Cancer | Cells | MCAT | Khan Academy

Most cells in the human body
just go about their business on a daily basis in a fairly
respectable way. Let’s say that I have
some cell here. This could be maybe a skin cell
or really any cell in any tissue in the body. As that tissue is growing or
it’s replacing dead cells, the cells will experience mitosis
and replicate themselves, make perfect copies of each other. And then those two maybe will
experience mitosis, and then if they realize that, gee, you
know, it’s getting a little bit crowded. There are other cells
in my neighborhood. They’ll recognize that, and say,
you know, I’m going to stop growing a little bit. That’s called contact
inhibition. And so they’ll just
start growing. And then let’s say one of them
experiences a little defect, and he says, you know what, gee,
something’s a little bit wrong with me. I, the cell, recognize this in
myself, and the cells will actually kill themselves. That’s how good of cellular
citizens they are. They’ll kind of make way for
other healthy cells. So this guy might even kill
himself if he realizes that there’s something
wrong with him. There’s actually a cellular
mechanism that does that called apoptosis. And I want to make
this very clear. This isn’t some type of outside
influence on the cell. The cell itself recognizes that
it’s somehow damaged and it just destroys itself,
so apoptosis. So that’s the regular
circumstance even when there is a mutation. And just to give you an idea,
even if mutations are relatively infrequent. And I don’t know the exact
frequencies at which mutations occur. I suspect it’s of different
frequencies in different types of tissues. There are on the order
of 100 billion. Let me do it in a
different color. There are on the order of 100
billion new cells in the human body per day. So even if a mutation only
occurs one in a million times, you’re still dealing with
roughly 100,000 mutations, and maybe most of the mutations,
maybe they’re just some little random things that don’t
really do a lot. But if the mutations are a
little bit more severe, the cell will recognize it
and destroy itself. And I want to make a very
clear point here. I’m talking about the cells of
the body or most of the body. This could be the cells in my
eye or the cells in my brain or the cells on my leg. These aren’t my germ cells. So these mutations, even if the
cell survives, will not be passed on to my offspring. That’s an entirely different
discussion when we talk about meiosis. These are all my body cells and
they’re replicating, and we’ve gone over this
with mitosis. So any mutations here, they’ll
either do nothing, or the cells might malfunction a little
bit, or the cells might hurt themselves or hurt me,
but they’re not going to affect my offspring. And I want to make that
point very clear. Now, you’re saying, hey, Sal,
100 billion new cells a day? That must mean like every cell
in my body has created, well that just gives you an idea of
how many cells we have. We actually have on the order of,
and you know it’s obviously not an exact number, but
actually in the human body, there’s on the order of
100 trillion cells. And if you look at it that way,
you say on average, one thousandth of your cells
replicate each day, but the reality is some cells don’t
replicate that frequently at all and some cells replicate
much more frequently. Just to take a little side note
here, this gives you an appreciation, I think, for the
complexity of the human body. I mean we think of our own
world economy and world society as so complex, it’s made
up of 6 billion humans. We’re made up of 100
trillion cells. Let me rewrite 100 trillion
in billions. 100 trillion can be rewritten
as 100,000 billion cells. And each one of those 100,000
billion cells are these huge– I know I shouldn’t use the word
huge– but they’re these complex ecosystems in and of themselves with their nucleuses. And we’ll talk about all the
different organelles they have, and we talked about
cellular replication, DNA replication and how the
cell replicates. So these things aren’t jokes
and they have all of these complex membranes that take
things into them. They are creatures to
themselves, but they live in this complex environment or
society that is each of us. So that’s just a side note just
to appreciate how large and how complex we are. But you can imagine, and this
is how I got off on this tangent, if we’re making on the
order of 100 billion new cells every day, you’re going
to have a lot of mutations, and maybe some of the mutations,
you know I said some of them don’t
do anything. Some of them, the cell
recognizes that the cell is just going to be kind of dead
weight so the cell kind of eliminates itself. But every now and then, you have
mutations where the cell doesn’t eliminate itself and
it also deforms the cell. So when you have that, let’s
say I have some cell here. I have some cell and it’s
got some mutation. I’ll do that mutation with
a little x right here. That’s in its DNA. Maybe it’s got a couple
of mutations. So one of the mutations keeps
it from experiencing apoptosis, or destroying itself,
and maybe one of the mutations makes it replicate
a little bit faster than its neighbors. So this cell, through mitosis,
it makes a bunch of copies of itself or a ton of
copies of itself. And this kind of body of cells
that essentially has a defect, they’re all from one original
cell that kept duplicating and then those duplicating, but all
these are defective cells. If you were to look at them
compared to the tissue around it, it would look abnormal
in some way. Maybe it wouldn’t function
properly. This is called a neoplasm. Now, a lot of neoplasms,
well they don’t have to form a body like this. Sometimes they might somehow
