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


DNA, or deoxyribonucleic acid, is the molecular basis for inheritance, discovered in the mid-1800s and understood in 1953 with the double helix structure. Composed of a sugar-phosphate backbone and base pairs (adenine-thymine, guanine-cytosine), DNA stores genetic information, replicates, and expresses traits. The human genome contains 6 billion base pairs across 46 chromosomes, making it a compact and efficient information storage system. Created by Sal Khan.

Want to join the conversation?

  • piceratops ultimate style avatar for user Nathan Shapiro
    Does DNA have any "limits" to length and size? I know it has to be small enough to fit in a cell, but as long as it's small enough to do so, does it have a limit to how big it can get? If it does, the size would depend on what type of cell it is, right?
    (38 votes)
    Default Khan Academy avatar avatar for user
  • mr pants teal style avatar for user Wrath of Academy
    Does the size of the organism affect how many base pairs it has?
    (18 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user Reece M
      All organisms have the same composition of nitrogenous bases. The nitrogenous bases are typically classified into two groups, pyrimidine and purine. Pyrimidines include uracil, thymine, cytosine consisting of a single ring structure. Purines include adenine and guanine consisting of a double ring structure. While there is no variation in base pairs in an organism, there is variation in the number of chromosomes in different organisms but this is not a result of the size of the organism. In a sense the size of the organism will affect the number of base pairs due to the organism having more cells containing DNA but the same base pairs will be used repeatedly in all organisms. This link shows the variation in the number of chromosomes in different organisms.
      (28 votes)
  • starky tree style avatar for user Worthee Ikah
    Does Adenine have to pair with Thymine and Guanine have to pair with Cytosine? Would there be some problems with the genes if say Thymine pairs with Guanine?
    (17 votes)
    Default Khan Academy avatar avatar for user
  • piceratops ultimate style avatar for user Nathan Shapiro
    Are genes only determined by protein synthesis?
    (13 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user amuro12
    Why are there only four types of nucleobases in the DNA? Why is there uracil in RNA, but thymine in DNA? Also, I read in a magazine that scientists created a new, artificial nucleobase. Could this be added to the natural DNA, and if so what would happen?
    (13 votes)
    Default Khan Academy avatar avatar for user
    • male robot donald style avatar for user noah reeves
      To have a structural piece of DNA or RNA the nucleotides consist of a nucleic acid (differing Uracil in RNA from Thymine in DNA) a deoxygenized sugar (DNA) or oxygenized sugar (RNA) and a monophosphate (PO4) The bases are the 3 structures (nucleic acid, ribose and phosphate) bond together with a strong bond called a phosphodiester bond. The bases (A---T) bond through hydrogen bonding. In transcription RNA is used which used Uracil instead of Thymine during the synthesis of DNA the uracil is methylated by folic acid which converts it to thymine. Methylation is a defense mechanism from an enzyme found in some bacteria and viruses (nucleases) and all DNA are methylated. Uracil is unique and can bond to other uracils in RNA giving it the ability to differentiate its structure for survival purposes. An artificial nucleobase sounds cool! I would love to see what types of structures and processes it could do. If it were added to natural DNA the structure would alter and the DNA would create different amino acid sequences leading to different proteins made and different life forms essentially. I would love to see the research on it.
      (20 votes)
  • duskpin ultimate style avatar for user valentine7Ethan
    How do mutations happen if Adenine only pairs with Thymine and Guanine only pairs with Cytosine? It seems like the mutation would have to be the same as the correct genes for it to work. Is there some exception?
    (10 votes)
    Default Khan Academy avatar avatar for user
    • male robot johnny style avatar for user Saksham Kaushal
      Great Question! During DNA Replication and Repair there are MUTATIONS/ERRORS in coding, and it is of several types: (1) the substitution of one base pair for another, (2) the deletion of one or more base pairs, and (3) the insertion of one or more base pairs.
      Among them, (1)The substitution of one base pair for another is a common type of mutation. But in contrast after DNA Replication even if there is an error,
      DNA polymerase II comes into action and replaces it with the correct base pair. So, there is a very very very less chance of Mutation. Mostly, Mutation occurs by Sex-linked Inheritance. Hope you found it Useful. Thanks!
      (17 votes)
  • eggleston green style avatar for user Stockfish9
    I heard that the strength of the bonds within the bases differ depending on the different chemicals. For example, Adenine and Thymine have a strong bond, while Guanine and Cytosine have moderately strong bonds. Is that true?
    I also heard that Adenine and Guanine could bond, although their connection is weak. I learned this when coding mRNA, so this information may only apply to RNA. Since the information seems similar, is the same true for DNA?
    (11 votes)
    Default Khan Academy avatar avatar for user
    • piceratops seed style avatar for user Eesha Sachdeva
      I've heard similar things that when coding mRNA, it may be possible for A and G to hybridize (not bond) if there are lots of G-C bonds and A-T bonds nearby. The strength of the surrounding "correct" bonds outweighs the "mistake" pair.

