Genes are pieces of DNA that code for a protein. That sounds great…but how does it do this? DNA is made out of nucleotides (A, T, C, and G) and proteins are made out of amino acids. To “crack the code” and transform the DNA code into the protein code, you need an intermediary. That’s a molecule called RNA! messenger RNA (or mRNA, for short) to be precise, because it is the message that communicates the information from the DNA to the protein.
Let’s revisit our car blueprint analogy to discuss the role of mRNA. If DNA is the blueprint for the steering wheel, and the protein is the steering wheel, mRNA is the person in the middle who reads the blueprint and builds the steering wheel.
So let’s see how this actually works. The first step is to understand what RNA is actually made out of. DNA stands for DeoxyriboNucleic Acid and RNA stands for RiboNucleic Acid, and if they sound like they are related, you are correct. Here are the three main differences
- DNA is double-stranded and RNA is single stranded (see the photo here)
- DNA is made from A, T, C, and G and RNA uses A, U (standing for uracil), C, and G
- In DNA, A always matches with T, whereas with RNA, A always matched with U
Because they are made up of similar components, the cell can copy the code from the DNA into the single-stranded mRNA molecule. Also because these two molecules are related, this process is call transcription – like what you would do when copying the words in a book from one place (DNA) to another (mRNA).
The mRNA is then what codes for the protein. It does this just like you would expect any code to – using a key. In this case, each set of 3 nucleotides codes for an amino acid. The cells have machinery (called a ribosome) that “reads” the mRNA strand. The combination of “ATG” always tells the ribosome to START making a protein. It then reads the three digit code, adding the appropriate amino acids along the way, until it reads one of the three codons that tells the ribosome to STOP making protein. At this point, the protein can fold up to make the 3D structure so it will function. Because this process takes nucleotides and codes them into amino acids (a totally different molecule), this process is called translation. Just like translating a book from English into French, this process translates mRNA into a protein.
Wow what a lot to take in! This process from DNA (the gene) to mRNA to protein is called the “central dogma” of molecular biology. This process was originally described in 1956 by Francis Crick (the same guy who along with Watson discovered the structure of DNA). Why is it called this? Probably because when it was described, they realized that this was a fundamental process for all life. It’s also important for us to know for so many reasons. If you know that DNA codes for RNA which codes for protein, any changes in this process can result in a protein that isn’t quite right. What if you have change the code of the DNA? This will change the protein! What if you change the amount of the mRNA? You will change the amount of protein! And what can this do? Cause problems in your cell that cause disease!