How do viruses change?

In this post I will try to answer three questions that are related to each other. How do new viruses evolve, how do viruses spread from animals to humans, and can viruses change when they start to spread.

In my last post, I talked about how viruses contain a strand of DNA or RNA which holds all the information needed to make new copies of the virus. When a virus reproduces, part of the process is making a new copy of either the RNA or the DNA.

DNA, which stores information.

If you’ve ever had to copy something out, you’ll know that from time-to-time, you make a mistake. The same thing can happen when new copies of the virus’ DNA or RNA are made. These mistakes are called replication errors. The altered DNA or RNA can still be used to make new viruses, but they will be different – this is called a mutation.

Replication errors do not produce completely new viruses. Instead, they produce new versions of the original virus – these are called strains. Viruses which store their information as RNA have more replication errors than those with DNA. There are two reasons for this. RNA is designed to store information for shorter periods of time, so it is less stable than DNA. Also, remember, the virus is using the organelles of a cell to copy itself. The organelles in a cell are designed to copy DNA, not RNA. They have all sorts of safety mechanisms to prevent replication errors when copying DNA, but these won’t work for RNA. The common cold is caused by RNA viruses, this is why there are so many different strains of it.

Flu is also caused by an RNA virus. As well as replication errors, flu viruses can do something called re-assortment. Basically, if two different strains of flu infect the same person or animal, they can mix their RNA together and then recombining it, giving a new strain which is very different to the old ones. The most recent example of this was swine flu in 2009.

So, what about coronavirus COVID-19? It’s an RNA virus, so whenever it reproduces there may be replication errors. Most replication errors actually cause changes that make viruses weaker, and these strains die out very quickly. Very occasionally a replication error will give a new strain that survives.

So, is COVID-19 a completely new virus? The answer is no, and this relates to the second question, which is about how viruses spread from animals into humans. If you’ve read my post about how viruses work, you’ll know that they invade cells and take them over in order to make new viruses. The first step in this is to stick onto the surface of a cell.

Spikes on the virus must match exactly with receptors on a cell.

The envelope of a virus is covered in proteins, called spikes, which it uses to attach itself to a cell. These proteins will attach to proteins on the cell surface called receptors. For this to happen, the shape of the spike has to match up with the receptor – like a key fitting into a lock, or two jigsaw puzzle pieces fitting together. If the key, or the puzzle piece, is even slightly the wrong shape, it won’t fit.

There are a lot of viruses that affect animals but which humans can’t catch – this is because the spikes are the wrong shape to attach to human cells. Every so often though, a mutation will occur which alters the shape of the spikes, so that they can attach to human cells. Humans can now catch the new strain of the virus.

COVID-19 spread from animals when a mutation changed the shape of the spikes so they now match receptors on human cells.

So, how did this happen with COVID-19? Viruses are more likely to cross from animals into humans if there is a lot of contact between them. Wuhan, in China, where the virus was first seen, has a large market where live animals are sold. The city is also very overcrowded, so it’s easy for viruses to spread. Scientists think that the virus spread to humans from animals being sold in the market. They aren’t sure which animal, but coronaviruses are common in bats, and in an animal called a pangolin – both of those were on sale in Wuhan.

The final question is, do viruses change when they start to spread? The answer is, yes. The more a virus spreads, the more it reproduces, so the more opportunity for replication errors. With a virus like flu, there is also more chance of the same person or animal being infected by two different strains, leading to re-assortment. So, how will COVID-19 change as it spreads? The answer is that any changes are likely to be very small, because they will be due to replication errors – coronaviruses can’t undergo re-assortment like flu viruses do. Changes are likely to be very minor and will not alter how the virus affects humans. Scientists working on a vaccine will also be making sure that it will still work if there are changes to the virus.

I hope you have found that useful! In my next post, I will be looking at how scientists study new viruses and find out how they work.

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!