What is a virus?

This is the first in a series of posts where I hope to answer some questions I’ve been asked about coronavirus by children. The first question I will try and answer is: what is a virus?

To understand viruses, we first need to understand a few things about cells. Your body is made up of millions of cells, which are the smallest form of life.

Human cells from inside the cheek,
seen under a microscope.

Cells are tiny – they are about 100 millionths of a metre in diameter. You can see them under a light microscope – if you are in Year 7 or 8 you have probably done this at school.

There are lots of different types of cells, and some look very different to the picture on the right. But there are certain structures, called organelles, that nearly all cells have, and these are shown in the diagram below.

The cell membrane is around the outside of the cell and controls what goes in and out of the cell. The inside of the cell is called the cytoplasm. Mitochondria are the cell’s power source – they take glucose (from our food) and react it with oxygen (from the lungs) to provide the energy the cell needs. Ribosomes are the cell’s factories – they make all the various substances that the cell needs. The nucleus contains the instructions the cell needs to function, stored on a molecule called DNA. More about that in another post.

The structure of a virus is very different to a cell. To start with, they are much, much smaller. Viruses are about 1000 times smaller than human cells. The goal of a virus is simply to reproduce – to make as many copies of itself as it possibly can. That is literally all they exist to do. The structure of a virus is a lot simpler than that of a cell, as shown in the diagram below:

Structure of a virus. A nanometre is one billionth of a metre
Influenza viruses seen with an electron microscope. You can clearly see the spikes on the envelopes of each virus.

As with cells, there are lots of types of virus and some look different to the one in the diagram. But they all have certain structures in common. All viruses have a protective capsule, called the envelope, which protects the contents. On the surface of the virus there are protein molecules sticking out – these are called spikes, and help the virus attach itself to a cell.

Inside the envelope is a strand of either DNA or RNA. These are molecules which carry all the information the virus needs to make copies of itself. Basically, the DNA or RNA is an instruction manual for making more viruses.

But, viruses have a problem. They don’t have any organelles, and they need them in order to reproduce. They need a nucleus to process the DNA or RNA (a bit like reading the instructions). They need mitochondria to provide energy, and they need ribosomes to make and assemble the new viruses. They solve the problem by taking getting inside a cell (called the host), taking over its organelles and using them to make new viruses. Here’s how its done:

  1. Attachment. The spikes on the surface of the virus stick to the surface of the cell. Have you ever been for a walk in long grass in summer, and come back with those spiky seeds stuck to your clothes? It’s a bit like that.
  2. Penetration. Once the virus has attached itself to the cell surface, it can penetrate the cell membrane and get inside.
  3. Uncoating. The envelope of the virus breaks apart, releasing the contents into the cell.
  4. Biosynthesis. Synthesis is basically a scientific word for making something. Biosynthesis is when the virus takes over the cell’s organelles and uses them to make everything needed for a new virus.
  5. Assembly. The virus uses the cell’s organelles to put the new viruses together.
  6. Release. The virus destroys the cell membrane so that the cell bursts open and releases the new viruses. They then go on to infect other cells.

So, how do viruses make us ill? Partly by the damage they do to our cells, but also as a result of the things our body does to fight them off. More of that in a later post.

I really hope you have found this post useful. Coming up tomorrow: how viruses change, and how viruses move from animals into humans. If you have any questions you’d like me to try and answer, please send them to me by comment, email or Facebook!