Mitosis, meiosis and sexual reproduction - Biology, Class 11 Video Lecture - Class 11 | Best Video for Class 11

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.