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Biology: Cytokinesis

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About this Lesson

  • Type: Video Tutorial
  • Length: 9:01
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 97 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: An Intro to the Cell Cycle & Mitosis (4 lessons, $6.93)

Cytokinesis is the division of the cytoplasm that takes place after mitosis, producing two daughter cells. Professor Wolfe walks you through this process in both animal and plant cells. In an animal cell, the well membrane elongates due to the contraction of microfilaments actin and myosin. This contraction creates a cleavage furrow that will eventually produce two separate cells known as daughter cells. In plant cells, the same process isn't possible, because of the rigid, non-cleavable cell wall. The Golgi apparatus in plant cells actually migrate to the middle of the cell, where they synthesize a new cell membrane. Then, proteins from the golgi vesicles are able to synthesize a new cell wall, creating two distinct cells.

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
An Introduction to the Cell Cycle and Mitosis
Cytokinesis
So in mitosis, it seems like the most difficult part of the whole story is getting those chromosomes doubled or getting
your chromatin doubled, somehow compacting that into chromosomes, and making it organized so that you can split
those chromosomes, as in the whole chromatid story, so that the chromatids can become eventually chromosomes.
Remember that what we want to talk about here is, how are we going to split the cell? And remember what we have.
We now have a cell with two nuclei in it, and those nuclei are starting to decondense. The chromosomes are going
down and they’re becoming chromatin again. Your nucleolus has redeveloped. Well, how are we going to split this
cell? Because eventually what we want to form from this is what we call two identical daughter cells. That’s our
eventual goal. And that sounds like quite a trick. It almost makes mitosis pale by comparison.
And remember where we are here in our little cell cycle. We’ve been through a normal cell lifespan. We’re in that 10
percent. We’ve gone through what I’ve labeled here as M, the mitosis part of the mitotic phase, and now we’re
heading into cytoplasmic division. We need to divide that cytoplasm. So let’s take a look at cytoplasmic division from
two perspectives: the perspective of the animal cell and then the perspective of the plant cell.
How does an animal cell divide its cytoplasm? Well, here’s our dilemma. Our dilemma is this: when we observe it
under a microscope, we see that what happens is you go from a relatively round cell and the cell begins to elongate
into almost an oval shape. And how does a cell elongate? How does a cell do that? Well, logic dictates that a cell,
being a fluid or plasma membrane, there’s really no levers. When I move an arm, for example, I’m levering it. When I
do this and I elongate my hand, I’m using bones and muscles as levers. Well, to a degree, there is none of that in a
fluid membrane. But let’s not forget something that you know about from your lessons in cell biology, and let’s not
forget that entire cytoskeleton. And let’s not forget the fact that there are microtubules and microfilaments that
connect this cell in an unbelievably intricate way, and that these microfilaments are indeed anchored across the cell.
In fact, if you know your cell biology, you know of two microfilaments called actin and myosin. These things really
come up a lot whenever we talk about motor molecules. You know, you could link to the lesson now on the muscle.
Muscles work by the interaction of thin filaments and thick filaments which kind of slide on each other. To make a
short story long, what happens with muscles is you get this thick myosin filament that looks like that, and then a ring of
actin filaments around it like so. These filaments slide on each other by motor molecule anchors. And so what you
can get here is the same situation here. You can actually form sliding filaments, and if you can slide filaments, guess
what you can do? You can shorten those filaments.
Now, think of that. Can you form an oval from a round circle by sliding filaments on each other? Let’s see. Let’s
picture my cell. Let’s go to one side of the cell. It’s round, but I have microfilaments across. Imagine I pull those
microfilaments in that direction, particularly at the center. Well, what’s going to happen then is, wow, this thing is
going to bend inward like that. And if we look at the other side and we do the same thing—remember it’s round, and
we bend it inward like so. It doesn’t have to necessarily be the same microfilament. This microfilament could be
pulling in one direction and the one right next to it could be pulling in the other direction. What’s going to happen
here? Well, gee whiz, the same thing is going to happen. That round membrane is going to bend in. And now let’s
connect these, and look what you have. You have the beginning of what I referred to as the cleavage furrow in animal
cells. And indeed, that’s exactly what begins to happen.
We see under electron microscope studies that microfilaments are literally pulling this thing in. And some
microtubules are involved in this, too, but I really want to stress these microfilaments. And so what we begin to get is
an amazing thing—and let me just show you a photograph that just knocks me out. You actually get the beginning of
cell bending. This is taken from a developing frog’s egg. This egg has been fertilized and has begun the process of
development. And the first thing that happens in development is the beginning of that cleavage furrow which will
eventually fuse.
Let me make sure you understand what I mean by this whole idea of fusion. Eventually these two cells are going to
fuse, and let’s see how that’s going to happen—or these two parts are going to fuse. Here is the one cleavage
furrow—and I’m going to make these very pronounced—and there’s the other one. Now, remember what we did back
in the mitosis part. We have our nucleus. The chromatin is now an amorphous—we’re in way late telephase now.
We have our nucleus and the nucleolus and the chromatin. And now look what can happen here. If we simply pinch
those together, let them merge, you’ll eventually grow into two of what I’ve referred to as daughter cells. Everyone
refers to them as daughter cells, to be quite honest with you. And these two daughter cells will be identical. And so in
an animal cell, we see therefore this whole idea of the cleavage furrow and we see this whole idea of the splitting of
the cell thanks to the contraction of microfilaments.

But what about plant cells? In plant cells we’re going to have a completely different story. Why? What do plant cells
have around the outside of them? Cell walls. That’s a dilemma. Why is that a dilemma? Well, let’s take a look. If we
take a look at a plant cell wall, here’s what we have: a rigid, non-flexible, non-cleavable structure. It’s got cellulose
and pectins, and you’re not going to bend that. But what a plant cell has is a cell membrane. But we can’t do a
cleavage furrow in a cell membrane. It’s going to leave the wall. But what can we do? Well, here we are. We’re in
late telephase. We’ve made our nuclei. We’ve got our nucleoli. What can we do? Well, we can synthesize new
membrane. What’s the organelle that synthesizes membrane? That’s right, the Golgi apparatus. So we could pop off
Golgi vesicles. And remember, there’s a whole bunch of these little highways called microtubules and microfilaments
that Golgi vesicles might be able to migrate along. And so they could migrate toward the center of the cell from both
cells and start to form a chain of Golgi vesicles. Now, that being said, what can happen to these Golgi vesicles is they
can fuse with each other, and look what’s going to happen. What are Golgi vesicles made out of? Membrane. And
what’s in those Golgi vesicles? Proteins, maybe enzymes—maybe enzymes that can actually build a new cell wall.
And look what I’ve just formed, folks. I’ve just formed a new cell membrane. Let’s fuse those; let’s fuse those. Two
plant cells back to interphase. And then of course, the plant cell can grow and then eventually add secondary cell
walls and harden off, and go through the process again.
So I just want to leave you with one last thing. Don’t make mitosis a mystery. Don’t make cytokinesis a mystery.
Learn the process. It’s the basis of the way cells reproduce.

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