Base Pairing - Video Tutorials & Practice Problems
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concept
Base Pairing Concept 1
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Hey, everyone. So when we talk about DNA or RN A base pairing, we need to talk about the intermolecular force of hydrogen bonding. Now, hydrogen bonding between the bases provides a or produces a stabilizing effect towards the overall integrity of the structure. Now, you're going to say that individually, hydrogen bonds are relatively weak but collectively, they are strong. And this is one of the landmark kind of ideas when it comes to hydrogen bonding. When we talk about DNA later on, it's just that there are so many hydrogen bonds that exist within DNA that overall together, they are strong, they hold the DNA molecule together. Now, here with this idea, we have what's called complementary base pairing. So basically, this talks about the bonding preferences between different bases, this base bonds or hydrogen bonds to this particular base. Here, we're going to say the bonding preferences of A with when we talk about T or you and then when we talk about G with C. Now, here we're going to say that these complementary base pairings have a set number of hydrogen bonds that they do with one another. So we're gonna say A with T or U will make two hydrogen bonds and G with C makes three hydrogen bonds. So remember we talked about this stabilizing effect A with T or A with U, they make two hydrogen bonds, which helps with overall integrity when you collectively add them together. But G with C, you're making three hydrogen bonds here, that's even more stability involved. So you'll tend to see that places within DNA where we see G with C forming three hydrogen bonds are areas of increased strength. Now, here, if we take a look in this image, we have different nitro spaces, we're showing their bonding preferences as well as the number of hydrogen bonds they make with each other A with T, then making two G and C, then making three hydrogen bonds in DNA. We're gonna say here that A pairs with T but in R and A, remember we have the nitrogenous space of uracil here a pairs with you instead and then with both of them C will always pair up with G. So just remember when it comes to DNA, it's A with T, when it comes to RNA A does not bond with T A bonds with you instead because in RN A, we substitute al thymine for your cell. So just remember that distinction and when it comes to DNA or RN A cytosine will always hydrogen bond to Guan
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example
Base Pairing Example 1
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Here in this example, it says right, in the missing bases and hydrogen bonds from the given image. Now, if we take a look at this image, we can see that we see you in ball. So U stands for Uil. So this will represent base pairing with RN A. All right. So let's just go over the missing nitrogenous spaces first. So a again, since this is RN A, it would a hydrogen bond to you here, we have you. So this would be an A over here, we have AC. So this will be G here, we have a G. So this would be AC then coming back, remember that cytosine and guanine C and G, they form three hydrogen bonds to each other, you'd have to show three hydrogen bonds here. It's fine, fine. We have hydrogen bonds with all of these and then down here a with you, they form two hydrogen bonds. So this is what we'd say in terms of our missing nitrogenous bases, as well as our missing hydrogen bonds.
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Problem
Problem
Four species shown below give the percentages of A–T pairings vs G–C pairings. Based on only the information given, which species would have the most significant strength in their base interactions?
A
Drosophila melanogaster (fruit fly) (55% : 45%)
B
Zea mays (corn) (51% : 49%)
C
Neurospora crassa (fungus) (46% : 54%)
D
Escherichia coli (bacteria) (49% : 51%)
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example
Base Pairing Concept 2
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Now, with DNA based pair, we have Char's rule. Now, in the early 19 fifties, Irwin Chargaff made an important discovery related to double stranded DNA. Now here, Chagall's rule says that for each species, no matter what it is, the percentage of A and T bases are roughly equal as are the percentage of G and C bases. Remember with DNA A and T will hydrogen bond to each other. So if they're hydrogen bonding with each other, we want them to have the same number. We have X number of A, we'd have to have the same X number of T. If we take a look here at this scale, let's just imagine that in this given species, this represents all of our a nitrogenous spaces and this would have to represent all our t nitrogenous spaces. If they're gonna hydrogen bond to each other, the numbers need to be equal. So the percentages are equal in the same way G and C, they're gonna hydrogen bond to each other. So they need to have equal numbers of both. So putting it on a scale, G and C would have to be equal to each other. And this is Char's rule because these base pairings exist, you have to have equal numbers of these pairings. So A and T would have to be equal in percentage, G and C would have to be equal in percentage. Collectively, they represent 100% of all the nitrogenous bases within a given species. And also remember here, you can see that the number of A and TS are different from the numbers in G and CS. It's these two being equal to each other in amount of percentages and these being equal to each other in amount of percentages. Ok? So it doesn't mean that they're all equal to each other, right? So we'll take a look at when it comes to mathematical questions when it's talking about Charal rule and how we can apply to understanding the percentages of a given nitrogen in space within any given species. But just remember, fundamentally A and T percentages have to equal each other. G and C percentages have to equal each other.
5
example
Base Pairing Example 2
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Here in this example question, it says human DNA is comprise of approximately 20% of adenine A and it says approximately what percentage of the nucleotides in a human DNA sample will be guanine G. So we're gonna say A T and then we have G and C, these are our base parents. We're gonna say collectively they represent 100% of all the nitrogen and spaces within a given species. In this case, a human being remember that the percentages between A and T have to be the same. So if Adam is 20% that means thymine has to be 20%. So this equals 20% this equals 20%. So we have 40% from just these two. If we subtract that from the 100% that means we have 60% remaining. The 60% remaining represents G and C parents. Again, they have to be equal to each other in percentages as well. So of the 60% 30% would have to be G and 30% would have to be seen. Again. Chagos rule, their percentages have to be equal. If A is 20% T has to be 20%. They rough. They have to be roughly the same percentage. And then if you subtract it from 100% that'll tell you your percentage left for G and C, they themselves also have to be roughly equal to each other. So just divide this number by two, you would see that each one will be 30%. If we look at our options, the answer would have to be a human DNA sample of Gui would have to be 30%.
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Problem
Problem
Cytosine (C) makes up 42% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine (T)?