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Biology: Disjunction and Meiosis II

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  • Type: Video Tutorial
  • Length: 10:57
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 118 MB
  • Posted: 02/10/2009

This lesson is part of the following series:

Biology Course (390 lessons, $198.00)
Biology Review (19 lessons, $27.72)
Biology: Cell Reproduction - Mitosis and Meiosis (16 lessons, $23.76)
Biology: Meiosis (5 lessons, $8.91)

In this lesson, Professor Wolfe starts out with an overview of Meiosis and then discusses and explains the processes of both Meiosis I and Meiosis II. During the stages of meiosis I, homologous chromosomes pair and are segregated into separate cells. These stages include prophase I, metaphase I, anaphase I and telophase I. Professor Wolfe will explain what happens durring each of these different phases. He will focus specifically on what is happening with the cell's chromosomes during these phases. In prophase I, homologous chromosomes synapse and form tetrads. In metaphase I, homologous chromosomes organize and line up. In anaphase I, homologous chromosome pairs separate, and in telophase I, a cleavage furrow forms, creating two cells. Each created cell has one chromosome from each homologous pair.

During the stages of meiosis II, the doubled chromosomes are divided and move into separate cells as in mitosis. Meiosis II is also made up of stages (prophase II, metaphase II, anaphase II and telophase II), and you will also learn what happens in each of these phases, again with a focus on what is going on with the chromosomes. In prophase II, the nuclear envelope breaks down and spindle fibers form. In metaphase II, chromosomes line up on the metaphase plate. In anaphase II, chromatids separate, and in telophase II, haploid cells are eventually created. After meiosis II there are four cells, each containing the haploid genetic complement. This is the objective of Meiosis: to reduce the number of chromosomes by half (to form haploid cells) and to segregate homologous chromosomes.

Taught by Professor George Wolfe, this lesson was selected from a broader, comprehensive course, Biology. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/biology. The full course covers evolution, ecology, inorganic and organic chemistry, cell biology, respiration, molecular genetics, photosynthesis, biotechnology, cell reproduction, Mendelian genetics and mutation, population genetics and mutation, animal systems and homeostasis, evolution of life on earth, and plant systems and homeostasis.

George Wolfe brings 30+ years of teaching and curriculum writing experience to Thinkwell Biology. His teaching career started in Zaire, Africa where he taught Biology, Chemistry, Political Economics, and Physical Education in the Peace Corps. Since then, he's taught in the Western NY region, spending the last 20 years in the Rochester City School District where he is the Director of the Loudoun Academy of Science.

Besides his teaching career, Mr. Wolfe has also been an Emmy-winning television host, fielding live questions for the PBS/WXXI production of Homework Hotline as well as writing and performing in "Football Physics" segments for the Buffalo Bills and the Discover Channel.

His contributions to education have been extensive, serving on multiple advisory boards including the Cornell Institute of Physics Teachers, the Cornell Institute of Biology Teachers and the Harvard-Smithsonian Center for Astrophysics SportSmarts curriculum project. He has authored several publications including "The Nasonia Project", a lab series built around the genetics and behaviors of a parasitic wasp.

He has received numerous awards throughout his teaching career including the NSTA Presidential Excellence Award, The National Association of Biology Teachers Outstanding Biology Teacher Award for New York State, The Shell Award for Outstanding Science Educator, and was recently inducted in the National Teaching Hall of Fame.

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Cell Reproduction
Meiosis
Disjunction and Meiosis II

