The base pairing in DNA between molecules of guanine and cytosine is shown in the given structure. How many hydrogen bonds can be formed between molecules of guanine and cytosine?
We are asked about hydrogen bonds between base pairs guanine and cytosine in DNA. DNA, which stands for deoxyribonucleic acid, is a giant, organic, double-stranded molecule found in the cells of living organisms. It holds the codes for protein synthesis, and therefore DNA controls many processes in living cells. Each strand is made of a sugar–phosphate backbone with nitrogenous space compounds attached along the chain.
There are four nitrogenous bases found in DNA, whose names we simplify with A for adenine, T for thymine, G for guanine, and C for cytosine. Adenine on one strand and thymine on another strand pair up. In the same way, adjacent cytosine and guanine molecules on opposite strands pair up too. The base pairs form hydrogen bonds with each other. And this is how the two strands are held together. The question asks how many hydrogen bonds are formed between molecules of guanine and cytosine. We are not told which molecule is guanine and which is cytosine. Nevertheless, we can still determine the number of hydrogen bonds that can form between these two nitrogenous bases.
A hydrogen bond is a dipole–dipole interaction that exists between a hydrogen atom covalently bonded to a strongly electronegative atom and an electron lone pair on another strongly electronegative atom. Strongly electronegative atoms that we will consider are oxygen, nitrogen, and fluorine. In other words, when a hydrogen atom is covalently bonded to a strongly electronegative atom — for example, oxygen or nitrogen or fluorine — a dipole, two poles, will form because of the difference in electronegativity between the two atoms.
And the 𝛿 positive hydrogen can electrostatically interact with, or hydrogen-bond to, the electron lone pair of a highly electronegative atom on another molecule, for example, oxygen or nitrogen or fluorine. And this is because lone pairs give that atom a 𝛿 negative charge. I have only shown the relevant portions of the two molecules which are hydrogen-bonding with each other.
Now, let’s have a look at our two bases to see where hydrogen bonds may occur. Here is a hydrogen atom covalently bonded to a strongly electronegative atom. The difference in electronegativity will give us a 𝛿 positive and a 𝛿 negative dipole. In a similar way, a dipole forms between the nitrogen and hydrogen on the first base, as well as over here. This oxygen atom has two lone pairs, and oxygen has a fairly high electronegativity relative to the carbon atom to which it is bonded. And so oxygen has a 𝛿 negative charge. In the same way, this oxygen atom has a 𝛿 negative charge too. And this nitrogen atom has one lone pair and is fairly high in electronegativity. And so it also has a 𝛿 negative charge.
Now, we are ready to see how many hydrogen bonds can form between these two molecules. A hydrogen bond can form between the 𝛿 negative oxygen atom on the first molecule and the nearby 𝛿 positive hydrogen atom on the second molecule. In the same way, a hydrogen bond can form between this hydrogen and this nitrogen and between this hydrogen and this oxygen atom.
How many hydrogen bonds can be formed between molecules of guanine and cytosine? The answer is three hydrogen bonds.