DNA Replication (Updated)

DNA Replication (Updated)


Captions are on! Click CC at bottom right to turn off. DNA. We talk about it so much—it is the ultimate
director for cells and it codes for your traits. With a molecule that has a function like that,
it makes sense that when you make another cell—like in cell division—you would also
need to get more DNA into the new daughter cell. And that introduces our topic of DNA replication,
which means, making more DNA. First, let’s talk about where and when. First where—well if it’s in a eukaryotic
cell, it occurs in the nucleus. However, remember, not all cells have a nucleus. Such as prokaryotic cells. They don’t have a nucleus. Still both prokaryotic and eukaryotic cells
do DNA replicationm but there’s some differences between the two that this clip doesn’t go
into. Next, when. When does this happen? Well a cell is going to need to do this before
it divides so that the new daughter cell can also get a copy of DNA. To get specific, in a eukaryotic cell, that’s
going to be before mitosis or meiosis in a time known as interphase. I think DNA replication would actually make
a great video game. Still waiting for that to be invented. I’m going to introduce the key players in
DNA replication so that you can get some background information. Now, remember, these are just some major key
players. There’s a lot to this process. Many of the key players are enzymes. In biology, when you see something end in
–ase, you might want to check as it’s very possible that it’s an enzyme. Enzymes have the ability to speed up reactions
and build up or break down the items that they act on. So here we go with the key players. Helicase- the unzipping enzyme. If you recall that DNA has 2 strands, you
can think of helicase unzipping the two strands of DNA. Helicase doesn’t have a hard time doing
that. When unzipping, it breaks through the hydrogen
bonds that hold the DNA bases together. DNA Polymerase- the builder. This enzyme replicates DNA molecules to actually
build a new strand of DNA. Primase- The initializer. With as great as DNA polymerase is, DNA polymerase
can’t figure out where to get started without something called a primer. Primase makes the primer so that DNA polymerase
can figure out where to go to start to work. You know what’s kind of interesting about
the primer it makes? The primer is actually made of RNA. Ligase- the gluer. It helps glue DNA fragments together. More about why you would need that later on. Now, don’t feel overwhelmed. We’ll go over the basics of this sequence
in order. But remember, like all of our videos, we tend
to give the big picture. There are always more details and exceptions
to every biological process that we can’t include in such a short video. DNA replication starts at a certain part called
the origin. Usually this part is identified by certain
DNA sequences. At the origin, helicase (the unzipping enzyme)
comes in and unwinds the DNA. Here’s the thing though: you don’t want
these strands to come back together. So SSB Proteins (which stands for single stranded
binding proteins) bind to the DNA strands to keep them separated. And topoisomerase—I always have to slow
down when I say that enzyme’s name—keeps the DNA from supercoiling. Supercoiling might sound super and it can
be when you’re trying to compact DNA, but it’s something that needs to be controlled
during DNA replication. Supercoiling can involve an over-winding of
the DNA, and you need the DNA strands to be separated for the next steps. Primase comes in and makes RNA primers on
both strands. This is really important because otherwise
DNA polymerase won’t know where to start. In comes DNA Polymerase. Ok, before we go on, remember how we said
DNA has two strands? They’re not identical; they complement each
other. In our video that covers DNA structure, we
talk about how the bases pair together with hydrogen bonds. The base adenine goes with base thymine and
the base guanine goes with the base cytosine. These strands are also anti-parallel so they
don’t go in the same direction. What do we mean by direction? Well, with DNA, we don’t say North or South. We say DNA either goes 5’ to 3’ or 3’
to 5’. What in the world does that mean? Well, the sugar of DNA is part of the backbone
of DNA. It has carbons. The carbons on the sugar are numbered right
after the oxygen in a clockwise direction. 1’, 2’, 3’, 4’ and 5.’ The 5’ carbon is actually outside of this
ring structure. Now you do the same thing for the other side
but keep in mind DNA strands are anti-parallel to each other. So let’s count these—again, clockwise
after the oxygen. 1’, 2’ 3’, 4’ 5’. And the 5’ is out of this ring. This strand on the left runs 5’ to 3’
and the strand here on the right here runs 3’ to 5’. We’ll explain why all that matters in a
moment. So let’s take that knowledge there and look
at DNA replication here. In this image, I labeled the top original
strand 3’ to 5’. I labeled this bottom original strand 5’
to 3’. That’s the original DNA that is going to
be replicated. DNA is unwinding here thanks to helicase. In this example, it will keep unwinding in
this direction. Primase places primers. DNA polymerase is building the new strands. Now the thing about DNA polymerase is, when
it’s building a new strand, it can only build the new strand in the 5’ to 3’ direction,
meaning it adds new bases to the 3’ end on the new strand. See how it’s being built in the 5’ to
3’ direction? This one is called the leading strand. But, take a look down here. So DNA polymerase once again is building a
new strand in the 5’ to 3’ direction. But there’s a bit of a problem here. See, as DNA unwinds, because DNA polymerase
can only build the new strand in the 5’ to 3’ direction, it has to keep racing up
here next to where this unwinding is happening. You can see why then this new strand is known
as the lagging strand. On this lagging strand, primers have to keep
being placed in order for DNA polymerase to build. These fragments that result are known as Okazaki
fragments. Primers have to get replaced with DNA bases
since the primers were made of RNA. Ligase, the gluing enzyme as I like to nickname
it, has to take care of the gaps between the Okazaki fragments, sealing them together. At the end of this replicating, you have two
identical double helix DNA molecules from your one original double helix DNA molecule. We call it semi-conservative because the two
copies each contain one old original strand and one newly made one. One last thing. Surely you have had to proofread your work
before to catch errors? In this process, you don’t want DNA polymerase
to make errors. If it matches the wrong DNA bases, then you
could have an incorrectly coded gene…which could ultimately end up in an incorrect protein—or
no protein. DNA polymerase is awesome; it has proofreading
ability. Meaning, it so rarely makes a mistake. Which is a good thing. So, remember how we said there is far more
detail to this process to explore? The detailed understanding of DNA replication
has led to some lifesaving medical treatments that can stop DNA replication in harmful cells
including pathogenic bacteria or human cancer cells. We encourage you to explore beyond the basics;
check out the further reading suggestions in the video details to explore more! Well, that’s it for the Amoeba Sisters and
we remind you to stay curious.

