DNA vs RNA: Differences in form and function

Stated Clearly presents: What is the difference between DNA and RNA?

In biology it’s often said that form equals function, or more accurately, form allows for function. The form of your front teeth, for example, allows them to function as food chopping tools, while the form of your molars allows them to function as food smashing tools. Similar connections between form and function can be found all the way down to the level of molecules like DNA and RNA: Physical molecules with well known atomic structure, groups of atoms stuck together in a specific pattern.

We will compare form in a moment but first it’s worth a quick refresher on the contrasting functions of DNA and RNA in modern cells – animal cells like our own.

Today’s video builds on our previous animations: “What is DNA and how does it work”, and “What is a chromosome”.

A chain of DNA is made of a sequence of paired links. There are 4 different types of DNA links (A, C, T, and G) called nucleotides. A gene is a long stretch of DNA, a sequence of nucleotides that code for something. The specific sequence of nucleotides is what’s really important here. Usually a gene codes for a distinct protein, or a group of proteins, but in some cases a gene will simply act as a template for a functional chain of RNA.

Long story short: Genes tell amino acids how to line up to form proteins, proteins interact with other molecules to make living cells, cells form tissues, tissues make organs, organs combine to make you and me.

DNA does not code for proteins directly. Instead, it more or less just sits there, safely guarded inside the nucleus (or centerpiece of your cells), waiting for other molecules to make RNA copies of its DNA genes. Those RNA copies (and by “copy” here, I mean they share mostly the same nucleotide sequence as the stretch of DNA they were copied from) those RNA copies are the things that can actually leave the nucleus where they either perform a function on their own, or they will be read by ribosomes. Ribosomes are the protein building machines we learned about in earlier animations. In these cases, the RNA copy of the DNA gene functions as a set of instructions telling the ribosome how to build a specific type of protein, that protein is what then goes on to perform an active function in the cell.

You can imagine DNA genes as extremely valuable books in a library, so valuable that the library staff refuses to let anyone check them out. Instead, they use the books to make photocopies, and distribute those copies out into the world. A chain of RNA in this analogy is like a photocopy of a DNA book.

Modern cells use DNA for long term information storage. RNA copies of DNA are what go out into the cell for active use.

So now let’s compare their similarities and differences in form:

The first difference you might notice here is that RNA is normally single-stranded, whereas chains of DNA normally exist as a double-stranded spiral.

I will untwist these here so they’re easier to compare. This difference in structure has nothing directly to do with the innate properties of RNA and DNA. Chains of RNA are single-stranded because our cells build RNA chains single stranded. This allows RNA to easily interact with other molecules inside the cell. Our cells build DNA chains double-stranded which, among other things, enhances their stability. Remember, DNA is carefully guarded inside the cell. It acts as information storage.

It’s worth noting that some viruses use RNA instead of DNA for their information storage. In many of those viruses, their RNA is also double-stranded for stability.

If we zoom out we see a second obvious difference: RNA chains are shorter than DNA chains but this also has little to do with RNA’s intrinsic properties. It’s mostly a consequence of how the cell uses RNA. Cells only make RNA copies of individual genes, rather than entire chains of DNA.

Fun side note: Even though DNA chains are longer there’s roughly 3 times more RNA than DNA by weight, because there are just so many RNA copies of DNA genes in use at any given moment.

RNA and DNA are both made of repeating units called nucleotides.

Zooming in we more clearly see that nucleotides consist of 3 main parts:

1. There is a carbon/nitrogen base (labeled here with different colors and letters).
2. There is a backbone made of a special sugar called ribose.
3. And there is a phosphate linker.

In real life, nucleotides are more like what we’re seeing here. These are space-filling atomic models. Each orb represents an individual atom.

The phosphate linker is identical in both RNA and DNA, which is why I painted them the same color in the cartoon model.

The sugar backbones, however, are slightly different between RNA and DNA. Can you spot the difference?

The difference is right… here!

DNA is missing an oxygen/hydrogen group… an “OH” group!

This seemingly tiny difference has huge consequences! It’s so important, in fact, that this difference is reflected in the names of these molecules:

RNA stands for “Ribonucleic acid”. “Ribo” because the backbone is a normal ribose sugar.

DNA stands for “Deoxyribonucleic acid”. “Deoxy” because its ribose sugar has been deoxygenated. One of its oxygen/hydrogen groups has been ripped away and replaced with a single, stable hydrogen atom.

So what’s the big deal about that tiny OH group in RNA? Why does it matter? Well, OH groups are highly reactive. You could say they’re sticky. This extra stickiness allows RNA to be easily pulled around through the cell by proteins and other molecules which is great, but this extra maneuverability comes at a price. That reactive Oxygen/Hydrogen group makes RNA easier to break. If a chain of RNA bends just right, its OH groups can even cause RNA to react violently with itself, bursting in two!

Deoxyribose (the modified sugar that DNA’s backbone is made of) is naturally less reactive and far more stable. Compared to RNA, DNA is innately tough.

To make up for RNA’s rowdy, fragile nature, cells constantly digest RNA chains, and constantly rebuild RNA chains. They use those stable DNA genes as templates for the RNA.

RNA is fragile and disposable, DNA is tough and repairable.

The last noteworthy difference between DNA and RNA is found in their nucleotide bases.

There are 4 types of RNA bases, which scientists have labeled A, C, G, and U: Adenine, Cytosine, Guanine and Uracil.

3 of those 4 are identical in DNA but DNA uses Thymine instead of Uracil.

The reasons we think cells evolved to use Thymine for DNA instead of Uracil are complicated, but in short, the lack of Uracil in DNA helps cells detect common forms of DNA damage and repair that damage. Again cells treat RNA as disposable and replaceable, they guard DNA chains and repair them when damage is found.

So to recap, even though DNA and RNA are nearly identical there are several important, easy to remember differences

Chains of RNA are relatively short and often single-stranded. They are active all throughout the cell. They are Highly fragile. They’re digested and rebuilt when damaged. They are used for active functions such as protein coding.

Chains of DNA are far longer and usually double-stranded. DNA is trapped inside the nucleus (except for in bacteria and other organisms that don’t have nuclei). DNA is relatively stable. DNA is carefully guarded by cells and then repaired when damaged. Finally, DNA is used as information storage

Just like the shape of your front teeth make them better for chopping, while the shape of your molars makes them better for smashing, the atomic structure of DNA makes it naturally more stable than RNA. Your cells have evolved to exploit and enhance these innate differences. Modern cells use these two types of molecules for very different functions.

The molecular structure of DNA was first worked out in 1953. The field of genetics has come a long way since then but plenty of mysteries still remain. People interested in the origin of life and its chemistry – scientists like Dr. Anthony Poole are trying to understand how RNA and DNA first came about. Did the earliest cells on earth use RNA both as active molecules in the cell, and for information storage, similar to how some viruses use RNA today? Could a cell really function with RNA alone? If so, what were the evolutionary steps leading to the RNA/DNA system that cells use today? These are just some of the mysteries currently being worked on, many more exist, mysteries that you, dear viewer, might someday help solve!

I am Jon Perry and that is RNA vs DNA, Stated Clearly