What is mRNA? The messenger molecule that is billions of years old in every living cell is the major ingredient in some COVID-19 vaccines.

One surprising star of the coronavirus-pandemic response was the molecule called mRNA. It is the major ingredient in the Pfizer and Modern COVID-19 vaccines. But mRNA itself is not a new invention of the laboratory. It evolved billions of years ago and is naturally found in every cell in your body. Scientists believe that RNA originated in the earliest life forms, even before DNA existed.

Here is an crash course in exactly what mRNA is and the important task it does.

Meet the genetic middleman

You probably know about DNA. This is the molecule that contains all your genes that are spelled out in a four-letter code – A, C, G and T.

DNA is found in the cells of every living thing. It is protected in a part of the cell called the nucleus. The genes are the details in the DNA blueprint for all the physical traits that make you unique.

But the information from your genes has to go from the DNA in the nucleus to the main part of the cell – the cytoplasm – where proteins are made up. Cells rely on proteins to perform the many processes needed to make the body function. This is where messenger RNA, or mRNA for short, comes in.

Sections of the DNA code are transcribed into abbreviated messages that are instructions for making proteins. These messages – the mRNA – are transported to the main body of the cell. Once the mRNA arrives, the cell can produce specific proteins from these instructions.

The structure of RNA is similar to DNA but has some important differences. RNA is a single strand of code letters (nucleotides), while DNA is double stranded. The RNA code contains a U instead of a T – uracil instead of thymine. Both RNA and DNA structures have a backbone of sugar and phosphate molecules, but RNA’s sugar is ribose and DNA is deoxyribose. DNA’s sugar contains one oxygen atom less and this difference is reflected in their names: DNA is the nickname for deoxyribonucleic acid, RNA is ribonucleic acid.

Identical copies of DNA are in every cell of an organism, from a lung cell to a muscle cell to a neuron. RNA is produced as needed in response to the dynamic cellular environment and the immediate needs of the body. It is mRNA’s job to fuel the cellular machinery to build the proteins encoded by the DNA that are suitable for that time and place.

The process that converts DNA to mRNA in protein is the basis for the functioning of the cell.

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Programmed to be self-destructive

As an intermediary, mRNA is an important safety mechanism in the cell. This prevents invaders from hijacking the cellular machinery to produce foreign proteins, because any RNA outside the cell is immediately targeted for destruction by enzymes called RNases. When these enzymes recognize the structure and the U in the RNA code, it erases the message and protects the cell from false instructions.

The mRNA also gives the cell a way to control the rate of protein production – to turn the blueprints “on” or “off” as needed. No cell wants to produce all the proteins described throughout your genome at once.

Messenger RNA instructions are set to self-destruct, such as a text or Snapchat message that disappears. Structural features of the mRNA – the U in the code, its single-stranded form, ribose sugar and its specific sequence – ensure that the mRNA has a short half-life. These features combine to make the message “read”, translate into proteins and then quickly destroy – within minutes for certain proteins to be strictly controlled, or up to a few hours for others.

Once the instructions disappear, the production of proteins stops until the protein factories receive a new message.

Use of mRNA for vaccination

All the properties of mRNA have made it of great importance to vaccine developers. The purpose of a vaccine is to make your immune system respond to a harmless version or part of a germ, so if you come across the right thing, you are ready to ward it off. Researchers have found a way to enter and protect an mRNA message with the code for a portion of the protein on the SARS-CoV-2 virus.

The vaccine provides just enough mRNA to make just enough of the ear protein for a person’s immune system to generate antibodies that protect them when they are later exposed to the virus. The mRNA in the vaccine is quickly destroyed by the cell – just like any other mRNA. The mRNA cannot end up in the cell nucleus nor can it affect the person’s DNA.

Although these are new vaccines, the underlying technology was initially developed years ago and over time incrementally improved. As a result, the vaccines have been well tested for safety. The success of these mRNA vaccines against COVID-19, in terms of safety and efficacy, predicts a bright future for new vaccine therapies that can be quickly adapted to new, emerging threats. Early clinical trials using mRNA vaccines have already been performed for influenza, Zika, rabies and cytomegalovirus. Certainly, creative scientists are already considering and developing therapies for other diseases or disorders that could benefit from a similar approach to that used for the COVID-19 vaccines.