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.
