Base Pairing - Online Tutor, Practice Problems & Exam Prep

Base Pairing Concept 1

Hey, everyone. So when we talk about DNA or RNA base pairing, we need to talk about the intermolecular force of hydrogen bonding. Now, hydrogen bonding between the bases provides or produces a stabilizing effect towards the overall integrity of the structure. Here we are going to say that individually, hydrogen bonds are relatively weak, but collectively they are strong. This is one of the landmark ideas when it comes to hydrogen bondings when we discuss DNA later on. It's just that there are so many hydrogen bonds that exist within DNA; overall together, they are strong. They hold the DNA molecule together. With this idea, we have what's called complementary base pairing. Basically, this discusses the bonding preferences between different bases. This base bonds or hydrogen bonds to this particular base. Here we're going to say the binding preferences of A with, when we talk about, T or U, and then when we talk about G with C. 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 going to say A with T or U, will make 2 hydrogen bonds and G with C makes 3 hydrogen bonds. Remember we talked about this stabilizing effect. A with T or A with U makes 2 hydrogen bonds, which helps with overall integrity when you collectively add them together. But G with C, you're making 3 hydrogen bonds here. That's even more stability involved. You'll tend to see that places within DNA where we see G with C forming 3 hydrogen bonds are areas of increased strength. Here, if we take a look, in this image, we have different nitrogenous bases. We're showing their bonding preferences as well as the number of hydrogen bonds they make with each other. A with T, they're making 2, G and C, they're making 3 hydrogen bonds. In DNA, we're going to say here that A pairs with T, but in RNA, remember, we have the nitrogenous base of uracil. Here, A pairs with U 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 U instead. Because in RNA, we substitute out Thymine for uracil. So just remember that distinction. When it comes to DNA or RNA, Cytosine will always hydrogen bond to Guanine.

Base Pairing Example 1

Here in this example, it says write 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 U involved. So U stands for uracil. This will represent base pairing with RNA. Alright. So, let's just go over the missing nitrogenous bases first. So A, since this is RNA, would hydrogen bond to U. Here we have U, so this would be an A. Over here we have a C, so this will be a G. Here we have a G, so this will be a C. Then coming back, remember that cytosine and guanine, C and G, form 3 hydrogen bonds to each other. It's fine. We have hydrogen bonds with all of these, and then down here, A with U, they form 2 hydrogen bonds. So, this is what we'd say in terms of our missing nitrogenous bases as well as our missing hydrogen bonds.

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%) 

Base Pairing Concept 2

Now with DNA base pairs, we have Chargaff's rule. In the early 1950s, Erwin Chargaff made an important discovery related to double-stranded DNA. Chargaff's rule states 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 the scale, let's just imagine that in this given species, this represents all of our A nitrogenous bases and this would have to represent all our T nitrogenous bases. If they're going to 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 going to 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 Chargaff'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. Also, remember here, you can see that the number of A and T's are different from the numbers in G and C's. It's these two being equal to each other in amount or percentages and these being equal to each other in amount or percentages. Okay. So it doesn't mean that they are all equal to each other. Right. So we'll take a look at when it comes to mathematical questions when it's talking about Chargaff's rule, and how we can apply it to understanding the percentages of a given nitrogenous base 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.

Base Pairing Example 2

Here in this example question, it says human DNA is comprised of approximately 20% of adenine (A). It asks approximately what percentage of the nucleotides in a human DNA sample will be guanine (G). So we're going to list A, T, and then we have G and C. These are our base pairings. We're going to say collectively, they represent 100% of all the nitrogenous bases 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 Adenine 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 100%, that means we have 60% remaining. The 60% remaining represents G and C bases. 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 C. Again, according to Chargaff's rule, their percentages have to be equal. If A is 20%, T has to be 20%. They roughly have to be the same percentage. And then if you subtract that 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 2 you would see that each one will be 30%. If we look at our options, the answer would have to be A. A human's DNA sample of guanine would have to be 30%.

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)?

• A. 8%
• B. 16%
• C. 21%
• D. 60%