At the beginning of this pandemic like everyone I was hearing lots about viruses, but realised
I didn’t know that much about what they are.
So I did a load of research and have summarised what I learned in these nine images.
Okay let’s get into it.
Viruses are the most abundant form of life on Earth.
But, are they life?
Not really.
They can’t reproduce on their own so they have to invade other cells to take over their
molecular machinery to reproduce using the steps shown here which I’ll go into more
detail later.
There’s an unknown number of different kinds of virus, definitely in the millions, if not
higher.
But only about six thousand have been studied in detail.
They vary wildly, invading all kinds of cells, animals, plants, bacteria and archaea and
none of them work in precisely the same way.
Despite this bewildering range it is possible to make some categorisations, although please
be aware that there are exceptions to any category that I’m defining.
That’s the nature of any subject as complex as virology.
With that in mind viruses always contain a genome, the instructions for making more viruses,
and a protective shell made of proteins called a capsid which keeps the virus safe and helps
the virus stick to and enter the cells they’re invading.
Some viruses are also coated in an envelope, a greasy cover taken from the membrane of
the last cell they infected.
Viruses are incredibly small.
For example an average human cell is a bit smaller than a tenth of a millimetre or a
hundred micrometres.
And viruses are around a thousand times smaller than that, ranging from about 20 nanometres
upwards.
Here are a few virus particles drawn to scale.
Hepatitis A, hepatitis C, coronavirus, influenza, hiv and the humongous ebolavirus.
For some viruses their genome is really small coding for just two proteins, but other viruses
code for up to 2500 proteins for the largest genomes.
There are a few ways to classify viruses.
The genome of a virus is either based on DNA or RNA.
And is either single stranded or double stranded.
Viruses come in a wide range of shapes.
The capsid of viruses are made of many of the same shaped proteins that self assemble
into different shapes like helical which is a spiral, icosahedral, made of shapes like
triangles or hexagons or pentagon to make a shape like a twenty sided dice or soccer
ball.
Prolate which is the same kind of thing but stretched out.
Complex which covers a wide range of viruses that don’t fit into the other categories,
and finally enveloped viruses which we mentioned earlier.
There is a scientific way to classify viruses by following the official taxonomy.
Another useful classification technique is the baltimore classification, which looks
at the genome of a virus, and it’s pathway to encoding messenger RNA or mRNA which is
a single stranded molecule that the host cell uses to make the virus proteins.
There are seven classes in the baltimore classification which represent different forms of single
and double stranded DNA and RNA.
For an example et’s look at the coronavirus that caused the 2020 pandemic.
It is an enveloped virus, meaning that it is very vulnerable to soap which rips of the
outer protective coat destroying it, which is why we’ve been told to wash our hands
so often.
And in the baltimore classification it is a class four virus which is a type of single
stranded RNA virus.
Interestingly this is different to seasonal flu which is a class five virus, so the two
are not related.
Now let’s look in a bit more detail about how viruses get into cells, how they reproduce
and how they get back out again.
There are three main ways viruses get their genetic material into a cell.
Enveloped viruses attach to receptors on the surface of the cell which then fuses with
the membrane.
Remember they are made of the same stuff because the virus took it’s envelope from the last
cell it was in, so the new cell doesn’t see that it is an invader.
Viruses without an envelope essentially trick the cell into thinking that the virus is a
harmless resource like nutrition, and engulfs it.
This is called endocytosis and viruses with an envelope can enter a cell this way too.
This virus then has to break out of this vesicle to release its genetic material.
A less common method of entering a cell is genetic injection where the virus punches
a hole in the cell membrane and directly injects its genetic material into the cell.
Bacteriophages do this to bacteria, but not to human cells.
Replication is a very complex process so I’m having to break it down into perhaps an overly
simplistic description, but at least it’s a start.
In general RNA viruses replicate in the cell cytoplasm, and DNA viruses replicate in the
cell’s nucleus, but there are exceptions to this.
The host cell then starts transcribing the viral genetic material without knowing that
it is not native to the cell.
The virus codes for essentially two things, an enzyme called polymerase and proteins which
form the capsid shell.
Polymerase creates copies of the viral genome.
This is also when mutations can happen to the virus.
In general viruses mutate very fast, RNA viruses more quickly than DNA viruses because RNA
is inherently less stable than DNA.
