Ken Tao is the MedSchoolCoach expert on MCAT, and will discuss introns and exons, and help you remember them as Exons are expressed, and Introns are in the trash.
Welcome back to another MCAT Mnemonic Monday. I’m Ken and I’m an MCAT expert with MedSchoolCoach. Today we have another biology mnemonic for you and it’s on Introns and Exons.
You probably recall hearing these terms when you were studying genetics for the MCAT. Let’s go ahead and start by doing a quick review of these terms.
During gene expression, we take DNA and we transcribe it into RNA using RNA polymerase. So you can see here, we have the double-stranded DNA that is then used to produce single-stranded RNA.
This RNA molecule that is produced in eukaryotes is called pre-mRNA. That’s because this pre-mRNA molecule is not going to be translated directly into proteins. In fact, the pre-mRNA has to undergo what is called pre-mRNA processing to form the mature form of mRNA.
During pre-mRNA processing three events occur. First, when you’re looking at pre-mRNA, there are some parts of the pre-mRNA molecule that are called exons (in blue) and other parts that are called introns (in red) in this diagram. The introns gets spliced out leaving only the exons.
So you can see here, we have the exons and the final mRNA molecule, but no more introns.
In addition, we add a 5′ GTP cap as well as a 3′ poly-a tail. Now the focus here is on exons and introns. So you want to keep in mind for the MCAT that introns get removed and exons are kept in the final mRNA molecule.
So a good mnemonic to help you memorize that is “exons are expressed.” So they’re kept in the final mRNA molecule, whereas “introns are in the trash.” So because they’re in trash, the introns are not in the final mRNA molecule.
So hopefully that’ll help you keep track of exons and introns for the exam.