DNA Basics

Why is it that there is a T base to complement an A base in DNA, but a U base in the case of RNA?

-A curious adult from the India

August 17, 2006

This was a lot of fun to answer and something I hadn't thought much about before. The answer has to do with mutations and the letter C.

As you know, DNA is an instruction manual for life. It is written in 4 letters -- A, C, G, and T.

Sometimes one of the letters gets changed and we end up with a typo in our instructions. This typo is called a mutation.

These mutations aren't necessarily bad. My blue eyes are probably from a distant mutation as is my ability to drink milk as an adult. But some mutations are definitely harmful.

Some can kill you outright. Others lead to cancer or other diseases. And the more mutations you have, the more likely one of them will be a bad one.

So our bodies have all sorts of spell checker software to make sure our instructions stay as free from typos as possible. This software is actually made up of proteins that are constantly looking over our DNA for mistakes. When they find a mistake, they remove the offending letter and put in the right one.

So what does this have to do with your question? Well, a very common way to get a typo happens when C's spontaneously turn into U's.

Imagine that our DNA had U's instead of T's. And the C's start turning into U's in our 6 billion letters of instructions. How could the body tell that the U shouldn't be there?

The way the body knows what letter should be where is by looking at what is across the helix from it. For example, if there is a G across the helix, our editing proteins know a C should be across from it. But what if it sees a U instead?

If our DNA contained U's instead of T's, we would have some very confused software. Some of the time it would replace the U and sometimes the G. This would cause lots of problems pretty quickly.

To get around this, our bodies have a protein that removes U's and replaces them with C's. This solves the problem of C's spontaneously turning into U's. Because U's are foreign to DNA, our cells know to replace them with C's.

So now we know why DNA has T's instead of U's. But why does RNA have U's? This is a simple cost issue.

It takes a lot of energy to turn a U into a T. Given that we'd die if we used U in our DNA, it is worth the cost for DNA.

But RNA is a different story. Instead of just the two copies we have with DNA, there are thousands of copies of RNA. If one goes bad, this is much less of a big deal.

Also, RNA doesn't stay around very long. Our bodies are constantly making and breaking apart RNAs. So if an RNA has a mistake, it'll soon be eliminated anyway.

Which is totally different than DNA. Your DNA is the final repository of all of the knowledge about how to make you. Change it, and you change a part of you. And your future kids.

But if one RNA of many thousands has a mistake, this is tolerable. Making T's in RNA just isn't cost effective for the cell.

There you have it. DNA has T's because we can't suffer the high mutation rate that would come with it. But, given the cost of turning a U into a T, mutations are much more tolerable in RNA.

By Dr. Barry Starr, Stanford University

More Information


If DNA had U's, the rate of mutation would be too high.

DNA has T's because C's can change into T's.