How are the two strands of nucleic acid in DNA held together?

Question

Accepted Answer

All right. Hello everyone. So this question is asking us which of the following correctly describes how the two nucleic acid strands in DNA are held together. Option A says covalent bonds between nitrogenous bases. Option B says ionic bonds between the sugar phosphate backbones. C says hydrogen bonds between complementary nitrogenous bases. And D says peptide bonds between nucleotides. So first and foremost, right, recall that nucleic acids like DNA are polymers, which means that they're made up of monomers and those monomers are known as nucleotides. Now, there are three main components to any given nucleotide. The first is the sugar backbone which is deoxyribose for DNA and ribose. For RNA. From there, we have the nitrogenous base connecting to the one prime carbon of the sugar backbone. And we also have a phosphate group connecting to the five prime carbon of the sugar backbone. So as mentioned before, these nucleotides would make up a given strand of DNA. Now two strands of DNA that run in opposite directions are held together to form ultimately the double helix. So here in terms of the backbone, that is the combination of or rather actually the phosphate groups are going to be on the outside of the double helix. And so what we're concerned about are the interactions that are holding the inner portion together, thereby stabilizing the double helix to begin with. So if we have the phosphate group here on the outside of the double helix, we have the sugar backbone somewhere in the middle. And that leaves us now with the nitrogenous bases in the interior of the double helix. And so now the question is what type of interaction holds these nitrogenous bases in the interior of the double helix together such that it stabilizes the double helix itself. Well, we can actually go ahead and eliminate some answer choices. First. The first one we can get rid of is option D because peptide bonds are part of proteins, not nucleic acids. And also we can eliminate option B because as mentioned before, the sugar phosphate backbones are in the exterior of the double helix. And don't actually participate in holding the double helix together. Rather the negative charge of the phosphate group is essential for the DNA itself to interact favorably with the environment that it's in due to its charge due to its charge interacting with the mostly aqueous environment. Will this leaves us now between option A and C, are there covalent bonds or hydrogen bonds taking place between the bases themselves? And the answer is going to be hydrogen bonding. Now, it's specifically going to be hydrogen bonding between pairs of complementary bases. In other words, Adenine will always pair with thymine and vice versa. While cytosine will always pair with guanine and vice versa. So our answer is going to be option C in the multiple choice because it's hydrogen bonds between complementary nitrogenous bases that hold the two strands together and there you have it. So with that being said, thank you so very much for watching and I hope you found this helpful.

How are the two strands of nucleic acid in DNA held together?

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Key Concepts

Base Pairing

Hydrogen Bonds

Antiparallel Strands