Hi. It's Mr. Andersen. Welcome AP Biology Lab 3. This one is on mitosis and meiosis.
In this mitosis portion what we're going to do is we're going to look at cells in a root.
In this case onion root and see them actively dividing and figure out how much time they
spend in each of the different phases of mitosis. And then meiosis, we're going to be looking
at ascospores produces by a specific type of fungus called sodaria. And we're going
to figure out percent of cross-over. And thereby we can figure out how far the genes are found
apart on the chromosome. And so first let's talk about what mitosis and meiosis are. Mitosis
is basically division of the nucleus. But it's equal division of the nucleus. And so
let's say that we have a typical diploid cell. In us we'd have 46 chromosomes in here. But
in this one they only have 2n=4. So basically in mitosis what we're going to do is we're
going to duplicate the DNA. They're going to line-up in the middle during metaphase.
They'll pull apart during anaphase. And then we essentially have two cells at the end that
are both diploid and their both identical to this first cell. And so mitosis is the
way during the cell cycle that we produce identical cells. That's how you went form
one cell to the billions and trillions of cells that are inside your body right now.
What's meiosis then? Meiosis is basically, starts the same way. We start with a diploid
cell. We copy the DNA. But then instead of just splitting in half once, it splits in
half twice. We also have this crossing over that occurs. And so essentially instead of
getting duplicate cells, we get haploid cells. They have half the amount of genetic information.
And then they have chromosomes that have never really existed before. Those chromosomes are
a combinations of the chromosomes of the parent cell. And so that's meiosis. And so let's
start with mitosis. In this lab you could use either meristems. Those are going to be
indeterminate parts of a plant. In other words you could think of them almost like plant
stem cells. They're cells that haven't decided what they're going to become. Or blastulas.
Blastula is going to be a ball of cells. We either use whitefish blastula. But I have
more luck just using the onion root. So basically how does a root grow? There's an apical root
meristem down here. And basically it's going to copy those cells over and over again. And
the root is going to get longer and longer and longer as those cells divide. And so basically
what you can do is you can look at the cells. And this is just a diagram. We can look at
the cells. What phase they're in. Count the number of cells and all the phases that they're
in. We could figure out how much time they spend in each of those different phases. Some
kids are confused at this. They think that somehow the cells are growing as they watch
them. Just think of it this way. Let's say we were to just take a snap shot of every
kid in my high school. So there's like 1900 kids in our high school. If I were to take
a snap shot of them right now and count the number of them who are sleeping or texting
or taking notes or doing a lab or whatever. Basically if I counted the percent of those
who are doing each of those activities then I could kind of extrapolate and say that's
how much time during the day, class day, they're spending doing each of those activities. So
basically what you do in this, use a partner is you go through and you count the phases.
So this one right here would be an interphase. An interphase. An interphase. An interphase.
Interphase. And prophase. And interphase. And this one right here would be, this one
looks like almost getting to, I would say anaphase maybe. Anaphase here. This would
be a prophase here. This would be a metaphase here. So basically what they do is they go
through. Count hundreds of cells. They figure out how much time they're spending in each
of those. We then get the classroom data set as well. Figure out the percent of the time
they're spending in there. And then we just make an old school pie chart. So in this one
we had about 73.8 percent of the time is spent in interphase. So we counted thousands of
cells. And so 73.8 percent of the time they were in interphase. That means they're spending
about that much time during the course of a day in interphase. Next would be prophase.
Metaphase. Anaphase. Telophase. And so basically in this lab you can figure out how much time
they're spending in each of those. And so this would be the cell cycle. And you'll see
this as you study mitosis and meiosis. This, it almost looks like a washing machine where
they're in this phase. And then in this phase. And then in this phase. We put cytokinesis
kind of right in here. And so basically in this lab you're able to see how much time
they're spending in each of those different phases. That's just the mitosis portion. Now
the meiosis portion what we're using is we're using a fungus called sodaria. So basically
there are two different phenotypes. There's the dark. And then the tan. You let them grow.
And then you're going to grab spores from this area where those two will come together.
Or where those two are going to interact. Basically most sodaria are going to be this
dark color. There going to make these dark colored spores like this. And then there's
a mutant which is going to be the tan. They're going to make the tan spores. But where they
grow together you'll get spores that are a combination of the two. So the chromosomes
are coming together. If there is no cross-over between those, two they're going to line themselves
in this 4 to 4 or 4 to 4 pattern. That means no cross-over existed. But if there is crossing-over
that exists then you're going to get a 2 to 2 to 2 to 2, or a 2 to 4 to 2, or 2 to 4 to
2. And so if see ascospores that look like this that means crossing-over has occurred.
So the cool thing about this lab is that basically you can figure out frequency of cross-over.
You take those spores. Put them underneath glass. Push on it with your finger and you're
going to kind of pop out all of these spores. It's going to look something like this. And
so now you can go through and you can count the number of spores that are crossing-over
and not crossing over. So if we were to look at this one, we're going to say not crossing-over,
not crossing-over, not crossing-over. But this one right here would be cross-over. So
it's going to be cross-over in that one. It's going to cross-over in that one. There's going
to be cross-over in that one. So you can go through the whole thing and figure out the
frequency of cross-over. I'm not going to count all of these. But let's say that roughly
50 percent of them have cross-over. So 50 percent cross-over. Since when they produce
these spores they'll double them. Basically what we have to do is we have to take that
number, 50 percent. We have to divide it by 2 and that's going to tell us the number of
map units. And so in this case we'd have about 25 map units. What does that mean? Well if
a chromosome looks like this. And there's a centromere right here. Basically it means
that those two genes are going to be 25 map units apart. Something like that. And remember
all the way is going to be 50 map units on the whole of the thing. And so if that makes
no sense, make sure you look at frequency of cross-over. Especially the work of Thomas
Hunt-Morgan. Maybe it makes a little bit more sense. But I hope that's helpful.