Structure of DNA and RNA: A Comparison

If you find the concept of DNA and DNA confusing, you are not alone. In fact, many people would agree with you. Not only do the two sound a lot alike, but they are also both made up of nucleic acid, one of the few macromolecules that are vital for life on Earth.

The first structural difference between the two macromolecules (large molecules) is visible to the naked eye. DNA is double stranded. This means that it is made up of two strands that intertwine, making the double helix structure that it is most famous for. RNA, on the other hand, is only one single strand (with the exception of some viruses that have double stranded RNA). Basically, that is the most fundamental structural difference between the two. However, there is more to that seemingly simple difference and you are about to find out all about it.

The two strands of the DNA molecule are made up of four bases, namely adenine, guanine, cytosine, and thymine. The backbone of the DNA molecule is made up of the sugar deoxyribose and phosphate. The backbone of RNA is made up of the sugar ribose as its backbone (ribose has one more oxygen atom than deoxyribose) and a slightly different set of bases, namely adenine, guanine, cytosine,and uracil.

Wow! That is a lot of information to digest in two little paragraphs. Let’s distill that a bit.

Let’s talk about the sugars of the two molecules. First of all, both sugar molecules are rings that are made up of five carbon molecules. The ribose sugar of the RNA molecule is considered a “normal” sugar, meaning that each of its carbon atoms is attached to an oxygen atom. Its formula is C5H10O5. On the other hand, the sugar of the DNA molecule, deoxyribose, is missing one oxygen atom—hence the name DEOXY. Its formula is C5H10O4, so one less oxygen. In fact, the deoxyribose is made from the ribose sugar by the loss of an oxygen atom. Overall, the difference between the two molecules in the number of oxygen atoms is what enables enzymes to differentiate the two from one another. Also, some researchers have shown that the lack of the oxygen atom is what makes the DNA molecule a lot more stable than the RNA molecule. Conversely, the oxygen in the RNA molecule makes it more reactive—read: it is a lot more unstable.

So, how does all of this relate to the actual strand? Each of the two sugars is a constituent of nucleic acids that were mentioned a little earlier in this post. Each nucleic acid is composed of three molecules, namely a sugar, a nitrogenous base, and a phosphate group. We talked about the sugars, so how about the nitrogenous bases? What are nitrogenous bases, in the first place? A nitrogenous base is a molecule that contains a nitrogen atom, and it has the chemical properties of bases. When it comes to the nitrogen-containing molecules of our two strands, these nitrogenous bases are divided into two groups, namely pyrimidines and purines, and they also differ in structure.

thymine and cytosine, are molecules

Pyrimidines, such as thymine and cytosine, are molecules that are made up of one ring that has six carbons. In terms of RNA that does not contain thymine, its other pyrimidine is uracil. However, purines, such as adenine and guanine, are a bit more complicated. They are made up of two rings fused to each other, namely a ring with six carbons and one with five. However, these rings are not only made up of carbon and nitrogen molecules. They also contain hydrogens.

So, let us look at nucleotides in a bit more detail. Now that you know that each of them not only contains different types of nitrogenous bases but also sugars (depending on the strand), you have probably predicted that this contributes to the difference in structure.

And how does all of that contribute to the overall shape of the two strands? Well, the strands are made up of nucleotides that are linked together by something called a covalent bond, or a chemical bond that revolves around sharing two electrons between the atoms of a molecule. In a bit more detail, each sugar is covalently bound to a phosphate group, and this means that by virtue of the two sugars being different, so will be the covalent bonds they form. When it comes to DNA, each side or backbone, of the strand, chain or double-helix is made up of these covalent bonds between sugars and phosphate groups. Earlier in this post, we said that DNA is a double helix. And now you will understand what it means to have two strands come together. In other words, each backbone of one strand is made up of covalent bonds. Attached to the sugars on the other side are the nitrogenous bases and these are connected in the center of the DNA molecule by virtue of hydrogen bonds. Each guanine binds to a cytosine via three hydrogen bonds, while each adenine binds to a thymine (or uracil in the RNA world) via two hydrogen bonds. Overall, given all this complexity, it comes as no surprise that one difference in a sugar molecule can cause differences in structure—they do, but mentioning biophysics is beyond the scope of this blog post.

The DNA helix coils and as such forms grooves on its sides that are different in shape. They are known as major and minor grooves and are different because of the different nitrogenous bases as well as the number of hydrogen bonds that are shared between the nucleotides.

The RNA helix also coils, but not as much. Also, while there is only one DNA strand, there are different types of RNA strands that can exist, depending on their function. Some are directly involved in protein synthesis or its regulation. Last, but not least, RNA molecules are much shorter than DNA.

Laura Day