Genetic Goofs

Yet another geneticist’s pick-up line: If I had to choose between RNA and DNA, I’d choose RNA – because RNA has U in it.

Buckle in, folks, because we’re diving once again into the murky molecular world of genetics.  We’ll be focusing our attention on DNA (DeoxyriboNucleic Acid) and RNA (RiboNucleic Acid), two indispensable molecules that make us – and by us I mean all living organisms on Earth – what we are.

We’ve discussed DNA before.  DNA is the molecule that contains the instructions for building a living thing.  Every one of your cells – except mature red blood cells – stores a complete blueprint for making you.  That blueprint is encoded in DNA.

The beautiful thing about this genetic code is its simplicity.  Everything in your genes is spelled out using just four letters: A, T, C, and G.  These are the initials of four molecules called adeninethyminecytosine, and guanine.  DNA, you may recall, is shaped like a twisted ladder.  The rungs of this ladder are made of two of these molecules in complementary pairs.  For example, adenine and thymine – A and T – always pair up together in a rung, and so do cytosine and guanine – C and G.

DNA base pairs

At the risk of digressing from the main topic, that seems pretty incredible!  How can just four letters make a you?  After all, you’re way more complex than that, right?

The letters tend to work in triplets called codons.  So for example, one side of a DNA molecule might read:


And its complementary strand would read, in the same direction:


Each triplet codes for a particular amino acid, and the sequence of amino acids makes up a protein, which is a major structural and functional component of living cells.  Working proteins handle every other function the cell needs to live, from manufacturing molecules to digesting nutrients.

Here’s an analogy; if the cell were a city, then DNA would be blueprints for important facilities and machines to build the city.  Each machine is fine-tuned for an important function, so it’s important that its blueprints are stored, interpreted, and copied as accurately as possible.

Human cells keep DNA in the nucleus – the membrane-bound central region of the cell.  The nucleus is like a vault.  It is vital to the safety of these all-important blueprints that DNA never leaves the relative safety of the nucleus.  How, then, do the genetic instructions get to the cellular machinery and infrastructure that interpret them?

That’s where RNA comes in.  RNA is built in the cellular nucleus, transcribed directly from DNA.  DNA unzips, and complementary bases are laid in along the exposed bases.  After transcription, RNA is shipped outside the nucleus, where it dictates the instructions for building proteins to special protein-building units called ribosomes.

DNA Transcription

As you can see, RNA is single-stranded, unlike double-stranded DNA.  But that’s not the only difference.

RNA does not contain thymine.  When the DNA-to-RNA transcribing machinery encounters an adenine base on the parent DNA molecule, it adds a different base, uracil, U, to the growing RNA strand.  So if a DNA strand reads:


The complementary RNA strand reads:


Now here’s the big question: Why?  Why does RNA have U in it?

The better question, as we shall see, is why doesn’t DNA have U in it?

As with all of life’s problems, the answer has to do with chemistry.  All of your cell’s important molecules are bathed in a soup of chemicals with varying degrees of reactivity.  Plus, we amble about in an environment awash in radiation; ultraviolet, gamma rays, X rays – you’re sure to absorb a few hits from ionizing radiation in your day-to-day activities.  It’s just common sense that your DNA is going to take a beating.

For example, the base cytosine, C, is easily converted to uracil, U.  Left unchecked, this simple substitution could wreak havoc on your genetic code.  Fortunately, all of your cells have a repair mechanism to hunt down and correct errors just like this one.  Your cells “know” that uracil doesn’t belong in DNA, so they convert it back to cytosine whenever they find it.

This is no small problem, by the way.  It happens about 100 times per day, per cell.  The enzyme that handles this repair, Uracil-DNA glycosylase, or UDG*, has its work cut out for it.

So DNA cannot contain uracil as one of its bases, because if it did, then UDG would have no way of knowing which uracils to keep and which to replace with cytosine.  That repair pathway would not work.

But hold on, the savvy reader will ask: if U isn’t good enough for DNA, why is it good enough for RNA?  Can’t cytosine get converted to uracil in RNA just as it is in DNA?

Yes, but RNA isn’t meant to exist for very long; just long enough to transfer the genetic code from the nucleus to the ribosomes, where proteins are assembled.  See, DNA gets copied and transcribed over and over; its code has to be durable.  A persistent error in DNA can kill the cell, or worse, lead to cancer.  But RNA is a short-lived throw-away molecule. If an RNA molecule suffers a cytosine-to-uracil mutation, the worst that happens is a couple of proteins don’t get made correctly.  It doesn’t matter in the long run; there will be hundreds or thousands of RNA molecules that don’t get mutated.  Business will carry on as usual.

So we know why thymine is preferable to uracil in DNA, but we haven’t discussed why uracil is preferable in RNA.

It’s because uracil is energetically “cheaper” than thymine.  It costs less energy and resources to manufacture and use uracil than it does for thymine.  So thymine is only used in DNA, whose accuracy is tantamount to the cell’s – maybe even the entire organism’s – survival.  And cheap, easy uracil is used in cheap, disposable RNA.  It’s true what they say: you really do get what you pay for.

Now that we know how DNA and RNA use thymine and uracil, respectively, is this really an effective pick-up line?  If the intended recipient of your woo knows anything about genetics, probably not.  In essence, you’re telling him/her that they are cheap and easily replaced.  These are not the words of a lover.

*UDG is notable as an initialism in that one of its letters stands for another initialism.  Which is a shame, because I think it would be fun to talk about UDNAG.


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