Hi. It's Mr. Andersen and today
I'm going to talk about meiosis. Meiosis is the process by which gametes are formed. And
so in humans that's going to be sperm and egg. When the first microscopes were made,
people looked at sperm under the microscope and they assumed that there were tiny little
humans tucked away in the head of each sperm. We know that this is ridiculous. And what's
really tucked away in the head of a sperm is going to be nuclear material or chromosomes
that are transferred from males to females. And so the one thing I want to start with
today is the importance of meiosis. And so this is a picture of my family when I was
a little kid. And so this is my dad. And this is my mom. These are my siblings. And this
right here is me as a little kid. And if you look at this family the thing that will jump
out right away is that my mom has red hair. And none of the rest of us do. And we all
look different than our parents. And so that's the function of meiosis. That's the function
of sex. It's to make sure that your offspring are different than you. Because if we're all
the same, then we're all susceptible to the same diseases. And so before we can talk too
much about meiosis we should get this out of the way. What is diploid and what is haploid.
Diploid essentially means that you have two complete sets of every gene. Or two complete
sets of the chromosomes. So in humans we have 46 chromosomes. That means we have 23 pair.
You can see this right here as a karyotype. And if it was a male it would be one X and
one Y chromosome. And if it's a female it has two X chromosomes. But basically you have
two copies of chromosome 1. Two copies of chromosome 2. Two copies of chromosome 3.
And so if we only had 3 chromosomes, this is basically what we'd look like. And our
sperm and egg would be haploid. They'd have one set of all the chromosomes. And then our
somatic cells or all the other cells inside our body are going to be diploid. Or they're
going to have two complete sets of chromosomes. And so basically life starts when haploid
cells combine. They form a diploid zygote and that eventually forms you. And so what
I'm really talking about is the human life cycle. In other words through a process called
meiosis we're going to be make gametes. Those gametes will eventually combine back together
again through a process called fertilization. And that's going to make something called
a zygote or a fertilized egg. Now that one egg is going to quickly divide into two and
four and eight and sixteen and 32. So through the process of mitosis that's going to form
a new organism again. And those new organisms are going to produce gametes through a process
called mitosis or meiosis and this whole thing goes over and over again. And so if you look
on this diagram, everything up here in the gray is going to be haploid. And everything
down here is going to be diploid. So the zygote is diploid. You are diploid. And the only
things that aren't are going to be the gametes that are formed. And so cartoon style, this
is what it looks like. Through meiosis males make sperm, females make eggs and those sperm
are going to fertilize an egg to make a zygote. And then through mitosis it makes us again.
Now know this that not all organisms are going to have the same life cycle. Sometimes when
you look at, for example, a liverwort, your mostly going to see haploid organisms. And
you're not going to see diploid organisms. But generally in humans we're diploid and
so are zygotes. And so before we get to meiosis, let's talk about mitosis. And we're going
to go real simple to start. And so basically think of this as the nuclei in a very simple
cell. In this one we're going to say 2n=2 because it's a diploid cell. In other words
it has two copies of it's one chromosome. You can see that this one's pink, because
it comes from mom. And this one we'll say is blue because it comes from dad. And so
as we go through the mitotic divisions, basically what happens is those two chromosomes are
going to make copies of themselves. So we call those sister chromatids. And they'd be
connected here in the middle in something called a centromere. Next thing that will
happen is that they're going to meet together at the metaphase plate. So they're going to
line up along the middle. What you would really get is spindle fibers that are going to attach
to this one and are going to attach over to this one. And likewise you're going to have
spindle fibers down here. Remember in us we're going to have a lot more chromosomes in this.
But basically what happens is they're pulled to either side in mitosis. That would be anaphase.
