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  • Hi. It's Mr. Andersen. Today I want to talk about DNA. Before we get into

  • transcription, translation, mitosis, meiosis, Mendelian genetics, all of these things are

  • based on DNA. And so we should have a real good understanding of what DNA is. Now this

  • animation here is just gorgeous as DNA spins and it gets this reputation for being a kind

  • of this mysterious magical kind of a molecule. But it's actually pretty simple compared to

  • proteins. It's pretty straight forward and so you should understand the parts of it.

  • And so basically the building blocks of DNA are nucleotides. And so just like carbohydrates

  • are made of sugars and protein is made of amino acids, the building blocks of nucleic

  • acids, both DNA & RNA, are going to be nucleotides. And so here's one nucleotide right here. A

  • simpler one if we were to look at basically building blocks of RNA, you've seen something

  • that looks like a nucleotide before, because if you take the adenine and a ribosugar and

  • add three phosphates to it then you have ATP. So you've seen something that looks a lot

  • like this. But a nucleotide has three parts to it. So part one is going to be this purply

  • part. And that's going to be what's called a nitrogenous base. And so in DNA there are

  • four types of nitrogenous bases. This one pictured here is called guanine, but there

  • are three other types in DNA. So you've got guanine. You've also have cytosine, thymine

  • and adenine. So when you see these letters in DNA, what they're really referring to is

  • this nitrogenous base. And the reason it's called a nitrogenous base, just look at all

  • the nitrogen that we have inside there. So that's a nitrogenous base. They come in two

  • different types. Some of the nitrogenous bases like this one are called purines. Purines

  • are going to have two of these carbon chains. The purines are going to be guanine. So that

  • would be one purine. And then the other one's going to be adenine. We'll also have some

  • that are a little simpler. They're not quite as big and those are going to be called pyrimidines.

  • Pyrimidines. Pyrimidines, the ones in DNA are going to be cytosine and thymine. And

  • I'll show you those in just a second. So the first part of a nucleotide is going to be

  • the nitrogenous base. The second part is going to be this sugar right here. And in DNA that's

  • called deoxyribose sugar. Now in RNA that's going to be a ribosugar, but in DNA it's going

  • to be a deoxyribo sugar. And the reason why it that if you were to look right here in

  • a ribosugar found in RNA there'd be a hydroxyl group coming off of it. But in DNA you're

  • missing that oxygen and so we call it de- or missing oxyribo, so deoxyribose sugar.

  • So that'd be the sugar right here. And then the third thing that's going to be found is

  • a phosphate group. And so a phosphate group, we're familiar with that from ATP, that's

  • going to be a phosphate group right here. And so those are the building blocks of nucleotides.

  • So let me get my writing out of the way and we'll go to the next slide. And so now this

  • is maybe a little bit more familiar when you're looking at the structure of DNA. And so this

  • right here, this nucleotide that we just kind of went over the parts of it, that nucleotide

  • is going to be found right here. Now one more thing I should mention about nucleotides is

  • that if you look up here on DNA there's a 3 prime end to DNA and then there's a 5 prime

  • end. And so what does that mean? Well that's going to refer to the sugar itself. And so

  • the sugar itself is going to have, since carbon is so ubiquitous or it's found everywhere

  • in organic material, we don't even draw the symbol on here. So there would be a carbon

  • right here, a carbon right here, a carbon here, a carbon here and then a carbon here.

  • And so we simply know number of those carbons. And so this carbon right here is called the

  • 1 prime carbon. This one is called the 2 prime carbon. This one right would be the 3 prime

  • carbon. This'd be the 4 prime carbon. And then this would be the 5 prime carbon right

  • here. And so the 3 prime carbon is going to be coming off this side and 5 prime coming

  • off the other side and so basically when you're looking at DNA this right here would be the

  • 3 prime carbon and this down here would be the 5 prime carbon. And so when you hear DNA

  • and the idea that DNA flows from 3 to 5 prime, well if you look at all of these deoxyribose

  • sugar with the oxygen on the top, they're gong to flow in this direction, from 3 prime

  • to 5 prime. But if you look at the other side of the DNA, it's going to go in the opposite

  • direction and so it's going to go into 3 prime to 5 prime. So DNA is said to be anti-parallel.

  • So it's going to flow in one direction on one side and it's going to flow in the opposite

  • direction. They're parallel to each other but they're flowing in opposite directions.

