What is mRNA, and how does it work?

I am very, very happy and excited today! The reason? I have a date booked for my COVID vaccine. 23rd of March is the day (coincidentally, the anniversary of the first UK lockdown), and it can’t come soon enough for me. The speed at which the various COVID vaccines have been developed, tested and approved for clinical use is impressive and means that we should, finally, have the end of this pandemic in sight. Unfortunately though, the anti-vaxxers are coming out of the woodwork in droves, and there are all sorts of myths and misinformation being pedalled on social media.

This is the first in a series of posts I am planning about the COVID vaccines. Many of the vaccines, including Pfizer and Astra Zeneca, are mRNA vaccines. So, I am going to start with the basics: what mRNA is and how it works.

mRNA is one of a group of biological molecules called the nucleic acids. These are DNA, and various types of RNA. Nucleic acids consist of molecules called nucleotides, joined together in long chains. Each nuclotide consists of a sugar, a phosphate group and a nitrogenous base. The sugar and phosphate make up the backbone of the chain, and the nitrogenous bases make up the genetic code.

DNA stands for deoxyribonucleic acid. It is a stable, information storage molecule that contains the ‘instructions’ for making proteins. In humans, all the instructions (genes) for every protein the body needs to make are stored on 46 molecules of DNA, called chromosomes. These make up the genome, and are found in the nucleus of every cell. DNA nucleotides consist of a phosphate group, the sugar deoxyribose, and one of four bases: adenine (A), thymine (T), guanine (G) and cytosine (C).

DNA never leaves the nucleus of a cell. For one thing, it’s too big and cumbersome. For another, it needs to be protected against damages. So, when a cell needs to make a particular protein, the gene for that protein is copied in the form of messenger RNA or mRNA. Ribonucleic acid (RNA) differs from DNA because it is a short-term molecule used for the transfer and processing of genetic information. There are many types of RNA, of which mRNA is just one. RNA nucleotides consist of a phosphate group and the sugar ribose. Three of the nitrogenous bases are the same as those in DNA: A, G and C. However, thymine (T) is replaced with uracil (U).

When a cell needs to make a particular protein, an enzyme called RNA polymerase copies the gene in the form of a molecule of mRNA; this is called transcription. The mRNA leaves the nucleus of the cell and enters the cytoplasm, where it binds to a structure called a ribosome; the ribosome then assembles the protein. This is called translation.

Viruses like COVID-19 cannot carry out transcription and translation, since they do not have ribosomes and various other things that are needed. Viruses carry their genetic material in the form of DNA or RNA – RNA in the case of COVID-19. When COVID-19 infects a cell, the virus capsule breaks open, releasing the RNA into the cell’s cytoplasm. Ribosomes in the cell will bind to the viral RNA in the same way they bind to mRNA, and will manufacture the necessary proteins for producing new viruses.