DNA replication and RNA transcription and translation | Khan Academy
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DNA replication and RNA transcription and translation | Khan Academy


– [Voiceover] We’ve already
talked about how DNA’s structure as this double helix, this twisted ladder, makes it suitable for being the
molecular basis of heredity. And what we wanna do in this video is get a better appreciation
for why it is suitable, and the mechanism by which it is the molecular basis for heredity. And we’re gonna focus
on a conceptual level, I’m not gonna go into all of the, I guess you could say biochemical details. Really just give you the
conceptual idea of what happens. So right over here this
could be a fragment of DNA, I have, what, I have– This is eight base pairs depicted. And just to be clear,
and we talked about this in the introductory video to DNA, DNA is much more than, you
know, a handful of base pairs. The DNA molecule can be tens
of millions of base pairs long. So for example this might be a section of a much longer molecule, so the much longer strand of DNA, and even there I’m probably
not giving justice to it. But this might just be this
very, very small section, let me do this in a different color, this little section right over here, zoomed in. So once again it might
be part of a molecule that has not seven or eight base pairs, but might have 70 million base pairs. So just like that. So let’s understand what a
molecular basis of heredity would need to do. Well first of all it would
need to be replicable. Or we would need to be
able to replicate it. As a cell divides, the two new cells would want to have the same genetic material. So how does DNA replicate? And this process is called replication. And we covered this in the
introduction video as well, but it’s nice to see
the different processes next to each other. And replication, you can imagine taking either splitting these
two sides of the ladder, and actually let’s do that. So let me copy and paste, so if I take that side right over there, so let me copy and then paste it. And then there we go, a little bit of it is
dropping below the video but I think that serves the purpose. And then let’s copy and
paste the other side. So let me select that. And then I copy and then I paste, and it’s just like that. And so you can imagine if
you were to split these, these things you could call
them two sides of the ladder, that either side could be used
to construct the other side. And then you would have two strands, two identical strands of the DNA. And so let’s see what
that actually looks like. So let me get my pen tool out now, let me deselect this,
get the pen tool out. It’s a new tool I’m using, so let me make sure I’m doing it right. Alright, so from this side, from this left side, or at least what we are
looking at as the left side, you can then construct another right side based on this information. A always pairs with T if
we’re talking about DNA. So adenine pairs with
thymine just like that. Thymine pairs with adenine Let me do that a little bit neater. Thymine pairs with adenine, guanine pairs with cytosine, cytosine pairs with guanine, falling a little bit down here. And just like that I was able to construct a new right hand side
using that left hand side. So maybe I’ll do the new sugar phosphate backbone in yellow. And we can do the same thing here using the original right hand side. So using the original right hand side, once again the T is paired with the A, let me do that in adenine’s color. So we have an adenine and thymine, adenine and thymine, adenine and thymine. Thymine pairs with adenine, so thymine, adenine. Thymine, adenine. Guanine pairs with cytosine. And then cytosine pairs with guanine. So cytosine just like that. And so you can take half
of each of this ladder, and then you can use it to
construct the other half, and what you’ve essentially done is you’ve replicated the actual DNA. And this is actually a
kind of conceptual level of how replication is done before a cell divides and replicates, and the entire cell duplicates itself. So that’s replication. So the next thing you’re
probably thinking about, “Okay, well it’s nice to be
able to replicate yourself “but that’s kind of useless
if that information can’t be “used to define the organism in some way “to express what’s actually happening.” And so let’s think about how the genes in this DNA molecule
are actually expressed. So I’ll write this as “expression”. And actually that warrants
a little bit of a detour because you hear sometimes the words DNA and chromosome and gene used somewhat interchangeably, and they are clearly related, but it’s worth knowing what is what. So when you’re talking about DNA you’re talking literally
about this molecule here that has this sugar phosphate base and it has the sequence of base pairs, it’s got this double helix structure, and so this whole thing this
could be a DNA molecule. Now when you have a DNA molecule and it’s packaged together
with other molecules and proteins and kind of
given a broader structure, then you’re talking about a chromosome. And when you’re talking about a gene, you’re talking about a section of DNA that’s used to express a certain trait. Or actually used to code for
a certain type of protein. So for example this could be, this whole thing could be a strand of DNA, but this part right over,
let’s say in orange I’ll do it, this part in orange right
over here could be one gene, it might define information for one gene, it could define a protein, this section right over here could be used to define another gene. And genes could be anywhere from several thousand base pairs long, all the way up into the millions. And as we’ll see, the way that a gene is expressed, the way we get from the
information for that section of DNA into a protein which is
really how it’s expressed, is through a related molecule to DNA, and that is RNA. Actually let me write this down. RNA. So RNA stands for ribonucleic acid. Ribonucleic acid, let me write that down. And so you might remember that DNA is deoxyribonucleic acid, so the sugar backbone in RNA is a very similar molecule, well now it’s got its oxy, it’s not deoxyribonucleic acid, it’s ribonucleic acid. The R, let me make it clear
where the RNA come from, the R is right over there, then you have the nucleic, that’s the n, and then it’s a, acid. Same reason why we call