These mutations are random, most having little effect on the virus, but sometimes they can
make the virus more dangerous or less dangerous to the host.
The capsid proteins self assemble, from a couple of simple shapes they can form large
complicated three dimensional structures which enclose copies of the genetic material inside,
to form a new virus particle ready to leave the cell.
It’s worth mentioning retroviruses which are a type of virus that insert their viral
DNA into the DNA of their host.
This has happened a lot through our evolutionary history and in fact viral DNA sequences make
up 8% of our genome, a lot of which we share with our common ancestors.
There are three ways viruses leave cells.
Apoptosis results in the death of the cell through a self-destruct mechanism.
The viruses either burst out as shown here, or more usually the cell death is a more controlled
process where sections clump off to be absorbed by other cells like macrophages.
Budding is where the virus exits the cell, taking with it an envelope of the cell’s
membrane.
This doesn’t kill the cell, but will degrades it over time and will eventually lead to the
cell’s death.
A kind of opposite process is exocytosis where the virus has an envelope inside the cell,
which it got from the nucleus membrane, or another membrane in the cell.
Then the virus exits through the cell wall leaving the membrane behind.
And this doesn’t kill the cell.
I should also mention that some viruses can stay dormant inside cells for years at a time
which is called latency.
So that’s how viruses invade cells, now lets zoom out and look at a broader picture
of viral infections.
The range of a virus is how many different kinds of organism the virus can infect.
Plant viruses don't infect animals and most animal viruses can't infect humans.
In most cases viruses are adapted to a single species, but some like rabies have a larger
range and others can cross over to other species when they mutate.
If we look at humans we can group viruses into those that have been with us for a long
time, and those which have recently crossed over.
Equilibrium viruses have had a long time to adapt to our biology so tend not to be lethal
to us.
After all it is more effective for the virus to keep us alive to infect other people rather
than killing us.
Non-equlibrium viruses are a lot more dangerous because the virus is not adapted to our biology
and so the mortality rate from these infections is a lot higher as different people’s immune
systems react to them in many different ways.
Here are some examples of non-equilibrium viruses including of course the coronavirus
pandemic we are currently experiencing.
Influenza has got a segmented genome, a genome in many parts which means that different strains
can swap genes if they infect the same host.
This means they are more likely to cross over species and is called antigenic shift, which
is what happened in the 2009 H1N1 epidemic.
Our bodies have got sophisticated mechanisms to detect and destroy viral infections.
It starts when a virus is ingested by an antigen-presenting cell which breaks it down and displays portions
of the virus to activate T-helper cells.
The T-helper cells then activate two responses to the virus.
B cells produce antibodies that are targeted at the specific virus.
These antibodies bind to the outer surface of the virus neutralising it, they also tag
the virus to be destroyed if it enters a cell by enzymes within the cell.
Cells in our body continually display what proteins they have inside them on their surface.
If a cytotoxic T cell recognises a viral protein on the surface of an infected cell, it will
go and destroy that cell.
Our body’s B and T cell’s keep a memory of the virus which can make us immune to future
infections from that specific virus, but this immunity wears off over time.
Sometimes this immunity lasts many years, but other times like for the viruses that
cause the common cold and flu, our immunity is not very good because of the high variability
in these viruses, or because our antibody response is poor.
Vaccines are incredibly effective ways of preventing viral infections.
It is possible to completely eradicate viral infections of human equilibrium viruses which
we’ve managed with smallpox, and is possible with polio, measles, mumps and rubella if
only people would listen.
Vaccines contain a modified form of the virus that has been weakened in some way so that
they no longer cause an infection but they do stimulate an immune response, teaching
our immune system how to recognise the virus.
And where there’s no vaccine antiviral drugs can be used to treat viral infections.
Although development of these drugs is difficult because they only target a specific virus
which are continually mutating.
One example of an antiviral technique are drugs that fool the virus into incorporating
dummy DNA into their genomes which stops them from having the instructions to replicate
any more.
So that covers everything I wanted to talk about, if you want to find out more there
are a bunch of links in the video description.
You’ll also find a link to this image which I’d love for you to share around the internet,
as well as this video, I think the more people who get access to good information about viruses
right now the better.
This content wouldn’t be possible without the support of my patrons and people buying
posters.
So if you’d like help out please consider it.
I hope you are all doing okay and I’ll see you with more science soon.