Voiceover: So let's talk a little bit about
how we all came into being.
What we see right over here, this is a picture
of a sperm cell fusing with an egg cell.
So that's a sperm cell and this is an egg cell
or we could call this an ovum.
And even this scene depicted right over here,
this is the end of an epic competition because
this sperm cell is one of one of two to three
hundred million that is vying for this ovum.
So there's two to three hundred million of these characters
and they're all vying for this ovum and the one
that you see that's about to fuse for it,
this is the winner of this incredibly -
remember two to three hundred,
200 million to 300 million sperm are trying to get here
so this is a major victory and to some degree
we should all feel pretty good about ourselves
because we are all the by-product of that
one in 200 to 300 million sperm cells
that won this race getting to our mother's ovum.
So the sperm cell came from our father and the egg cell,
this is all happening inside of our mothers,
the egg cell is from our mother.
Now, once this happens,
let's talk a little bit about the terminology.
So once these two fuse, or the process of them fusing,
we call that fertilization.
Fertilization.
And it produces a cell that then differentiates
into all of the cells of our body,
so you can imagine that this is an important process.
So let's make sure that we understand the different
terminology, the different words for the different things
that are acting in this process.
So each of these sex cells, I guess we could say,
the sperm cell and the ovum, these are each called gametes.
So this right over here is a gamete
and the ovum is a gamete,
the egg cell is also a gamete.
And as we'll see, each gamete has half the number
of chromosomes as your body cells or most of
the somatic cells of your body so outside of your sex cells
that might be in your ovaries or your testes,
depending on whether you're male or female,
these have half the number
so let's dig a little bit deeper into what I mean there.
So let's just do a blow up
of this sperm cell right over here,
so a blow up of a sperm cell and I'm not
going to draw it to scale, you see the sperm cell
is much smaller than the egg cell but just
to get a sense, so let me draw the nucleus
of this sperm cell, so just like that.
If we're talking about a human being, and I'm assuming
you are a human being, so that might be
of interest to you, this will have 23
chromosomes from your father so let's do them.
One, two, three, four, five, six, seven, eight, nine,
10, 11, 12, 13, 14, 15,
16, 17, 18, 19, 20, 21, 22
and for the 23rd one, that's going to be
your sex-determining chromosome
so if your father contributes an x,
you are going to be female,
if your father contributes a y,
you are going to be male.
So these are the chromosomes in
the male gamete or I guess I should say the gamete
that your father's contributing, the sperm.
So this is a gamete right over here and that's going to
fuse with the egg, the ovum that your mother is contributing
and once again, I'm not drawing that to scale.
So this is the egg, and let me draw it's nucleus.
So that's it's nucleus, once again
none of this is drawn to scale.
And your mother is also going to contribute 23 chromosomes.
So one, two, three, four, five, six, seven, eight, nine,
10, 11, 12, 13, 14, 15,
16, 17, 18, 19, 20, 21, 22
and then she will contribute an x chromosome
for the sex determining so your sex determining
chromosomes are going to be xy, you're going to be male,
if this was xx, you're going to be female
so this is also a gamete here.
So a gamete is the general term for either a sperm or an egg.
Now once these two things are fused, what do we have?
Once they're fused, then we're going to have
you could say a fertilized egg
but we are going to call that a zygote
so let me draw that.
I'm going to do this in a new color,
and I'm running out of space and I want this all to fit
on the same screen so I'll draw it not quite at scale
and so let me draw the nucleus of the zygote,
I'm going to make the nucleus fairly large so that we can
focus on the chromosomes in it,
once again none of this is drawn to scale.
So you're going to have the 23 chromosomes
from your father, so let me do that.
One, two, three, four, five, six, seven, eight, nine,
10, 11, 12, 13, 14, 15,
16, 17, 18, 19, 20, 21, 22 and 23,
and then the 23 chromosomes from your mother.
One, two, three, four, five, six, seven, eight, nine,
10, 11, 12, 13, 14, 15,
16, 17, 18, 19, 20, 21, 22 and 23
so you got that x chromosome from your mother.
And as you might have notice, I've drawn them in pairs
so you now have a total, let me make it clear,
you have 23 chromosomes here,
23 chromosomes in the sperm,
you have 23 chromosomes in the egg
and now you have 46 chromosomes in the fertilized egg,
46 chromosomes, and now that we have
a full contingent of chromosomes
and then this cell can now keep replicating,
keep splitting and differentiating into all of what
makes you, you, we call this right over here,
we call this a zygote.
So one way to think about it, the gametes are the sex cells
that have half the number of chromosomes
and the zygote is the cell that's now ready to differentiate
into an actual organism that has double the number
or that has a full contingency of chromosomes,
that has 46 chromosomes, and you see that
I've made them in pairs and these pairs,
we call these homologous pairs
and in each of these pairs,
this is a pair of homologous chromosomes.
So what does that mean?
Well that means that in general, these two chromosomes,
you got one from your father, one from your mother,
they code for the same things, they code for the same
proteins but there are different variants of how they code
for those proteins, those traits that you have
so gross oversimplification is,
let's say that there is a gene on,
that one from your father that helps code for hair color
well there would be a similar,
there would be another variant of that gene
on the chromosome from your mother
that helps code for hair color as well.
So these are homologous chromosomes,
these two chromosomes code, in general,
for the same things and so the zygote now has,
you could say it has 46 chromosomes or you could say
it has 23 pairs of homologous chromosomes.
And this is, once again, this is the case for human beings.
If we're talking about some other species, instead of
23 pairs of homologous chromosomes or 46 chromosomes
in total, you might be talking about 10 pairs of
homologous chromosomes with 20 chromosomes in general.
Now to help biologists, to help clarify when they're
talking about the number of chromosomes for
a given species, they introduce two words,
haploid and diploid.
And haploid is referring to when you have half
the full contingency of chromosomes.
So for human beings, the haploid number is 23.
So, this is the haploid number, it is 23.
For another species it would be something else
and haploid is based on the prefix "hapl",
that's the prefix for single, so you have a single member
I guess you could think of it,
of each of the pairs and now you have both
of each pair, you have both chromosomes in each
pair or you have the full contingency and this 46 chromosomes,
this is called the diploid number for humans.
The diploid number right over here.
And when people talk in general, and we will speak in general
when we start talking about mitosis and meiosis
for a given species they will refer to the haploid number,
they will refer to the haploid number as n chromosomes
and they will refer to the diploid number as just twice that,
as two n chromosomes.
So hopefully this gets you familiar with some of the vocabulary
around fertilization and haploid and diploid
and zygotes and gametes and also makes you feel
a little bit better about yourself that, just to exist,
at least half of your chromosomes had to win
an incredibly competitive race.