  • And so if we were to look across from the 3 prime end here, we're going to have a 5

  • prime end over here. Now why is that important? Well, when you're building DNA, when we're

  • going to add another nucleotide on. So here's one nucleotide. Here's another nucleotide,

  • when we're going to add another nucleotide we can only add it on the 3 prime end. And

  • so I could add a new nucleotide on this side, but I can't add it to the 5 prime end. And

  • so when we get to DNA replication, how DNA copies itself, that's going to be really,

  • really important. We could add one down here to this 3 prime end because this is going

  • to be our other nucleotide right here, so we could add one here, but we can't add it

  • to the 5 prime end. Okay. So enough with the 3 prime and the 5 prime. Let's talk about

  • some other of the large parts of DNA. If we look at the backbones, so the backbone is

  • going to be on this side, and on this side. And it kind of, DNA, looks like a ladder.

  • In other words if this is the backbone, then the rungs of the ladder are going to be the

  • nitrogenous bases that go right down the middle. But if we look at the backbone itself, it's

  • simply a deoxyribo sugar, a phosphate, a deoxyribose, phosphate, deoxyribose, phosphate, deoxyribose,

  • phosphate, deoxyribose. So the back is going to be the same with every nucleotide. And

  • if we look on the other side, the other side it's going to be phosphate, deoxyribose, it's

  • going to be the same thing on the other side. So the backbone is going to be relatively

  • boring. That's not too exciting. But if we look to the inside, that's where we're going

  • to have these purines and the pyrimidines, these nitrogenous bases. And if you look right

  • here, the adenine, which again is a purine, so it's got these two rings, that's going

  • to be connected using these hydrogen bonds to the pyrimidine on the other side. And if

  • we have a purine on this side, we're going to have a pyrimidine on the other side. And

  • there's base pairing. In other words, adenine is always going to bond to thymine and if

  • we look down here, guanine is going to bond to cytosine. Or this would be a cytosine and

  • a guanine. And this is going to be an adenine and a thymine. Now what do I mean by they

  • bond to each other? Well the bonds are going to be right here in the middle. So if you

  • look at DNA, the structure of DNA, everyone of these atoms is going to be connected to

  • every other atom by a covalent bond. Except if we look right down here in the middle.

  • If we look right down the middle, these are actually hydrogen bonds. Remember hydrogen

  • bonds are very weak, so these are relatively weak bonds that go right down the middle.

  • Why is that important? Well if I pull DNA in either direction like this, the DNA will

  • unzip in the middle and we're just breaking those hydrogen bonds. If we let go of it,

  • it'll just go right back together again because those hydrogen bonds are going to form. And

  • just like adenine is covalently bond to the nucleotide below it, it's hydrogen bond between

  • the two. And so that'll be super important in like when we're doing DNA replication or

  • when we're making messenger RNA. These hydrogen bonds, super easy to break. And so that would

  • be the gross anatomy of deoxyribonucleic acid. Those are going to be the parts. What's important

  • is that we can store information in here. So when we talk about transcription and translation,

  • every three letters here are going to code for one amino acid. And so if we were to break

  • down the word deoxyribonucleic acid, you should be able to understand where that name comes

  • from. And so in the other words the deoxyribo part comes from this. It comes from the deoxyribose

  • sugar. The nucleic part comes from the idea that it's found inside a cell. The DNA is

  • going to be found inside the nucleus of the cell. And then the acid part actually comes

  • from the phosphate group. Phosphates are going to donate hydrogen and so that's going to

  • make it acidic which will become important in just a second. So here's our DNA. It's

  • just repeated nucleotides over and over and over. But we know the DNA looks like this

  • and so DNA is going to have this three dimensional shape. And scientists think that RNA was the

  • first genetic material on our planet and then DNA is kind of an upgrade to that. And so

  • if we see this is our DNA, it's essentially that same ladder, but that ladder has been

  • twisted into a helix. And the reason why it's not really drawn here is that there are also

  • going to by hydrogen bonds that are holding each of these, so there are going to by hydrogen

  • bonds here and hydrogen bonds here, and so basically what that does is it gives it this

  • three dimensional shape. And that makes it really, really stable. And so DNA has some

  • advantages over RNA. Number 1 it has a more stable three dimensional shape, but the other

  • nice part is that we could have a mutation on one side of the DNA and since mutations

  • are passed from generation to generation to generation, if it's in DNA then we have a

  • back-up copy on the other side, so if there's a mistake here we can actually look at the

  • DNA that's on the other side and then enzymes can actually cut this out and replace it on

  • the other side. And so that's DNA. Those are the major parts of DNA and it's really not

  • that complex. If you understand breaking it down to a nucleotide and the idea that it's

  • simply just a ladder. A ladder of information and that information essentially tells a cell

  • how to make a protein. And so that's DNA and I hope that's helpful.

Hi. It's Mr. Andersen. Today I want to talk about DNA. Before we get into

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什麼是DNA? (What is DNA?)

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    Cheng-Hong Liu 發佈於 2021 年 01 月 14 日
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