Protein Power: Transcription
Ask Science looks at different kinds of RNA and how our cells use them to make proteins.
Lee Falin, PhD
Listen
Protein Power: Transcription
A few weeks ago, in the episode called The Secret Life of Genes, I talked about…the secret life of genes.
In case you haven’t listened to that episode (and you should!), in it we met Addy, the nucleotide. Addy had a job he loved – helping transcribe DNA into messenger RNA, (or mRNA), the code that tells the cell how to build proteins.
In last week’s episode called Protein Power: DNA vs. RNA, we looked at RNA and how it differs from DNA. Make sure to check that one out too.
Today we’ll look at a couple of different kinds of RNA, and how the cell uses them to make proteins. ;
Special Delivery
When we last left our story, the RNA polymerase II enzyme had just finished making a copy of the gene by sticking a bunch of nucleotides together to form RNA. Well, now things really start to get interesting!
From Genes to Proteins
As I mentioned last time, RNA, or Ribonucleic Acid, comes in lots of different varieties. One of the kinds we need in order to make proteins is mRNA or, “messenger” RNA.Â
When RNA polymerase first creates the chunk of RNA needed to make a protein, it’s called pre-mRNA or precursor-mRNA. Before it can go on to be used to make a protein, however, a few more steps have to happen. To understand why, let’s take a look at some statistics.Â
For a long time, scientists thought that every gene contained the instructions for exactly one protein. Then, after the human genome was sequenced, we discovered that the human genome only has about 21,000 different protein-coding genes, while there are somewhere between 250,000 and 1,000,000 different proteins created in your cells. So what’s going on? The answer to this mystery is something called “alternative splicing.”
When RNA polymerase first creates the chunk of RNA needed to make a protein, it’s called pre-mRNA or precursor-mRNA.
Your genes actually contain two different types of DNA sequences: exons and introns. For the most part, exons are the parts of the gene that contain the protein sequence, while Introns are extra bits of DNA stuck in between the exons. In order to get the real protein sequence, your cells send in special proteins to cut out the introns and splice the exons back together.Â
For example, if I give you the instructions, “Go to the park CHEESE and play PICKLES football.” The words, “cheese” and “pickles” are like introns. We need to splice them out to get a sentence that makes sense. For a long time, scientists thought that these introns were just leftover relics of evolution, chopped out and discarded. However, recent research has shown that many introns actually play important roles in the regulation of our cells.Â
        Â
So now that we know about splicing introns, let’s get back to the discrepancy between the number of genes and number of proteins in your cell. In the late 1970s and early 80s, scientists realized that these instructions don’t always get spliced the same way. Sometimes, some of the exons (the sections of the gene containing the protein-building instructions) get chopped out as well. This is what’s known as alternative splicing.Â
So going back to our “Go to the park CHEESE and play PICKLES football” example, we could make lots of different instructions out of this, depending on what we splice out. Still chopping out the “introns” (cheese and pickles), but instead of just sticking with “Go to the park and play football,” we could also chop out other parts to make “Go play football,” “Go to the park,” “Go to the football,” or even “Park and play football.” There are lots of possibilities.Â
There are even some cases where introns are left intact, while exons are chopped out, though this is rare.Â
Feather in My Cap
Once the splicing is finished, there are two more parts needed for the transformation of pre-mRNA to full-fledged mRNA.
First, an extra guanine is attached to one end of the mRNA in a special way. This new addition is called the 5’ cap, (read as 5-prime cap). This cap serves a number of purposes. First, it allows the mRNA sequence to exit the nucleus so that it can find its way to the ribosome for translation (more on that next week). Second, it prevents the cell from breaking down and recycling its parts.
At the other end of the RNA sequence, a bunch of adenosine nucleotides are tacked on. This is called the “polly(A) tail,” which means a tail made of a bunch of As. Like the 5’ cap, the tail also protects the RNA from being broken down, and it also helps the ribosome know where the end of the sequence is.Â
Conclusion
Last week we talked about how your cells convert DNA into RNA as the first step to making a protein. This week we covered how that RNA is turned into messenger RNA or mRNA. Next week, we’ll finally see how that mRNA is used to make a protein.
If you liked today’s episode, you can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.comcreate new email.
Telomerase image courtesy of Shutterstock.
