Protein Power: DNA vs. RNA
This week, Ask Science begins looking at how proteins are made and what they’re good for. We start at the source of all proteins: DNA.
Lee Falin, PhD
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Protein Power: DNA vs. RNA
A couple of weeks ago, in the episode called The Secret Life of Genes, I talked about the fascinating lives of genes. Our friend Addy the nucleotide had a job he loved, helping transcribe DNA into messenger RNA, (or mRNA), the code that tells the cell how to build proteins.
This week, we’ll take a closer look at how that works by looking at the difference between DNA and RNA.;
It All Starts with Genes
As I mentioned in my episode on The Human Genome, each of your genes is a little section of DNA that contains the instructions for creating a protein. DNA is made up of 4 different kinds of nucleotides: Guanine, Adenine, Thiamine, and Cytosine. Since those names don’t exactly roll off the tongue, we usually just abbreviate them as G, A, T, and C.
The rungs of the DNA ladder are formed when associated bases on each strand stick together. Guanine binds to Cytosine and Adenine binds to Thiamine. Each of these ladder rungs is called a “base pair” because they are made of a pair of nucleotide bases.
When your cell wants to make a protein, it sends an enzyme, called RNA polymerase II to make a copy of your DNA (there are other varieties or RNA Polymerase that make other things, but we won’t discuss them today).
What’s in a Name?
When you’re trying to figure out the meaning of a scientific term like “RNA polymerase,” it’s helpful to break the word up into smaller parts, so let’s take a look at that name.
Anytime you see something biological ending in “ase,” it usually means that it is an enzyme. An enzyme is a special kind of protein that catalyses a chemical reaction. This means it allows the reaction to occur, or helps to speed it up.
The “polymer” part means that this particular enzyme catalyses a reaction that results in making a polymer. You’ve probably heard the world polymer before. It means something big made of a bunch of small somethings stuck together. Since this is RNA polymerase, the something big is an RNA molecule and the small somethings are nucleotides.
So putting that all together, what RNA polymerase does is catalyze the reaction where a bunch of nucleotides are stuck together to form an RNA molecule.
Running with RNA
RNA polymerase II does its work by running along the DNA molecule and taking advantage of the base pairing feature of DNA. Every time it sees a Guanine nucleotide, it sticks a Cytosine to it, and vice versa. When it sees a Thymine nucleotide, it sticks an Adenine to it. And just when you thought things might be making a little bit of sense, when it sees an Adenine, instead of sticking a Thymine to it, it slips in a Uracil nucleotide.
Why in the world does it do that? Let’s see if we can figure it out by looking at the difference between DNA and RNA.
RNA vs. DNA
RNA is lot like DNA, in that they both are made of a bunch of nucleotides stuck together. However there are a couple of key differences. First, one starts with “R,” and the other starts with “D.” This might sound like a dumb thing to say, but all of the rest of the differences in the two molecules can be traced back to this fact.
The “D” of DNA stands for “Deoxyribose,” whereas the “R” of RNA stands for plain old ribose. Let’s use our strategy of tearing apart big words to see what the difference is between these two.
Back in the 19th century, scientists isolated an interesting compound from gum arabic (a gummy sap that comes from the acacia tree). They called it arabinose. When a biological word ends in “ose,” that usually means it’s a kind of sugar (like fructose, sucrose, glucose, etc.). The “arabin” part comes from the fact that it was found in gum arabic.
Over time, scientists discovered that there were a few different kinds of this type of sugar, depending on which way you arranged the atoms. One of those kinds they called “ribose,” which is just a shortened version of arabinose.
Sugars are a kind of carbohydrate, meaning that they are made up of carbon, oxygen, and hydrogen atoms. Ribose’s chemical formula is C₅H₁₀O₅, meaning it has 5 carbons, 10 hydrogens, and 5 oxygens.
So now that we know more about ribose (the “R” in RNA), let’s take a look at deoxyribose, the “D” in DNA. Deoxyribose is ribose that’s been “deoxyied.” In science, words that start with “de” usually mean remove, and “oxy” almost always refers to oxygen. So “deoxy” means, remove an oxygen.
Putting that all together, deoxyribose means a ribose with an oxygen removed.
One of These Nucleotides Is Not Like the Other
As I mentioned earlier, one more important difference between RNA and DNA is found in the kind of nucleotides that RNA uses. While DNA uses Guanine, Adenine, Thiamine, and Cytosine (G, A, T, and C); RNA uses Guanine, Adenine, Uracil, and Cytosine (G, A, U, and C).
And now the reason for this is….nobody knows for sure!
However there are several possible reasons. One of the most likely is that Thymine is more stable. Since DNA is responsible for the long-term storage of your genetic code, stability is pretty important. RNA on the other hand, tends to only stick around until its job is done, so stability is less important.
Conclusion
So now you know more about the differences between DNA and RNA. Next time we’ll take a look at how your cells use RNA to make the workhorse of biology, the protein.
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Mitochondrion image courtesy of Shutterstock.
