AP Biology Lab 3: Mitosis and Meiosis


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.

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