the DNA nucleic acid. So you have this RNA. So what role does this
play as we are trying to express the information in this DNA? Well the DNA, especially if we’re talking
about cells with nucleii, the DNA sits there but
that information has to for the most part get
outside of the nucleus in order to be expressed. And one of the functions that RNA plays is to be that messenger, that messenger between
a certain section of DNA and kind of what goes on
outside of the nucleus, so that that can be translated
into an actual protein. So the step that you go from DNA to mRNA, messenger RNA, is called transcription. Let me write that down. And what happens in transcription, let’s go back to looking at
one side of this DNA molecule. So let’s say you have
that right over there, let me copy and paste it. So there we go, actually
I didn’t wanna do that. I wanted the other side. So actually I think I’m on the
wrong, let me go back here. And so let me copy and then let me paste. There we go. So let’s say you have
part of this DNA molecule, or you have 1/2 of it just like we did when we replicated it. But now we’re not just trying
to duplicate the DNA molecule, we’re actually trying to create a corresponding mRNA molecule. At least for that section of, at least for that gene. So this might be part of a gene Actually whoops, let me make
sure I’m using the right tool. This might be part of a
gene that is this section of our DNA molecule right over there. And so transcription is a
very similar conceputal idea, where we’re now going to construct a strand of RNA and specifically mRNA ’cause it’s going to take that information outside of the nucleus. And so it’s very similar except for when we’re talking about RNA, adenine, instead of pairing with thymine, is now going to pair with uracil. So let me write this down, so now you’re gonna have
adenine pairs not with thymine but uracil. DNA has uracil instead of the thymine. But you’re still going to have
cytosine and guanine pairing. So for the RNA and in this case the mRNA that’s going to leave the nucleus A is going to pair with U, U for uracil, so uracil, that’s the
base we’re talking about, let me write it down, uracil. Thymine is still going
to pair with adenine, just like that. Guanine is gonna pair with cytosine, and cytosine is going
to pair with guanine. And so when you do that, now these two characters can detach, and now you have a single strand of RNA and in this case messenger RNA, that has all the information on that section of DNA. And so now that thing
can leave the nucleus, go attach to a ribosome, and we’ll talk more about
that in future videos exactly how that’s happened, and then this code can be used to actually code for proteins. Now how does that happen? And that process is called translation. Which is really taking
this base pair sequence and turning it into an
amino acid sequence. Proteins are made up of
sequences of amino acids. So translation. So let’s take our mRNA or this
little section of our mRNA, and actually let me draw it like this. Now let’s see, I have it is U A C, so it’s gonna be U A C then U U then A C G okay? And then we have an A, let me make sure I change
it to the right color. We have an A there, and
then we have this U U A, C G, alright, now let me put a C right over there, I’m just taking this and
I’m writing it horizontally. I have a C here, not a G, it’s a C. And then finally I have a G. And of course it’ll keep
going on and on and on. And what happens is
each sequence of three, and you have to be very
careful where it starts, and so this is in some ways
a delicate and surprising, but at the same time
surprisingly robust process, every three of these bases
code for a specific amino acid. And so three bases together, so these bases right over here, these I guess you could
say this three letter word or this three letter sequence, that’s called a codon. And this is going to be the next codon. And we actually haven’t drawn
the next codon after that ’cause we need three bases
to get to the next codon. And how many possible codons do you have? Well you have one of four bases and you have them in
three different places, so you have four times four times four, possible codon words
I guess you could say. And four times four times four is 64. So you have 64 possible codons. Which is good because you
have 20 possible amino acids. So this is overkill and
allows codons to be used for other purposes as well. And they also, you might
have more than one codon coding for the same amino acid. So you have 64 possible codons that need to code for 20 amino acids. And so this codon right
over here with the ribosome, and we’ll talk more
about how that happens, can code for amino acid 1. So let me just write it
here, this is amino acid 1. And actually this amino
acid is brought to here, they’re actually matched
together by another type of RNA, this is mRNA we’re talking
about right over here. This is mRNA, but there’s
another type of RNA called tRNA that essentially brings these
two characters together. So the tRNA, and I’m just gonna, it’s
got some structure here, I’m not drawing it completely right, but it’s going to match right over here, where maybe it has an A, a
U, and a G right over here and on this end it was
attached to this amino acid, and so it matches them together. And then they’re gonna have another tRNA that might attach to amino acid 2, which I will do in purple, and that just happens to coincide with, so it can complement right over here, so it attaches in the right place, so it’s A A U right over here, this tRNA. And so it’ll construct the
sequence of amino acids. And as you put these amino acids together, then you’re actually
constructing a protein. So protein is essentially a bunch, a sequence of these
amino acids put together. And these proteins are
essentially the molecules that run life for the most part. Obviously you know if you eat an animal it’s going to be made up of
fat and sugars and proteins, but the proteins are the
things that actually do a lot of the whether they’re enzymes,
whether they’re structural, the muscle is formed from proteins, these are the things, and I’m just drawing a
small segment of them, they could be thousands or
more of these amino acids long. And they kind of form these
incredibly complex shapes and they have all of these functions. This is what’s kind of
doing the work of life. And this for the most part, and this is kind of how the
information for life is stored.

