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An introduction to what cancer is and how it is the by-product of broken DNA replication. Created by Sal Khan.

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  • leaf red style avatar for user amandasarizona
    How exactly do treatments like chemotherapy and radiation help Cancer, as there is no cure. Also, how do people recover from Cancer?
    (242 votes)
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    • blobby green style avatar for user matthew.swallow

      It is not true that there is not a cure for cancer. It depends very much on the type of cancer, the stage of the disease, the health of the patient. Treatment either has curative intent or paliative intent.

      People "recover" from cancer when no malignant cells are found within the body- this is termed "remission." Cancer may return after "remission"- but the longer a patient has been in "remission" the less likely the cancer is to return.

      Chemotherapy consits of systemically administered drugs that directly damage cellular DNA and RNA. It kills cells by promoting apoptosis (regulated cell death) and sometimes frank necrosis (disordered cell death). They work in a variety of ways: (1) promoting DNA damage, (2) inhibit DNA repair, (3) work against cell metabolism, (4) work against tubulin. As Khan has alluded to- it works on the prinicple that DNA will be dividing fastest in the cancer cells. The problem is that it is a systemic treatment and so actually damages all cells in the body.

      There are now more "targeted" regimens that we can use such as the tyrosine kinase inhibitors.

      Radiotherapy delivers energy to the tissue in the form of ionising radiation. This induces apoptosis. There are many different forms of radiotherapy (internal versus external). external beam radiotherapy is the most common. in some disease (e.g. prostate cancer) "beads" of radioactive material can be placed inside the cancer and irradiate it from within. Other examples include radio-iodine therapy.

      I hope this has helped :-).

      It is very complicated- and the understanding of the genetics is an evolving area of research. The genetics behing cancer is fascinating and when you start to understand it you really see the point of having done high-school biology!

      But as I said at the beggining it is important to know that cancer CAN be cured: particularly if it is caught in the early stages.
      (413 votes)
  • old spice man green style avatar for user Skywalker94
    How does chemotherapy target cancer cells? I know that chemotherapy hurts all of the cells in the body, but does it kill the cancer cells more than it kills the other cells of the body (due to blood flow or something like that)?

    Also, this might seem like a dumb question, but why exactly does chemotherapy cause people to lose their hair? Does it actually damage hair follicles so that they cannot get their hair back or is it a temporary condition?
    (59 votes)
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  • orange juice squid orange style avatar for user Alexander
    How does radiation cause cancer?
    (2 votes)
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  • leaf green style avatar for user pallaka Anjireddy
    How is cancer actually caused?
    (5 votes)
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    • leaf blue style avatar for user dysmnemonic
      There are four main causes: inherited cancer-causing genes, ionising radiation (including UV light), chemical damage to DNA (including some chemicals produced within the body), and random bad luck. Most of the mutations these cause are in regions with no known function, and most of them are fixed - but when enough mutations to certain essential DNA regions occurs, cells can start reproducing improperly, resulting in cancer.
      (3 votes)
  • hopper jumping style avatar for user Smart-guy
    I am littly bitt scared, after reading the comments. Can somoene please simply explain me what I have to do to reduce chance of getting cancer. For example how much I HAVE to exercise everyday( because somethimes I DONT EVEN WALK OUT OF THE HOUSE) and what should I eat for example which vegetables and what I not should eat? and if I get cancer is there a way to totaly remove the cancer from my body. And one of my relatives has liver cancer and it has probably spread to other places near the liver so is this possible to cure or not?
    (4 votes)
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    • piceratops ultimate style avatar for user AegonTargaryen
      Exercise does not really affect cancer rates at all but obesity can increase the chances of getting some cancers.
      To avoid getting skin cancer, you should wear adequate sunscreen during bright days.
      Stay away from radioactive isotopes (this should not be to hard) and make sure to monitor radon levels in your house, radon is a radioactive gas that can seep out of the ground and into your house, but high radon levels are very rare.
      DO NOT SMOKE, this is a leading cause of the fatal lung cancer.
      Asbestos can also cause lung cancer but asbestos is no longer legal as a product, though it can still be in the insulation of old buildings.
      Alcohol can cause seven types of cancers if drank in excess over a long period of time.

      There are some people that have a genetic predisposition to cancer making it likely to get one type of cancer no matter what they do. Many types of cancer are treatable and even more are curable. Using common sense can be sufficient to avoid most types of cancers.

      Once cancer starts spreading, it becomes harder to treat but it might still be cured, it depends on the case.

      And as T B mentioned, it is important to get regular check-ups after age 30 so that if you do have cancer, it can be found in its early stages and cured easily.
      (4 votes)
  • female robot grace style avatar for user Epic Brat GIrl
    if the cells relize there is something wrong then why does cancer exist??
    (2 votes)
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  • mr pants teal style avatar for user Filsan
    Is cancer something you can pass on to your offspring?
    (4 votes)
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  • leaf green style avatar for user cole
    i know that in every cell there is dna in it, but do we have the original dna still in us? if we do what i am wondering like in the case of cancer, is it possible to access that dna to "reset" to body, getting rid of the cancerous cells?
    (3 votes)
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    • female robot ada style avatar for user Pearl Northrup
      I think understanding cancer a bit more might help.
      I believe that most cancers are caused when the mechanism that signals a cell to stop replicating itself is stopped. A part of this mechanism is a series of nucleotides at the end of chromosomes that don't code for anything, called 'telomeres'. When DNA is replicated, one double helix is separated into two sides called 'template strands', a new double helix is formed from each of the two template strands. The very end of the newly created strands can't be copied because of the way that the enzyme that creates new DNA works (the enzyme is called DNA Polymerase). Telomeres work to prevent the loss of important genetic information from this end shortening effect. The telomere end on chromosomes becomes shorter with each cell replication and eventually become so short that the cell stops replicating. In cancer cells, short telomeres don't stop the cell from dividing. It continues to divide until important genetic information is lost from the end shortening of newly replicated strands. The DNA contained in stem cells could be considered original DNA. Stem cells can turn into any kind of cell. They have the ability to grow healthy cells and replace damaged cells. I hope I helped answer your question.
      (3 votes)
  • hopper happy style avatar for user Mariah  Pleban
    How do people recover from having cancer?
    (3 votes)
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    • starky ultimate style avatar for user Edge
      There are a lot of different kinds of cancer. But for simplicities sake we have two main classifications, malignant and benign. A benign cancer is a group of cells that just grow out of control, they stay where they are though, so we can cut them out or treat them with area specific drugs to cut off the blood supply. So with just a surgery, you could be clear of cancer for life, potentially.

      Malignant cancers are much worse, they dont stay where they are, and can spread through the body. If you have a thousand tiny tumors, surgery just isn't going to work, because you'll miss some or the surgery itself will be too invasive and destructive. I suppose its telling that wikipedia doesnt have a page for the treatment of malignant cancer, instead they have a page for the management of malignant cancer. There's a variety of techniques for destroying the cancerous cells with the aim of putting the cancer in "remission", which is defined as a temporary decrease in its severity. I think this comic sums it up far better than I can.
      (4 votes)
  • starky ultimate style avatar for user Daniel Choi
    I heard this from someone. Is it true that every one in the world have cancer cells in their body?
    (2 votes)
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    • male robot johnny style avatar for user N8-0
      Cancer is a mutation in cells. Yes, everybody has the possibility of getting cancer because they have cells... But the cells are normal until there's a problem with the splitting and that makes them grow faster or something. Hope this helps.
      (5 votes)

Video transcript

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.