      However, in DNA, because it is so important for DNA to be accurate for cell replication purposes, there are many mechanisms that will ensure that even if an A-G pairing does happen, it will be corrected to a C-G pairing. In DNA, correctness is really important because DNA is what is transmitted to offspring cells. Whereas, in RNA, correctness isn't as important because a bad mRNA transcript can easily be degraded, or a badly made protein can easily be degraded.
      (7 votes)
  • piceratops ultimate style avatar for user Nathan Shapiro
    In mitochondrial DNA, is it still made up of adenine, thymine, guanine, and cytosine?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user logan.cripssorger
    how come adenine doesn't pair up cytosine ?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • leafers seed style avatar for user PCMSIII
      DNA nucleotides are held together by hydrogen bonds that span the gap between the two strands. Adenine and Thymine have structures that allow for two hydrogen bonds to be formed across the gap. Guanine and cytosine have structures that allow for three H bonds to be formed. Due to sterics, A and C are not compatible, as there would be a bulge in the DNA strand, and the maximum # of H-bonds would not be permitted.

      Do a search for Adenine + Thymine images, and you'll see the bonds and specific shapes that I mention.
      (8 votes)
  • piceratops ultimate style avatar for user Adrian
    Is it true that Watson and Crick essentially 'stole' Rosalind Franklin's work? Because my encyclopedia on tech says they DID steal Franklin's work. Is my encyclopedia correct?
    (4 votes)
    Default Khan Academy avatar avatar for user

