Draw the structures of adenine and uracil (which replaces thymine in RNA), and show the hydrogen bonding that occurs between them.

Question

Accepted Answer

All right. Hello everyone. So this question is asking us or this question says that cytosine and guanine are two base pairs in DNA, which of the following accurately shows the hydrogen bonding between them. So recall first and foremost that a given nucleotide for either DNA or RN A includes three base components, right? We have the sugar backbone, we have the nitrogen space and we also have a phosphate group. Now, the reason why the double helix structure of DNA remains stabilized is because of hydrogen bonding between complementary bases on opposite DNA strands. Those hydrogen bonds are ultimately in the interior of the double helix and that keeps the structure maintained. So here, the complementary base pairing between cytosine and guanine is said to be more stable than that between Adenine and thymine. And the reason for this is because cytosine and guanine have three hydrogen bonds between them, whereas Adenine and thymine have two. So let me go ahead and scroll down. So we can consider the structures of both cytosine here on the left and Guan in here on the right, by understanding their structure, it's going to be easier to understand their hydrogen bonding So here, let's go ahead and talk about where these three hydrogen bonds are actually formed. So here, it's important to recognize that there has to be one hydrogen bond donator and one hydrogen bond acceptor on each base so that the hydrogen bond can actually take place. So hydrogen bond acceptors are going to be electron atoms with lone pairs of electrons. This includes oxygen fluorine and also nitrogen. Now, hydrogen bond donors include hydrogen atoms attached to electron atoms. That is the aforementioned oxygen fluorine and nitrogen. So considering the structures of both cytosine and guanine here, cytosine in particular has one hydrogen bond donor in the case of the free amino group attached to the ring. Now, next to the hydrogen bond donor, we have our first hydrogen bond acceptor, which is this nitrogen inside of the ring that has a lone pair of electrons. The second hydrogen bond acceptor is this car boal oxygen. So that's true for cytosine. Now, let's consider guanine. Now, guanine is unique because it has two fused rings. But it just so happens that the hydrogen bond donors and acceptors relevant to guanine are going to be within the six membered ring. So looking at our Carboni oxygen on guanine, we have our first hydrogen bond acceptor. Next to that, we have a nitrogen inside of the ring that has a hydrogen. So that's going to be an a donator. I should say it's a donator and our second hydrogen bond donor is actually going to be this free amino group on the next carbon over that I neglected to draw previously. But there it is right. So from here, in order for these hydrogen bonds to take place the donor of one molecule of one base should align with the acceptor of the other and vice versa. So here if I take the structure of Guan, rotate it just a little bit and then move these two basis together, we can see that the hydrogen bond donors and acceptors are aligned with each other. So now let's go ahead and consider our answer choices. So here we have four different answer choices and the one that's correct should as mentioned before, align those hydrogen bond donors with the acceptor of the other base and vice versa. So in this particular case, the one that's going to match is going to be option number three here, which is our correct answer and there you have it. So with that being said, thank you so very much for watching and I hope you found this helpful.

Draw the structures of adenine and uracil (which replaces thymine in RNA), and show the hydrogen bonding that occurs between them.

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Key Concepts

Nucleotide Structure

Hydrogen Bonding

Base Pairing Rules