphase II,
if you have one more or one less,
like I said it's almost always fatal.
Now there are exceptions.
For example, chromosome number 21.
It's one of the smallest
chromosomes that there is,
and that's the reason why this
has the highest survivability
rate about 85% survivability.
And this is what results in Down syndrome.
Down syndrome is when the
child has a normal set
of all chromosome pairs,
but they end up having an
extra chromosome 21, okay.
So what ends up happening is
this is why women over 40,
they actually have 10 times the chance
of having a child with Down
syndrome than women under 40.
Now it's not that you reached 40,
and all of a sudden it's 10 times,
they just split up the groups.
And when women are in their
last few years of fertilization,
they are 10 times more likely
to have this problem occur
in their production of the gametes.
Now this can occur in
males as well, like I said,
but usually not until a
couple of decades later.
So if it happens in males as well,
but we mostly look at females.
Now, there are others
that are less common,
but they only had like a 5% survival rate.
And these are, excuse
me, trisomy 18 and 13.
So if we go back to the karyotype,
that would be an extra 13 and an extra 18,
but those have a very
low survivability rate.
And all of the others and
the autosomes that we see,
pretty much the fetus supports.
Now that brings us to another concept.
Trisomy, what does trisomy mean?
Trisomy means you have
three of a homologous group
rather than two, okay.
Monosomy means you are missing one.
So in the case of here, this is trisomy,
but when one cell loses a chromatid,
then you ended up having
what we call monosomy,
where you only have one
of the chromosome pairs
instead of both of them.
Okay, now this is for the autosomes
what's interesting is it
is actually fairly likely
for the embryo to survive
if it occurs in the sex chromosomes.
So the sex chromosomes are less stringent
about having an extra chromosome.
In fact, in some cases it
makes no difference whatsoever.
For example, Jacob's syndrome,
about 17% of males have
an extra Y chromosome
and it doesn't do anything.
It doesn't make you more
aggressive as they once thought,
because really they
only saw this phenomena
when they were taking
the DNA of men in jail.
And they notice a very high percentage.
Well, then they started sampling
the general populace as a whole
of men that weren't in jail
and found the same statistics.
So it led them to believe at first
that males with an extra Y chromosome
tend to be more aggressive,
committed crimes
and other stuff like
that and that's not true.
So men can have an extra Y
chromosome and be just fine, why?
I mean, look how small that thing is.
It's just got a few hundred genes on it.
And most of the time,
an extra one of those really
won't cause any problems.
Now, Kleinfelter syndrome, XXY.
This is still a male.
And in a lot of cases,
again, they're fully normal.
Now this is not a
hermaphrodite. (chuckles)
It's not, it doesn't even lead to that.
There are situations
where men can have underdeveloped testes
or slight breast development,
but that's not hermaphroditism.
So I'll explain hermaphroditism later.
But XXY, in fact, a good portion of men
can have an extra X chromosome
and be just fine as well.
So it all depends,
or some dynamics during
the development process
that really make or break whether or not,
you're gonna have a
certain problems or not.
Now Triple X syndrome is a female
because there's no Y chromosome.
So pretty much you get a Y chromosome,
you're gonna develop male,
that's just how it is.
But if there's no Y
chromosome, you develop female.
So three X chromosomes,
there are certain abnormalities with this.
There are certain genetic defects
from infertility to some other aspects,
but it's actually fairly common too,
to have three X chromosomes
and the survivability is really high.
Now, the only case of
monosomy that we've ever seen,
remember monosomy means one
of the homologous pairs,
is what we call Turner syndrome.
This is where you have a female
with just one X chromosome.
Now it has a 1% survivability rate,
and they're always infertile.
So it's not very common.
This is the only case though,
of Montessori that we've ever seen,
where the female survives
to be born and mature.
Okay, it's always infertile.
There's always some type
of mental retardation
and developmental retardation,
especially in the sex
characteristics and whatnot.
So that's just that.
Now with men, you've got
to have an X chromosome.
There are over a thousand genes on this.
That's 5% of your genome.
You don't have an X
chromosome, you don't develop.
So there's never been
a case of monosomy male
with no X chromosome, okay.
So these are just some of
the examples of trisomy,
where you have three of a chromosome pair,
in this case, the same chromosomes
and this is the only case of monosomy.
So really Down syndrome
is the most prevalent case
of what we call autosomal nondisjunction,
these are in the non-sex
chromosomes, okay.
So that's one of those
that I'll test you on
as far as what trisomy
21 causes and what not,
and what trisomy is, okay.
But you'll also have another question
on defining nondisjunction.