Video transcript

- [Voiceover] As long as human beings have been around I could imagine that they have noticed that offspring tend to have traits in common with the parent. For example, someone might have told you, "Hey, you walk kind of like your dad," or, "Your smile is kind of like your mom," or, "Your eyes are like one of your uncles "or your grandparents." And so there's always been this notion of inherited traits. But it wasn't until the 1800 that that started to be studied in a more scientific way with Gregor Mendel the father of genetics. But even then, even Mendel who was starting to understand the mechanisms or he was trying to understand how inheritance happened, then you even could start to breed certain types of things. Even he didn't know exactly what was the molecular basis for inheritance. And the answer to that question wasn't figured out until fairly recent times, until the mid 20th century. Not until the structure of DNA was established by Watson and Crick and their work was based on the work of many others especially folks like Rosalind Franklin who essentially provided the bulk of the data for Watson and Crick's work, Maurice Wilkins and many, many, many other folks. But it's really the structure of DNA that made people say, "Hey, that looks like "the molecule that's storing the information." Just to be clear, DNA wasn't discovered in 1953. DNA was discovered in the mid 1800s. It was this kind of this molecule that was inside of nuclei of cells. And for some time people said, "Maybe this could be a molecular basis of inheritance." You could imagine what you would need to be a molecular basis of inheritance. It would have to be a molecule or a series of molecules that could contain information, that could be replicated, that could be expressed in some way. But it wasn't until 1953 wherein this double helix structure of DNA was established. The people said, "Hey, this looks like our molecule." So first, let's just talk about the structure here and then actually we'll talk about where this name, DNA, deoxyribonucleic acid comes from. And then we'll talk a little bit about why this structure lends itself well to something that stores information, that can replicate its information and that could express its information. We might go in depth on the expression of information in future videos. So this structure right over here and this is a visual depiction of a DNA molecule. You can view this as kind of a twisted ladder. It has these two, I guess you could say sides of the ladder that are twister. That is one side right over there and then it is another side. There is another side right over here. And in between those two sides or connecting those two sides of that twisted ladder you have these rungs. And these rungs are actually where the information, the genetic information is I guess you could say stored in some way. Because these rungs it's a sequence of different bases. And when I say bases, you're gonna say wait. This says acid, why are you saying bases right over here? Well, the word deoxyribonucleic acid comes from the fact that this backbone is made up of a combination of sugar and phosphate. And the sugar that makes up the backbone is deoxyribose. So that's essentially the D in DNA. And then the phosphate group is acidic and that's now where you get the acid part of it. And nucleic is, hey this was found in nuclei of cells. It is nucleic acid. Deoxyribonucleic acid. It is actually mildly acidic all in total but for every acid it actually also has a base, and those bases form the rung of the ladders. And actually each rung is a pair of bases and as I said, that's where the information is actually stored. Well what am I talking about? Well let me talk about the four different bases that make up the rungs of a DNA molecule. So, you have adenine. Adenine. And so for example, this part right over here. This section of that rung might be adenine. Maybe this right over here is adenine. This right over here. Remember, each of these rungs are made up by it's a pair of bases. And that might be adenine. Maybe this is adenine and I could stop there, I mean I'll do a little more adenine. Maybe that's adenine right over there. And adenine always pairs with the base thymine. So let me write that down. So adenine pairs with thymine. Thymine. So, if that's an adenine there then this is going to be a thymine. If this is an adenine then this is going to be a thymine. Or if I drew the thymine first, well say, okay it's gonna pair with the adenine. So this is going to be a thymine right over here. This is going to be a thymine. If I were to draw this, this would be a thymine right over here. Now the other two bases, you have cytosine which pairs with guanine or guanine that pairs with cytosine. So guanine and we're not gonna go into the molecular structure of these bases just yet, although these are good names to know because they show up a lot and they really form kind of the code, your genetic code. Guanine. Guanine pairs with cytosine. Guanine and cytosine. Cytosine. So actually if this is, let's say there's some cytosine there, let's say cytosine right over here. Maybe this is a cytosine, maybe this is cytosine, maybe this is cytosine, this is cytosine and maybe this is cytosine. Then it always pairs with the guanine. So, let's see, this is guanine then and this will be guanine. This is guanine, this is guanine. I actually didn't draw stuff here. This is guanine, I didn't say what these could be but these would be maybe the pairs of they could be adenine-thymine pairs and it could be adenine on either side or the thymine on either side, and they could be made of guanine-cytosine pairs where the guanine or the cytosine is on the other side. Actually just to make it a little bit more complete let me just color in the rungs here as best as I can. So those are guanines so they're gonna pair with cytosine. Pair with cytosine, pair with cytosine. When you straw in this way you might start to see how this is essentially a code, the order of which the bases are... I guess the order in which we have these or the sequence of these bases essentially in code the information that make you, you, and you could be. Well how much of it is nature