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.
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.