If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content


Cells replicate through a process called mitosis, making perfect copies of themselves. Sometimes, a cell's DNA can mutate during replication. Most mutations are harmless, but some can cause cells to grow uncontrollably, leading to cancer. Created by Sal Khan.

Want to join the conversation?

  • leaf orange style avatar for user aimeerpierce
    Once apoptosis occurs, what happens to the dead cells?
    (45 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user deilson.elgui
      Their pieces, called "apoptotic bodies", are eliminated by phagocitosis by nearby cells, such as macrophages. Apoptotic cells show changes in their plasma membrane (e.g., phospholipids flip from the inner to the outer lipidic layer) that mark them to be removed by other cells.
      (69 votes)
  • leafers tree style avatar for user SynthDef
    Do all cancer cells have the inability to carry out apoptosis?
    (37 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user Gavin Crowder
      I am no expert however I believe that no cancer cells can undergo apoptosis unless they mutate back to being able to which in my mind would be exceptionally rare.

      For cancer to occur apoptosis must be repressed in the cell in some way so that the cell has a chance to replicate and become a harmful tumor instead of simply killing itself. If the cell mutated in a harmful way but was still able to undergo apoptosis it would die and not harm anything and you would never know any mutation took place.

      Hope this helped I don't have the ability as Sal does to explain things in a very simple to understand manner.
      (31 votes)
  • male robot hal style avatar for user Konrad Taube
    So I don't really understand... why EXACTLY does cancer kill you? I understand that it has to do with the cells dividing uncontrollably, but what about that makes it so lethal?
    (24 votes)
    Default Khan Academy avatar avatar for user
    • orange juice squid orange style avatar for user CarlBiologist
      These cells that are uncontrollably dividing are also not doing their normal jobs. They are diverting bodily resources and not performing vital functions. For example, if you have tumor in your heart those cancer cells in that tumor are not doing what normal heart cells do, work together to pump blood. The actual immediate cause of death varies. For example, someone might die of a blood clot in their brain from a tumor blocking a vital blood vessel.
      (35 votes)
  • orange juice squid orange style avatar for user Alexander
    How does radiation cause cancer?
    (20 votes)
    Default Khan Academy avatar avatar for user
    • leaf blue style avatar for user Paolo Miguel Bartolo
      Radiation, specifically ionizing radiation, can cause cancer simply because they cause mutations. Ionizing radiation has enough energy to ionize an atom. Such changes caused by that high level of energy can change the molecular makeup of DNA (deactivating DNA base pairs, destroying whole sections of DNA), and therefore cause a mutation.

      A cell has genes that controls replication and cell division, thus preventing cell growth from going out of control. If it just so happens that those genes are the ones affected by the radiation, then the cell starts to replicate abnormally, causing cancer.
      (16 votes)
  • mr pants teal style avatar for user matthewtvu
    What if someone's body was able to adapt to their invasive cells?
    (12 votes)
    Default Khan Academy avatar avatar for user
  • female robot grace style avatar for user Epic Brat GIrl
    if the cells relize there is something wrong then why does cancer exist??
    (7 votes)
    Default Khan Academy avatar avatar for user
    • leafers tree style avatar for user Jacob Darley
      To begin with we must note that for a cell to divide it must first duplicate its genetic information and this info must be distributed in a manner that ensures that each new daughter cell will have a complete and unabridged copy of the DNA. This process is accomplished through a series of stages called G1, S, G2 and M phases. These phases are refereed to as the Cell Cycle. There are several "DNA damage checkpoints" in the cell cycle. The first is at the end of G1, the second is in late S phase, and the last is at the end of the G2 phase. These DNA restriction points work by inhibiting various Cdk-cyclin complexes. In addtion there is a Spindle checkpoint in mid M-Phase. before the spindle pulls the two sets of chromosomes to opposite poles of the cell, the spindle checkpoint ensures the chromosomes are properly positioned. In addition to these cancer safeties there is a restriction point in late G1 just prior to the G1 DNA damage checkpoint which ensures that the cell does not enter into S-Phase without the presence of growth factors. When the restriction point halts cell division the cell is said to enter into G0 phase. Typically when a cell undergoes a mutation in its DNA the cell will detect this during the various safe points, however it is possible (though EXTREMELY uncommon) for a mutation to slip past all 5 of these safeties, leading to a mutation that is passed to its daughter cells and then increases exponentially with further cell division.
      (12 votes)
  • leafers ultimate style avatar for user jmill643
    I know radioactivity is bad and all, but what makes it bad? like, what makes it so dangerous?
    (8 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user Nathan Shapiro
      In order to really understand how it is dangerous, you must know how radioactivity starts.
      Radioactivity is the result of an atom which has an unstable nucleus. In order to produce a stable nucleus, it must decay. This is where the term "radioactive decay" comes from. Three ways an atom can decay is beta decay, alpha decay, and gamma decay.
      Beta decay is where the nucleus of the atom actually changes one of it's neutrons into a proton (this is accomplished by emitting an electron).
      Alpha decay is where the unstable atom emits a "nucleus" from its own nucleus. It emits two neutrons and two protons. This is also the nucleus of a helium atom.
      Gamma decay is the result of the nucleus emitting a high energy photon.

      Every one of these methods of stabilizing an atom emit something. It turns out that each of these emitted particles contain a high amount of energy and are traveling at high speeds. When one of these hits a living cell, the energy can mutate or kill it. Just a few of these are fine (just as X-rays are fine if a limited amount of them are exposed to you), but when they get to large levels, it can be very dangerous. Your cells are being mutated and killed faster than your body can replace them.

      Interestingly, bread, microwaves, and even people, emit radioactive rays! However, it is such a small amount that it's not dangerous.
      (12 votes)
  • mr pants teal style avatar for user AA
    What are some ways that make cancer easier for you to get? Also is there different types?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • primosaur ultimate style avatar for user Thiam
    have scientists thought of taking the DNA of a normal cell, and put it in a solution that ends up replacing every DNA in every cell with the one from the normal cell?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • leafers ultimate style avatar for user ZEDZANO
      This could work in theory, but the problem is that you would need to somehow destroy the DNA of the cancerous cells without killing healthy cells or destroying the DNA you are trying to insert. Additionally, the human genome is approximately 3.2 billion base pairs which makes it hard to do something like this.

      There have been theories about the use of gene editing tools such as CRISPR to make small changes to the genome of cancerous cells so that they stop their growth and can be dealt with by the immune system, but this is also difficult to do in adult human cells. At the moment there are treatments such as radiation that can be used to damage the DNA of cancerous cells in targeted areas and potentially pair up with other treatments such as chemotherapy, surgery, or immunotherapy in order to fight against the cancer.
      (7 votes)
  • leaf green style avatar for user pallaka Anjireddy
    How is cancer actually caused?
    (6 votes)
    Default Khan Academy avatar avatar for user
    • 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.
      (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.