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Hi. It's Mr. Andersen and in this video I'm going to talk about cells.
Life is made of cells. It's the basic unit of structure and function. And also all cells
we have must come from preexisting cells. And so how does that work? It's through a
process called cell division. This is going to occur in all cells, both prokaryotic like
bacteria and eukaryotic like you. And through cell division one cell can become two. Which
can quickly become four and then eight and then sixteen. And so we get exponential growth
in cells. And to give you an idea how quickly this goes, you started life as one fertilized
egg. Or one zygote. But those were quickly dividing over time. And it eventually created
an adult that's going to have around 10 trillion cells inside your body. And they all came
from that original cell. Now to even increase that number quite a bit, just look in your
gut and we're going to find that the bacteria living in your digestive tract are going to
outnumber the cells in your whole body ten to one. There are going to be 100 trillion
bacteria just living in your gut. And they're going to be outnumbered about 10 to one by
viruses living in your gut. So we're going to have quadrillion viruses living in your
gut. Now viruses aren't technically cells, but everything else I've described here is
going to be reproducing or it's going to be created through a process called cell division.
And there are really three types of cell division. In prokaryotic cells, or in bacteria, they're
simply going to divide through a process called binary fission. That's where one cell, one
bacteria cell, is going to make two cells that are exact copies of the original cell.
In eukaryotic cells we have something similar to this. We're going to have cell division
or we sometimes just refer to it as mitosis. Technically mitosis is going to be division
of the nuclei. And then we're going to have cytokinesis which is division of the rest
of the cell. But we generally just say mitosis is when one eukaryotic cell makes two cells
that are exact copies of that cell. It's a lot like binary fission. And then the third
type of cell division is going to be meiosis. Meiosis is going to be when we create sex
cells. Or when we create gametes. Mitosis and binary fission insure that we have exact
duplicates of that original cell. Meiosis is totally different. Meiosis is going to
make cells that are different. They're going to have half the genetic information and they're
going to give us variability. And so in us sex is important because it gives us variability
in the next generation. In bacteria mutation and there's some other ways that they can
share information is more important. So let's start by talking about bacteria. They reproduce
through a process called binary fission. In this we're going to have this adult cell,
so we're going to have this parent cell. It's going to be made up of, you can see here,
ribosomes. It's going to have a plasmid here which is a little bit of extra DNA. But it's
genome or all of its DNA is going to be wrapped up into one loop. And so what do we want to
do in binary fission? We want to take one cell and we want to make two cells that are
identical to that. And so the process is pretty simple. What the bacteria cell starts doing
is duplicating all of its DNA. So the first thing it will do is DNA polymerase goes around
the whole loop and it's going to make an exact copy of all the DNA in the bacteria. One thing
not shown in here is that we're also going to duplicate our plasmids. And we're also
going to have, you know, ribosomes increasing as we increase the number of DNA as well.
What happens next is it's going to be pulled to the side of that cell. And really what's
going to happen is the DNA will actually attach to the cell wall. And then eventually that
cell wall is going to split in half and we're going to have two bacteria cells. How are
they different? They're not really different. They're exactly the same. The DNA in this
cell and the DNA in this cell is going to be exactly the same. And when we divide in
half, we're going to divide in half those ribosomes and the plasmids as well. And so
again, bacteria don't do mitosis. They don't have sex. They simply reproduce through binary
fission. And this happens incredibly quickly. So in a typical e. coli they can copy themselves,
make two copies of one cell, in 20 minutes. Which is just crazy how fast that occurs.
But again we don't go through binary fission because we're eukaryotic cells. And before
we talk about mitosis and meiosis, we should make sure you know the difference between
what a gamete is and then a somatic cell. And so eukaryotic cells are going to be everything
from the chimpanzee to the bees to the flowers to the mushroom to the yeast, they all have
a nuclei. And so there are two types of cells that we find inside eukaryotic organisms.
First are going to be the somatic cells. Somatic cells are going to be the body cells in your
body. And then we're going to have the gametes which are going to be the sex cells. So in
humans the only cells inside our body that are going to be gametes are going to be sperm
in males and then the eggs or the ovum that are found inside females. How are they different?
Remember somatic cells are the body cells that are going to be diploid in humans. They're
going to have two copies of every gene. And therefore two copies of every chromosome.
Gametes are only going to have one. So sperm and egg each provide you with half the DNA
that you get. So how do we duplicate a typical cell? Well first we should say when do we
do that? Well remember when you go from a zygote to an adult, we're doubling the cells
over and over and over again. And then we're constantly replacing the cells in our body.
So as cells become worn out or damaged or removed we have to replace those with other
cells. And we do that through mitosis. Again what is mitosis? It's technically the division
of the nuclei, but then we're going to have division of the rest of the cell which is
called cytokinesis. So what do we start with? And we're going to make this real simple,
this is a simple cell. It's a diploid cell. It's 2n=2. That means they have two chromosomes.
These two chromosomes are going to be homologous. That means that they're the same length. The
reason why they're different colors is you get on from one parent and one from another.