So what did we end up with? Well this would be one nuclei where 2n=2 and this is another
nuclei where 2n=2. And these two nuclei are exactly the same. And that's the function
of mitosis. We want to make exact copies of the cells. So when you go from that zygote
to the organism all the DNA in all the cells of your body is identical. Now we're going
to go through meiosis. So meiosis starts the exact same way. We start with a diploid cell
where 2n=2. One from mom. One from dad. We copy those chromosomes. So during the S phase
of interface we'll make exact copies. So this looks exactly the same as mitosis, but now
it gets a little weird. So basically what will happen is these two, we call those homologous
chromosomes are going to come together. They're going to meet in the middle. And so during
prophase they'll wind around each other. And they'll wind around each other so tight that
something's going to happen here. Watch really carefully on these chromosomes. So basically
what's going to happen is little bits of the female chromosome are going to switch places
with the male. In other words this little blue portion came from this one and this little
pink one came from this portion. We call this crossing over. In other words since they're
all the same length, since they have the same genes, although they may have different copies
of the genes, it's really easy for them to bond together and form new chromosomes. So
what happens next? Well they've met in the middle. There would be a spindle fiber here.
And so basically they'll pull apart. And as they pull apart those chromosomes are going
to be pulled in either direction. And then they're going to pull apart again. And so
the big difference that happened right there, I don't know if you caught it, is that in
mitosis there's only one division but in meiosis there's two divisions. And so what we've done
is we've essentially made four nuclei. Those nuclei are haploid now. Remember we started
with 2n=2 and now we just have n=1. And so each of these are a nuclei that would have
a different chromosome in it. And so each of these in males would form a brand new sperm.
In females it's a little different. In males you make sperm your whole life from the day
you're born to the day you die. From the day you go through puberty to the day you die.
You're going to be making sperm over and over again. But in females you've actually undergone
most of this meiotic process before you're even born. Males make trillions of sperm during
their life time. And they make them their whole life. Females just make a few eggs before
they're born. And they lavish them with attention. In females one of these nuclei will be the
chosen one and that will become the one egg. And the other ones don't really become anything.
And so if we do this is a quick little diagram where, basically this would be a cell where
it is 2n=4. So we're going to start where with 4 chromosomes. It's going to copy itself.
There's going to be crossing over. There's going to be a division and then another division.
And so in this case n=2. But the big highlights here is that we've got crossing over right
here where they're actually switching bits of the chromosome. You can see that there's
one division during meiosis 1 and then another division during meiosis 2. And so basically
we have four cells. In males each of these would become a sperm. In females one becomes
an egg. Now if you think about that, it doesn't look like we get much variety because we're
only getting four cells out of that. But the reason you are special, the reason you are
the only one, in the words of Barney the dinosaur, you're the only one like you is because of
these three processes right here. First one in independent assortment. Basically what
happens in independent assortment, well, if we look at these chromosomes they could have
lined up like this with the blue on top. Or the could have lined up with the blue on the
bottom. And so the odds of blue being on the top when they line up would be 1 in 2. And
the odds of blue on the bottom would be 1 in 2. And so you might think that's not much
variability at all. But if you think about it, how many chromosomes did we have? Do you
remember? We have 23. And so actually the possibilities that you can get through independent
assortment is 2 to the 23rd. So we can get billions of possibilities just by having each
of those chromosomes assort independently. And that's why we get just in a simple dihybrid
cross in the peas, we're going to get something that looks like this. Next thing is going
to be crossing over. Remember those chromosomes are going to cross over. So there will be
a coming together here, we call that synapsis. And basically what you get are chromosomes
that have never existed before. And then the last thing that happens is random fertilization.
That one sperm has to fertilize that one egg. And so the odds of that happening, with a
couple, the odds of two people with separate sperm and separate eggs creating organisms
that look exactly the same or people that look the same is almost impossible. It's 1
in 70 trillion times that that would happen. Now you could have a fertilized egg that divides
in half and have an identical twin. But basically all siblings are going to look different because
of independent assortment, crossing over and random fertilization. But this whole thing
is built on the process of meiosis. It makes offspring that are different. That gives them
protection against diseases and it's really the stuff of life. And I hope that's helpful.
