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.