Let’s remember the whole overall scheme of meiosis before we tie it all together. Remember, there are two things
you are trying to accomplish here. Number one, you want to go from 2N to N. You want to go from diploid to haploid.
You want to take your chromosomes and divide them up. But there is more than that. The second thing that you
want to accomplish is, you want to make sure that you are going to segregate your homologs. What does that
mean? You want to make sure that the homologous chromosomes, those chromosomes that you got one from your
mother, one from your father, those chromosomes that work together are sent separately on their way into your
gametes. You don’t want those things pairing up and going away together, because then you going to run into
situations at fertilization, where, you know, if they pair up and go together, and that is in a sperm, then you fertilize an
egg with that, you are going to end up with three homologs or four homologs. That is not going to work. So 2N to
N, segregate homologs, that’s what we are all about in meiosis.
That being said, let’s take a quick look at what happens during the phases of meiosis and remembering that it is
based on mitosis, which means that we double our chromatin first. Which means that we have an added burden,
because we have doubled. Look, think about it. If you are going to double and you want to go to half, that means that
you need two divisions, right? Think about it. If you just have a single amount and you want to go in half, you divide
in one. You divide once. We have doubled, so we have to half and then half again. So we have two divisions. We
saw that the first division is simply called meiosis one. It has the same phase names as mitosis, which would be
prophase, metaphase, anaphase and telophase. That is the good news. The even better news is prophase and this
upcoming metaphase are the crucial ones. So, let’s talk about what is going to happen in these phases and why they
become so important.
Now, here is the point and here is where we want to begin. What we want to begin with is a quick summary of what
happened in prophase one. In prophase one, we had a chromosome or we had a pair of chromosomes that actually
did, they synapsed, they came together and there was a little bit of crossing over and that is where the genetic
material exchanged. But, what is crucial here is the line up into tetrads of homologous pairs. Tetrads, homologous
pairs, that is find your partner, because we are going to “dosey doe” here. So we are going to find the partner and
now we have to get in line. See, remember my analogy was Sister Mary Camillus; boy, she rings that bell and we
freeze. She rings it again, we find our partner, she rings it a third time, and what did we have to do?
We had to go to metaphase one. And, what that meant, was we had to go to a situation where we lined up with our
partner in a double line, metaphase one. Prophase one is big. Metaphase one is as big. Let’s see why. Now, once
again, let me tell you what I am leaving out. I am living out everything you know about from mitosis. So I am not
going to tedious up this thing with the centrosomes go to either poles, because they do. The spindle fiber, the
spindles build. The kinetic core is attached, you know all of that already, and you should know all of that already. We
don’t want to clutter this. Chromosomes are the name of this game.
So, what we have here are homologous pairs. But now look, sort of like mitosis, they are lined up on the metaphase
plate, but what big difference do you guys see between this and mitosis? What you are seeing here is the fact that
the homologous chromosomes line up on the equator, not the single chromosomes that you would have seen back in
mitosis that would have lined up like this on the spindle. Here we are seeing the pairs, why? Well, once again, let’s
go back to my schoolyard. If my teacher wants half the class to go to room one and half the class to go to room two,
my teacher is not going to say, “Okay, half go to room one, half go to room two” because it would be chaos, because
we are just stupid little chromosomes. But, instead, she will say, “kids, form a double line, everyone on this side go
that way, everyone on this side go that way”. You are separating the homologs. How cool, that is just so cool. So
anyway, there is the homologs; you know what is going to happen next. So here we are. This is what, metaphase
one.
From metaphase one, we are going to move on and we are going to enter a stage called anaphase one. Just like
mitosis, have I said that before? So what is happening here now, is that the homologous chromosomes, the ones that
are hooked up by their kinetic cores are going to “pacman” their way to either pole. Think of the significance of this
step. It seems so simple, but evolutionary, how big? Instead of separating chromatids, you are now separating, still
separating, chromosomes. So, now that we have separated them, and they are going to either cell, we are going to
have a dilemma, but man have we accomplished something, because we have now made sure to segregate our
chromosomes, all right.
So check it out. We are now where? Anaphase one. What’s next, you guys, I know you know this; we are going to
have to go to telophase and in telophase one. What are we going to have to do? In telophase one we are going to
have to form our cleavage furrow, and in forming that cleavage furrow, we are going to form two new cells.
Now, here is the thing. Let’s look at these cells very closely. I have this illustration for you, so that you can kind of
see the crossing over, but what is the most important thing? What is the most important thing here, is that cells are
pinching in – and help me count chromosomes, because remember, the two goals of meiosis. Two goals, what’s the
scheme? Go from 2N to N and segregate homologs.
Have we accomplished segregating the homologs? You bet. Right here, you know if there is a gene for eye color
here, there is a gene for eye color here, and if there is a gene for body length here, there is a gene for body length
here. We have segregated our chromosomes. We have taken apart our homologous pairs.
Have we accomplished 2N into 1N? Well let’s think about it. This particular cell started out with four chromosomes,
remember? So you are saying to yourself, “Well that is kind of cool, we did it all”, because look. it started with four, I
would expect 2N to be 4 and N to be 2 and if you look at this telophase cell, we have gone from four to two, but do
you notice something about this? Why can’t we stop here guys? Just draw a line and say we are over, because the
chromosomes, you know this, the chromosomes are still what? They are still doubled? See, they still have two
chromatids in them. So what would you recommend?
Let’s go through a second division, so we can pull these apart. How easy? If you know mitosis, you got the rest of
the story, because this next one is even going to be more like mitosis than the first one.
So now we go to meiosis two. In meiosis two, let’s see what we are going to have. In meiosis two – oh look – look
familiar? I sure hope it does. Because we are now, and by the way, what about interphase, do we go back to an
interphase after telophase one? The answer is some cells do. Some organisms go into a very short interphase, other
organisms get right out of that telophase and go right into prophase two.
Prophase two, everything starts to happen again. By the way, like in telophase of mitosis, your nuclear membrane
started reform. Remember, it is based on mitosis. You actually do go very close to an interphase, but then when we
get to prophase, look the nuclear membrane starts to break apart again. The centrosome doubles and moves apart,
and we start to get our microtubule spindle forming. But now you predict what is going to happen next. I will give you
three seconds. Ready? Good. All right, what is going to happen next, is we are going to go to metaphase two. How
do you think the chromosomes are going to line up? Will they line up in homologous pairs? I hope you said that is
impossible, because if you did, you understand. There are no more homologous pairs. They have been separated.
So now, folks, we can line up our non-homologous chromosomes just like it’s going to be. By the way, notice that
there are two cells, just like it’s going to be mitosis. Predict what is going to happen next? Yeah, these are going to
go that way. The chromatids are going to split. If I can once again use my analogy of these guys and put them in a
cell, very much like I have here. We are going to line up on the equator. So if we make this, I will make this cell look
this way, here we will put the cell like this. So here is my cell. There is my metaphase plate. There are my
microtubules. What is going to happen next? What is going to happen next is, anaphase, with eventual cleavage
furrow formation. Get it? See how nice.
So, let’s finish this up. Next we are going to go, what did I say that was, metaphase? Now we are going to go to
anaphase. Get rid of this. There is the chromosome splitting apart. Anaphase two, just like mitosis. The chromatids
are separating, the chromatids are separating, see they are now called daughter chromosomes, but we have
separated those chromatids. What is next? You got it, the formation of cleavage furrows, with the eventual formation
of one, two, three, four cells, after telophase. What is unique about these cells? Have we accomplished our goal?
Oh boy, have we ever. Have we gone from 2N to N? Absolutely, from four, in this case, to two. Have we separated
the homologs? We did that back in anaphase one. You are ready now, for sexual reproduction, because you have
formed a single or a haploid cell with a 1N number of chromosomes. Great stuff.

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