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Biology: The Eukaryotic Cell Cycle


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About this Lesson

  • Type: Video Tutorial
  • Length: 9:13
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 99 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)

Professor Wolfe gives an overview of the full cycle of eukaryotic cells. There are consistencies in the cell cycles of almost all eukaryotic cells. Why do cells divide? All the cells divide to maintain their volume to surface are ratio. When cells divide, the genetic information in the offspring cells must be identical to the genetic information in the parent cell, so this genetic information must first be organized and doubled. This process is called 'packing' and forms chromasomes, which are 2 molecules of identical DNA. Ninety percent of a cell's life cycle is spent in the period called "interphase," which is where the cell grows (known as the G1 or Gap 1 phase), DNA replicates (known as the S phase for DNA synthesis), and the cell prepares to divide (known as the G2 or Gap 2 phase). A eukaryotic cell will only spend ten percent of its life cycle dividing, or "replicating."

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.

About this Author

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Cell Reproduction
An Introduction to the Cell Cycle and Mitosis
The Eukaryotic Cell Cycle

Have you ever wondered why cells divide? There are a lot of good reasons for it. Different cells divide differently.
However, we find when we study the cell cycle that there are literally consistencies throughout practically every kind of
cell. Now, once again, we have to make a special niche for our prokaryotic friends. With their one chromosome and
their singular chromosome and their lack of compartmentalization, they’re going to do cell cycle and cell division a little
bit differently than you and I. I want to talk to you about the eukaryotic cell cycle. I want you guys to get a good feel
for why this happens.
Now, one of the nightmares—you know, so often that when I talk to my students and I ask them for a little retrospect,
many students will say, “Man, in high school I never got cell division.” And then you ask them about college. “You
know, in college I never really got cell division.” And then you talk to teachers and they say, “You know, I had to teach
cell division 148 times before I really got it.” And I don’t get it why you don’t get it, why it’s so hard—no, actually I do
get it because I remember back when it was a struggle for me. And you know, something I’ve learned over the years
is: understand the process and you’ll get the words. Okay? Just a little advice from an old guy here. If you get the
process, it’s all going to make sense. And you’re going to see as we progress through cell divisions—and two
particularly different types of cell divisions—that your basis is learning the eukaryotic cell cycle and what happens, and
then it all stems from there.
Well, let’s start out with what’s important. If you were going to divide a cell, what’s important? Well, the first thing you
would probably say is, “Why bother? Why bother having a cell cycle?” Well, I think you know a little bit about this
story. Why don’t cells get to be giant? Well, cells don’t get to be giant because they depend on their cell membrane
for input of nutrients and for output of wastes, and it’s a simple question of geometry. Volume increases
proportionately faster than surface area. So the cell will get proportionately more volumetric or more volume, whereas
its membrane—and the mathematical thing is, it grows on a cubic basis—whereas the membrane only is a surface
area. So if you just plug in some numbers you find out that, wow, cell volumes get bigger a lot faster than cell
surfaces. It becomes a volume-to-surface-area ratio. Doesn’t work. So cells can’t get giant, which is a good thing for
people who swim in lakes, because amoebae stay on the bottom and they’re tiny. But let’s move on.
So now we know why it has to happen. What would you want to do? Well, if you’re going to make a cell duplicate,
you have to make sure that that cell can function. Think about it. I think I want to make a new skin cell. There’s a
skin cell and there’s its nucleus. Now, if you want that skin cell and this precursor to a skin cell to give rise to new skin
cells, you have to make sure of something. You have to make sure that the two offspring, if you will, of that cell are
going to be able to do a job. And what’s its job? To be a skin cell—not to be a liver cell, not to be a sperm or egg, to
be a skin cell. So therefore, we have to make sure that we get the genetic information that was in this first one, and
we make sure that it goes into this one and this one. So we have to make sure that our genetic information gets in
And so what has evolved over the course of the millennia—more than millennia. Over the millions and billions of
years that life has been around, what has evolved is this idea of, before we can replicate a cell, we have to get its
genetic material organized. So organization. You know, if you think of compartmentalization of cells, it’s an attempt at
organization, and a good attempt. Well, there is even organization within that organization. A cell must organize its
genetic information before it can pass it on. And here’s the thing: not only must it organize it, but it has to double it.
We know that. This is what we’re going to lay out for you over the course of this discussion on the eukaryotic cell
cycle: how does a cell organize its genetic material and how does it pass it on after doubling it? Well, let’s take a
quick look.
Let’s take a look at what happens to our genetic material. Now, I keep using the words “genetic material” because I
want you to think. What is the genetic material? DNA. And what we know about DNA is that our cells, to organize
DNA, go through a series of steps called packing. And we go from what is normally found in the nucleus of a cell,
called chromatin. And chromatin is DNA wrapped around proteins called histones and nonhistonal proteins. And
what happens is we go through a series of steps where this coils and coils and coils and coils, and goes through a
series of continued coils until eventually we get what we like to refer to as a supercoiled strand of DNA, which
eventually ends up in this thing here called a chromosome.
Now, we’re going to talk a lot about chromosomes as we move through the cell cycle but there are a couple of things I
want you to understand. A chromosome is literally two molecules of DNA. You see, chromosomes only exist when
the cell cycle is occurring or when cell replication is occurring. So what we just did is we went from a state called chromatin, which was that loosely-packed DNA, to a chromosome. But even more importantly, we did something
else. If you know anything about molecular genetics, you know that DNA can replicate. So in-between this idea of
chromatin and chromosome, we have doubled our DNA. And there is literally one molecule of DNA on the left side of
the chromosome and one molecule of DNA on the right that are identical to each other. So we’ve accomplished our
goal of doubling our DNA and organizing it. And what did I say is important? You want to make sure you organize
that genetic material, and number two, you want to make sure you get it out to the new cells that you’re making.
Which brings us to an overview of the cell cycle, and let’s talk about what that means.
The cell cycle is what a typical eukaryotic cell goes through. We’ve divided it into phases so that we can have a
picture and have something to talk about. The bottom line is, I know everybody thinks, “Oh, my gosh, we’re going to
learn about this hard thing called mitosis.” But if you take a look at the life of a cell before dividing, we’re going to see
that about 90 percent of a cell’s life is in this green section here called “interphase.” And indeed, interphase is divided
into three phases itself. We have a phase called G1, which stands for gap. You know, we’ve talked a lot in this
course about cell biology. We’ve talked a lot about how cells function. We’re going to talk a lot more about how cells
function. Guess what? Right there. G1. We have been G1 experts. But then at the end of G1, the DNA is
duplicated, and that’s when the cell is getting ready to do what I showed you a second ago, split into two. And now
we get a DNA duplication, a replication of DNA using enzymes like DNA polymerase—did you ever hear of that?
Maybe. And so our DNA is going to replicate.
And now the cell is ready to divide but it’s got to grow a little bit more. So after S, DNA synthesis, it goes into gap 2.
So when does it divide? Right here. Only 10 percent of a cell’s life is spent dividing. Obviously, the field of biology
spends an awful lot of time investigating what happens in the phases when a cell is doing cell things, but it’s very
important for us to now spiral in and take a look at what happens in this small little section here, which we’re going to
call “cell replication.”

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