100 thoughts on “DNA Replication (Updated)

  1. We hope you like our newly updated DNA replication vid! So what's different? You will find the script to be almost the same, except with a little more detail: we included topoisomerase activity and better explained leading and lagging strands. (Again, remember, there is still much detail to explore beyond this video!) Our art has also improved as we continue to practice and no MS Paint this time ha! You can see our milestones here: https://www.amoebasisters.com/milestones.html Also, no worries, we try to not delete old videos. The old DNA replication video, if you prefer it, is still here: https://youtu.be/5qSrmeiWsuc

  2. Hello! This video was really helpful but I still have a question lol! I really don’t understand the whole 5’ to 3’ thing because in the video the little polymerase is moving from the 3’ towards the 5’? Maybe I’m just being thick lmao but I’m so confused 😂

  3. Is that exactly how it works what I mean is do it perfectly works in asvery calculated manner like u r showing or just a chemical reaction that can make mistakes like glue molecules reaching to unzipping molecules I don’t know if this makes sense

  4. always out here watching your youtube videos even though english aint my first language haha they just make more sense lol thank you!!

  5. Amazing video!! Just a mistake I think the Okazaki fragments are only the DNA portion (not the whole thing with the primer). Thanks!!

  6. Thank you so much for making these videos. They help me so much in understanding what's going on in class. I really appreciate these videos. Please continue to make more!

  7. Your videos really help me with my exam, thank you so much for your hard work and dedication 💖

    Can you make videos about evolution, heredity pattern, and mutation? I will be really happy if you can make it😁

  8. please keep including more links to more detailed information, after the "cliffnotes" version it makes learning the detailed version digestable. thank you love you guys

  9. Are we sure that the primers of the Okazaki fragments are not substituted by DNA polymerase with deoxyribonucloeotides that come from the polymerization/enlongation of the nearby DNA fragment? That would change the point that ligase acts. Thanks for the wonderful video by the way!

  10. I just want to thank you for making these awesome videos!
    I am taking Microbiology right now, and your videos help me understand some concepts like prokaryotic cells vs eukaryotic cells.
    Please keep making these videos, they are so helpful! 🙂
    P.S. Can you make one about Gram-positive bacterias vs Gram-negative bacterias?

  11. It would be awesome if you make a video talking about RNA synthesis. I just read the theory for DNA replication and it was really difficult to visualized, but after your video, I can have a new read and understand it different. Now I am reading about RNA and its a kind of different, so I insist, it would be awesome if you make a video about RNA synthesis. By the way, you should create that game, that would be a perfect tool for people to learn and enjoy at the same time.
    ¡Great video!

  12. I've probably gone over DNA replication a good 4 times in college and NOW with this video is when I come to FULLY understand why there is a lagging strand lol

  13. I’m taking a break from my studying! And your video is very entertaining hehe helps a lot. I also like the drawings, really impressive and the cartoonish characters! Thanks for posting videos like this.

  14. Thank you Amoeba Sisters! You saved me! I don't understand what my book is telling me, but you just made it so clear! Thank you!

  15. Sukei! This is single-handedly the best video I have seen explaining in common-sense terms and simple graphics how and why the lagging strand must have Okazaki fragments inserted due to the directionality of helicase and the unwinding of the strands. My students and I are much appreciative of having a visual representation that breaks this down simply. You guys are amazing!

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