And so in mitosis, let's look first before we talk about the specifics of what we're
going to get out of mitosis. We're going to start with one cell looks like this. And we're
going to end up with two cells that look exactly like that first cell. So in mitosis we want
to duplicate the cell. And so let's watch that. What's the first thing that happens
is we have to duplicate all of the DNA. And this occurs during interphase. What's going
to happen is this chromosome right here is going to copy all of its DNA. And now we're
going to have what are called sister chromatids. And so we have a chromosome on this side.
A chromosome on this side. And they're exactly the same. So they're an exact duplicate of
that original chromosome. And you can see that we're going to have that on the other
homologous chromosome as well. What happens next? They're going to meet in the middle.
They're going to line up. These lines represent a spindle which is going to attach to either
side. Each of those sister chromatids and then they're going to be pulled apart. They're
going to be pulled apart. And eventually, so this would be the mitosis when we divide
the nuclei. And then the whole cell is going to split in half. We're going to divide that
cytoplasm and that's called cytokinesis. And so what do we end up with? We end up with
two cells that are both diploid. Our original cell was diploid as well. So let's take a
look at what this really looks like. We're looking right here at cells that are undergoing
mitosis. And so they're growing on a growth medium. And if we just play the video, this
is time lapsed. And so it's speeding it up quite a bit. But what will happen if we look
at like this cell right down here, is that you can see it kind of come together in the
nuclei's dividing up the genetic information. And then the rest of the cell is going to
divide. What do we end up with when we're done with that? We have two identical cells.
And those are going to grow. And then they're eventually going to divide the nuclei and
they're going to grow. And they're going to grow. And they're going to grow. And cells,
unless they're cancer cells, are going to keep growing on here until they fill up this
whole medium. But we need to talk about meiosis, which is the third type of cell division.
What happens in meiosis. Meiosis is when you take one cell and we're going to make four
cells out of it. Now let's look how this is a little different than in mitosis. In mitosis
remember we took one cell and we made two cells. In meiosis we're actually going to
have another division. But there's something even different that we didn't see in mitosis.
Remember in mitosis we want to make sure that each of the cells that we make are identical
to that original cell. You can see here that the four cells that we create at the end are
going to be totally different than that original cell. And they're going to have half the genetic
information. Remember I said the sperm and the egg have half the information that's found
in the original cell. And so what's going happen? Well let's watch how this begins.
The first thing that happens is you're going to duplicate each of these chromosomes. And
so we're going to make copies of the chromosomes. But then something odd happens. What happens
that's odd, we're going to take a little bit, in the case we're going to take a little bit
of this white chromosome here. And it's going to cross over with this red chromosome right.
So we're actually swapping chromosomes. And so that process is called crossing over but
what you're really doing is your crossing over or your making a new chromosome. It's
a combination of the chromosome you got from your mom and your dad. It's a chromosome that's
never been built before. So those will eventually line up in the middle. And then they're going
to divide. And then they're going to divide again. And so they'll divide in half. So this
is meiosis one and then they'll divide in half again. And that is called meiosis two.
So what do we create when we're done. We're going to create daughter cells. But each of
those daughter cells are going to be different. Each of these would create a new sperm or
a new egg. Now it's going to be much more complicated in us, in humans, remember, we're
not going to start with 2n=4. With 4 total chromosomes here. We're going to start with
2n=46. And so we're going to start with 46 chromosomes. So it's going to be really really
different when we get to the end. And so we can create literally billions and billions
of different types of cells through meiosis. And so that's going to give us variation.
So to summarize. What do we have? We have three types of cell division. We've got binary
fission. That's going to be found just in bacteria where you take one bacteria, first
copy the DNA and then the rest of the cell is going to divide through cytokinesis. Each
of these daughter cells is going to be identical to that first cell. If you're looking at eukaryotic
cells, we've got mitosis. What happens in mitosis? We start with a diploid cell. So
here are the reds and the greens are each going to come from different parents. So we
have four chromosomes. They're going to duplicate first during S phase of interphase. They'll
line up in the middle. And they'll be pulled to the side. Now if we look here, these two
cells that are produced in mitosis are identical to that original cell. And that's what mitosis
does. It makes exact copies. If we start with meiosis, meiosis is going to start with an
original cell that looks very similar. But instead of lining up like this where we separate
the sister chromatids, they're going to line up with the homologous chromosomes wrapping
around each other and swapping bits of that chromosome. They'll divide in half. And then
they're going to divide in half again. And what we're going to create is gametes. Those
are going to be sperm and egg that have half the genetic information in a new combination
of that genetic information. So again, this will be fertilized by a different sperm and
we get back to a zygote. Where do we go from the zygote? We go right back here to mitosis
where we can copy that cell over and over and over and over again. And so that's cell
division. It's found in all cells on our planet. It shows how one cell can go to all the cells
that we have to day. A good question might be where did that original cell come from?
But that's for another podcast. And I hope that was helpful.


Cell Division

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Why Why 2013 年 3 月 25 日 に公開
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