Base Pairing - Online Tutor, Practice Problems & Exam Prep

DNA and RNA base pairing relies on hydrogen bonding, where adenine (A) pairs with thymine (T) or uracil (U) through 2 hydrogen bonds, while guanine (G) pairs with cytosine (C) through 3 hydrogen bonds, enhancing structural stability. Chargaff's rule states that in any species, the percentage of A equals T, and G equals C, ensuring proper base pairing. This balance is crucial for the integrity of nucleic acids, with A and T, G and C contributing to the overall 100% of nitrogenous bases in DNA and RNA.

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. Individually, hydrogen bonds are relatively weak, but collectively they are strong. This is one of the landmark ideas when it comes to hydrogen bonding when we talk about DNA later on. There are so many hydrogen bonds within DNA that, collectively, they are strong and hold the DNA molecule together. With this idea, we have what's called complementary base pairing, which talks about the bonding preferences between different bases: this base hydrogen bonds to a specific base. Here, we discuss the binding preferences of 'A' with 'T' or 'U', and 'G' with 'C'. These complementary base pairings have a set number of hydrogen bonds they form with one another. 'A' with 'T' or 'U' make 2 hydrogen bonds, and 'G' with 'C' makes 3 hydrogen bonds.

Remember, we talked about the stabilizing effect: 'A' with 'T' or 'A' with 'U' make 2 hydrogen bonds, which helps with overall integrity when they are collectively considered. However, 'G' with 'C', making 3 hydrogen bonds, involves even more stability. You will tend to see that areas within DNA where 'G' with 'C', forming 3 hydrogen bonds, are areas of increased strength. We can see different nitrogenous bases showing their bonding preferences and the number of hydrogen bonds they make with each other: 'A' with 'T' making 2, and 'G' with 'C' making 3 hydrogen bonds. In DNA, 'A' pairs with 'T', but in RNA, remember, we have the nitrogenous base of uracil. Here, 'A' pairs with 'U' instead. And with both DNA and RNA, 'C' will always pair up with 'G'. Just remember, when it comes to DNA, it's 'A' with 'T'. In RNA, 'A' does not bond with 'T'; 'A' bonds with 'U' instead, substituting Thymine for uracil. Remember this distinction, and that in 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 again, since this is RNA, A 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 G. Here we have a G so this will be a C. Then coming back, remember that cytosine and guanine, C and G, they form 3 hydrogen bonds to each other. So you'd have to show 3 hydrogen bonds here. It's fine with all of these, and then down here, A with U, they form 2 hydrogen bonds. 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 pair, 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. If we have x number of A, we'd have to have the same x number of T. Let's 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 are going to hydrogen bond to each other, so they need to have equal numbers of both. Putting on a scale, G and C would have to be equal to each other. This is Chargaff's rule. Because these base pairings exist, you must 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. So, it doesn't mean that they're all equal to each other. We'll take a look at when it comes to mathematical questions regarding 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. And it asks approximately what percentage of the nucleotides in a human DNA sample will be guanine G. So we're going to mention 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 2. If we subtract that from the 100%, that means we have 60% remaining. The 60% remaining represents G and C pairs.

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, and 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%

Here’s what students ask on this topic:

What is the significance of hydrogen bonding in DNA and RNA base pairing?

Hydrogen bonding is crucial in DNA and RNA base pairing because it provides stability to the nucleic acid structures. In DNA, adenine (A) pairs with thymine (T) through 2 hydrogen bonds, and guanine (G) pairs with cytosine (C) through 3 hydrogen bonds. In RNA, adenine pairs with uracil (U) instead of thymine. Although individual hydrogen bonds are weak, collectively, they create a strong stabilizing effect, holding the DNA or RNA strands together. This stability is essential for the integrity and proper functioning of genetic material.

How does Chargaff's rule apply to DNA base pairing?

Chargaff's rule states that in any double-stranded DNA, the percentage of adenine (A) is equal to thymine (T), and the percentage of guanine (G) is equal to cytosine (C). This rule is based on the fact that A pairs with T and G pairs with C through hydrogen bonding. Therefore, if a DNA sample has 30% adenine, it must also have 30% thymine. Similarly, if it has 20% guanine, it must have 20% cytosine. This balance ensures proper base pairing and the overall stability of the DNA structure.

Why does adenine pair with thymine in DNA and uracil in RNA?

In DNA, adenine (A) pairs with thymine (T) through 2 hydrogen bonds, while in RNA, adenine pairs with uracil (U) instead of thymine. This difference is due to the presence of uracil in RNA, which replaces thymine. The hydrogen bonding preferences remain the same, with adenine forming 2 hydrogen bonds with either thymine or uracil. This substitution is a key distinction between DNA and RNA, reflecting their different roles and structures in the cell.

How many hydrogen bonds are formed between guanine and cytosine?

Guanine (G) and cytosine (C) form 3 hydrogen bonds when they pair together. This triple hydrogen bonding provides greater stability compared to the 2 hydrogen bonds formed between adenine (A) and thymine (T) or uracil (U). The increased number of hydrogen bonds between G and C contributes to regions of DNA with higher stability and strength.

What is complementary base pairing and why is it important?

Complementary base pairing refers to the specific hydrogen bonding between nitrogenous bases in DNA and RNA. In DNA, adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C). In RNA, adenine pairs with uracil (U) instead of thymine. This specificity ensures accurate replication and transcription of genetic information. Complementary base pairing is essential for maintaining the integrity of genetic material and for the proper functioning of biological processes such as DNA replication and RNA transcription.