circulate in the body, but most of the time they form
this kind of big lump. And if they get large enough,
they’re noticeable. And that’s when we
call it a tumor. So if this is actually a lump
of kind of differentiated tissue that’s definitely
abnormal, that’s what you call a tumor. So the term neoplasm and
tumor are often used interchangeably. Tumor is the word we use more
in our everyday vocabulary. Now, if this lump just kind of
grows to a certain size, it’s just there, it doesn’t really
do anything dangerous, it’s not replicating out
of control. I guess it’s not replicating
a lot faster than its neighboring cells and it’s
just hanging out, maybe growing a little bit, but not in
any significant way harming our environment, we
call that a benign tumor or a benign neoplasm. And benign essentially
means harmless. Benign tumor. That means that’s good. You want to hear that. If you got a lump– God forbid
you have a lump either way– but if you do and it’s a benign
tumor, that means that lump, it can kind of stick
around, no damage done. But if these DNA mutations, and
maybe some of these are, it is benign, but maybe one of
the benign ones has another mutation in it that starts
making it grow like crazy. And not only does it
grow like crazy, but it becomes invasive. And invasive means that
it doesn’t care what’s going on around it. It just wants to infiltrate
everything. So let’s say that guy
grows like crazy. Let me do it in a
different color. And he starts infiltrating
other tissue, so he’s invasive. So super growth,
he’s invasive. So he doesn’t care
what’s going on. He’s all of a sudden turned into
some type of a cellular psychopath. And even worse, his descendants,
it’s not just one cell anymore. He just keeps duplicating and
passing on this kind of broken genetic information that makes
it want to replicate. And then maybe there could be
more and more things that break down in its I guess
offspring or the DNA that comes from its replications. And actually, that’s a good
likelihood, because the same parts of its DNA that broke
down, some of the DNA that broke down in this guy, some
of the mutations might have actually hurt the DNA
replication scheme, so that mutations become
more frequent. So more frequent mutations. So as these replicate, more and
more mutations appear, and then maybe eventually one of
the mutations appears that allows these cells to break off
and then travel to other parts of the body. And then those parts of the body
start to take over and start taking over all
of the cells. And this process is called the
cell has– this is one of the hardest words for me to say,
something wrong with my brain– but the cell
has metastasized. You might have heard the word
metastasis, and that’s just the notion of these run amok
cells all of a sudden being able to travel to different
parts of the body. And I think you guys know what
we call these cells. These cells that aren’t
respecting their cellular neighborhood. They’re growing like crazy. They don’t experience that
contact inhibition. They’re invasive. They start crowding out
other cells and hogging up the resources. And they keep mutating really
fast because they have all of these genetic abnormalities. And eventually they might even
break away and start infiltrating other parts
of the body. These are cancers
or cancer cells. And so you might have
an appreciation for why this is so hard. Cancer is such a hard disease
to quote, unquote, cure. Because it really isn’t
just one disease. It’s not like one type of
bacteria or one type of virus that you can pinpoint and
say let’s attack this. Cancer is a whole class of
mutations where the cells start exhibiting this fast
invasive growth and this metastasis. So you might look at one type of
cancer and be able to say, hey, let’s target the mutation
where the cells look like this and you’re able to knock
out some of them. Let me do this in this color. So maybe you’re able to
knock out that guy, that guy, that guy. But because their DNA
replication system might be broken in some way, they
continue to mutate, so eventually you have one version
that’s able to not be knocked out by whatever
method you get. And so you have this kind of new
form of cancer, and then that new form of cancer is
even harder to kill. So you can imagine that
cancer is kind of a never ending fight. And you kind of have to attack
the general idea behind it. Chemotherapy and radiation, all
of these type of things. They try to attack things that
are fast growing because that’s the kind of
one common theme behind all of the cancers. And we could do a whole playlist
on what cancer is and how people are attacking it, but
I wanted to at least show you in this video that cancer
really is just a byproduct of broken mitosis, or even more
specifically, broken DNA replication. That we have all of these cells
replicating themselves every day on the order of 100
billion, and every now and then something breaks. Usually when they break, either
nothing happens or the cell kills itself. But every now and then, the
cells start replicating even though they’re broken. And sometimes they start
replicating like crazy. If they just replicate, but
they’re really not doing any harm, it’s benign. But if they start replicating
like crazy, taking over resources and spreading through
the body, you’re dealing with a cancer. So hopefully, you found
that interesting. You already know a good bit of
the science that kind of deals with what is probably one of
the worst ailments that we deal with as creatures. I mean, obviously, we’re not
the only people who can experience cancers. Even plants have cancers.

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