DNA | Biomolecules | MCAT | Khan Academy
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DNA | Biomolecules | MCAT | Khan Academy


– [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.

32 thoughts on “DNA | Biomolecules | MCAT | Khan Academy

  1. Ahhhh man if you would have posted this like just 3 weeks ago, I would have gotten better than a C on my bio test. I can still use this for finals coming up but either way, I just want to say that you're the best Mr. Khan, I don't know what I'd do without you and Patrick JMT's video's. You've both helped me out so much through this semester. Y'all the real mvps :')

  2. Hey guys so it's pretty hard to get noticed on youtube but I was really hoping that you guys would give my channel a look. I make educational videos in hopes that it will help students with their classes. You don't even have to subscribe if you don't want to but it would mean a lot to me if you took a look! Thanks so much for your time!

  3. This has sparked more curiosity in me to understand, what are gene mutations and how they might be triggered/caused ; are gene mutations just a tool for natural selection !? ; Can gene mutations be "corrected" ; If gene mutations are the basis of all cancers, then rising cancer cases throughout the world hints at rising number of gene mutations … ?

  4. OMG thank you for mentioning Rosalind Franklind and Wilkins!!!! I think She ought to be nominated for a postumous nobel prize.

  5. Hey Sal, love all the video's and keep up the great work at Khan Academy! At around 12:13 you draw a sequence of basepairs to illustrate how bases always find a complementary nucleotide. I took the sequence you made up, and ran it through NCBI's free BLAST tool(Basic Local Alignment Search Tool, it's like a "google" search for known DNA sequences), and it turns out that random sequence of basepairs you used is known to exist in over 100 different DNA and mDNA sequences in over 25 different organisms, including the bacteria staphylococcus pyogenes,, macaca fascicularis (the crab-eating macaque), peromyscus maniculatus (the deer mouse) and solanum tuberosum (the common potato)!!!

  6. in one of your other video you said human genome has 3 billion base pairs but in this one you told that there are 6 billion. which one is correct?

  7. Sal, you blessed mystical unicorn you, thank you for blessing us with your knowledge

    – from every kid in america

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