versus nurture and when people say nature, you know, it's literally genetic, and that's an ongoing debate, an ongoing debate but it does code for things like your hair color. When you see that your smile is similar to your parents it is because that information to a large degree is encoded genetically. It affects a lot of what makes you you and actually not even just within a species but also across species. Humans have more genetic material in common with other humans than they do with say a plant. But all living creatures as we know them have genetic information. This is the basis by which they are passing down their actual traits. Now you might be saying well, how much genetic information does a human being have? And the number will either disappoint you or you might find it mind-boggling. The human genome and every species has a different number of base pairs to large degree correlated with how complex they are although not always. But the human genome has 6,000,000. Sorry, not 6,000,000, 6,000,000,000. 6,000,000 would be disappointing, even billion might be disappointing. 6,000,000,000 base pairs. 6,000,000,000. 6,000,000,000 base pairs. And when you have your full complement of chromosomes and this is in most of the cells in your body and outside of your sex cells, the sperm or the egg cells. This is going to be spread over 46 chromosomes. 46 chromosomes or I guess you could say 23 pair of chromosomes. If you divide 6,000,000,000 by 46 you get a little over on average 100,000,000. I think it's a 100 and something million base pairs per chromosome. And some chromosomes are longer, actually the longest are over 200,000,000 and some might be shorter. That's just on average. Now this number might to some of you might be exciting. You're like, "I thought I was a simple creature. "I didn't know I was this complex." 6,000,000,000, that's a lot of base pairs. That feels like a lot of information. For others of you it might not feel so great. You might say, "Hey, wait I could store "this much information on a modern thumb drive "or on a hard disk. "I thought I was more unique than that." And of course we all are special and unique. You're gonna say 6,000,000,000 base pairs. I thought I was, you know, I was infinitely complex and whatever else. There's some arguments for that along some other directions, but this is the approximate length I guess you could say or the approximate size of the actual human genome. And when we talk about chromosomes and we'll talk about chromosomes in much more depth, imagine taking this zoomed in thing that you have right over here and you know, over here, I don't know how many we have, Like one, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19. We have about 20 base pairs depicted here. Imagine if you had about 200,000,000 of these base pairs and then you were to take this thing and you were to kind of coil it up into that thing is a chromosome. It is a chromosome and you're saying, "Wait, "I have that much information in "most of the cells of my body. "This thing must be incredible compact." And if you said that I would say, "Yes, you are correct." This, the radius, the radius of the DNA molecule is on the order of one nanometer. One nanometer which is a billionth of a meter. So you can start to assess kind of the scale of this thing. This is a very dense way to actually store information. But just to have an appreciation of and you might have seen it when I was coloring in on why the structure lends itself to being able to replicate the information or even to be able to translate or express the information. Let's think about if you were to take this ladder and you were to just kind of split all the base pairs. So, you just have 1/2 of them. So you essentially have half of the ladder. And so if you only have half of the ladder, you're able to construct the other half of the ladder. Let's take an example, let's say and I'll just use the first letter to abbreviate for each of these bases. Let's say you have some... So let's say this is one of the, this is the sugar phosphate backbone right over here. So this could be one of the sides. Let's say there's some adenine. Actually we do in the right color. So you got some adenine, adenine. Maybe some adenine right over here and maybe there's an adenine there. And maybe you have some thymine, thymine, maybe thymine right over here and then you have some guanine, guanine, guanine. And then let's say you have some cytosine and you have some cytosine. So with just half of this ladder I guess you could say, you're able to construct the other half, and this is actually how DNA replicates. This ladder splits and then each of those two halves of that ladder are able to construct versions of the other half, or versions of the other half are able to constructed on top of that, on top of that half. So how does that happen? Well, it's based on how these bases pair. Adenine always pairs with thymine if we're talking about DNA. So if you have an A there, you're gonna have a T on this end, T on this end. T's right all over here, T right over there. If you have a T on that end you're gonna have an A right over there. A, A. If you have a G, a guanine on this side, you're gonna have a cytosine on the other side. Cytosine, cytosine, cytosine. And if you have a cytosine you're gonna have a guanine on the other side. Hopefully that gives you an appreciation of how DNA can replicate itself. And as we'll see also how this information can be translated to other forms of either related molecules but eventually to proteins. And just to kind of round out this video, to get a real visual sense what the DNA molecule looks like or I guess a different visual depiction from this. I found this animated gif that, you know, if you haven't fully digested what a double helix looks like, this is it. And you see here, you see your sugar phosphate bases here. You see kind of the sugars and phosphate, the sugars and the phosphates alternating along this backbone, and then the rungs of the ladder are these base pairs. So this is one of the bases, that's the corresponding, that's this corresponding, I guess you can say partner. And you can see that along all the way up and down in this molecule. Very exciting.