About James Carlton

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100 thoughts on “DNA replication and RNA transcription and translation | Khan Academy

  1. why cant professors slides be as understanding and clear as khans academy. i understood everything from here. smh wasting my money paying no good professors.

  2. I am from Germany and i could not find a good video in german who describes RNA transcription as good as you did. So, thank you very much for saving my ass😅 I am doing a training as a nurse and in a week i am writing my first exam.

  3. Wait how did he arrive at 64 possible codons at 12:54? Sorry if this is a dumb question- I've been studying for 5 hours dont jugde me

  4. Ridiculous discussion , especially concerning sense ( coding )( non template ) , & antisense ( non coding )( template ) strands , you didn't mention that !!!

  5. this 15 minute video has helped me more than my bio teacher, spending 135 minutes (3 class periods) on. this. THANK YOU!

  6. so life isn't about the DNA, it's about the work performed by the proteins generated from the sequence of amino acids encoded in the nucleotides. Got it.

  7. Khan Academy does a great work. I have proposed them to school boys & girls who really want to learn with a simple & excellent explanation at their own pace which may never happen at usual shools. They have my respect.

  8. Wait so what about at the end the last codon… does anyone know what happens to CG? and what it translates to?

  9. What it takes my professor weeks to teach me (and I still struggle to understand) is suddenly made so much more clearer in 15 minutes. Great stuff.

  10. I feel like this guy enjoys drawing a little too much, he just kinda of takes it a bit too far and draws way to slow..

  11. Quite an impressive method to teach the concepts and make them more understandable. This is really uncomplicated and helpful.

  12. Ur concepts are good. Though the way ur expressing it to the viewers are not clear i hope. Which means try to just tell the points which u wanna tell rather than speaking much and also try to be slow and explicit

  13. how do you fit all this knowledge of all these videos in that brain, it’s like you are a computer that stores not only main ideas but details.

  14. all this lecture has is A pairs with T and G pairs with C. This little part on a squiggly line is a gene. A pairs with U in RNA. And also nice colors

  15. I’m glad to know about how this is able to get me through a lot in such a short amount of time. Considering how my first trimester finals are coming up, this will definitely help me pass Biology

  16. You're a great help man!! Thank you I really appreciate all your work you do on here and also on the whole khanacademy website 💜

  17. I swear going to school for 4-5 hours a week and this video explain that in 15mins….education is so over rated. why can we have instructors like this In schools?

  18. I had A very real dream 2 weeks ago ' were I was in my body watching my cells being zapped bi mini lazers ' and wen I woke I heard DNA to RNA "… Wondering watt it meant ' thanks "❤️❤️❤️⚔"

  19. I do not know who this guy is but he has helped me SO MUCH!! SO MANY TIMES! I love his voice, he explains everything so well. I love being able to rewind when I need to try to figure it out no matter how many times it takes me.

  20. Yeah! I really like this application "Khan academy" but this video is not so effectual and not give satisfatory information.

  21. I got a better way to remember the DNA letters and what it pairs to-
    Guanine-cytosine = George cloony
    Adenine-thymine = Ali and Tyson

  22. It is great that you have new bases pairing up with the bases of the two sides of the DNA molecule that has just split; however the new bases must come from somewhere. Can anyone say where? Thin air is certainly not an option.

  23. Your voice is so calming, and relaxing which helped me calm down! I had all kinds of panic attack before finals 😭❤️ Thank you I understood everything! Why is college not teaching us but Khan academy is! Rather pay them! 🙄

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