Leah here from leah4sci.com/mcat.
And in this video, we're going to begin our discussion on viruses.
When you hear the word virus, what's the first thing that comes to mind?
Are you thinking infection?
Swine flu, coronavirus, a general contagious disease?
Let's take a step back.
Infectious agents that cause disease or germs are known as pathogens.
Common pathogens can include bacteria, fungi, parasites, the small critters that cause infection
and viruses.
Bacteria, which are unicellular prokaryotes, are not all bad.
Only some are pathogenic.
For example, you have millions of bacteria living in your intestines producing vitamin
K, which is helping you survive.
But then we have a small amount that are pathogenic and responsible for the E-coli that keeps
popping up, scaring everyone from eating lettuce, a fungus.
on the other hand, is a eukaryote and can include things like mold or yeast.
And once again, we have some that are very beneficial and some that are pathogenic.
Many antibiotics come from mold like penicillin.
And who doesn't love a delicious sourdough bread or some alcohol that relies on yeast
to do the fermentation?
Fungi can also be pathogenic.
Think of toxic mold that grows in human areas or itchy yeast infections when looking at
pathogenic parasites, there are many that live within the body or outside the body and
can range from worms, ticks, mosquitoes and so much more.
And finally, there are viruses.
Virus comes from the Latin word that means poison and is the smallest infectious particle
compared to everything we looked at here.
A typical virus will range in size from 20 to 400 nanometers, where a nanometre is 1
times 10 to the minus 9 meters, which is a thousand times smaller than a bacteria that
can be about a thousand nanometres or one micrometer.
Forgot how to interconvert between the units?
Make sure to review my cheat sheet link below or visit leah4sci.com/conversions.
Unlike the other live pathogens like bacteria, fungi or parasites, viruses are actually not
alive, although some will argue that they are.
Characteristics that define the living organism includes must be made up of a cell or a group
of cells.
Obtain food to grow and survive, adapt to the environment, and most importantly, be
able to reproduce, be able to have children.
The argument for viruses not being alive.
First of all, a virus is not made out of cells.
As we'll see in a moment.
A virus is simply genetic information surrounded by a protein coat, something live must be
able to obtain food to grow and survive.
Viruses don't eat.
The cells that replicate the viruses have to eat.
Those cells are alive, but the virus that infects the cell doesn't rely on food.
And finally, something live must be able to reproduce and have offspring and viruses definitely
cannot reproduce.
Wait a minute, Leah, isn't this why the world is panicking over the seasonal flu and corona
virus?
Because it keeps growing and spreading?
Actually, no.
The virus hijacks a living cell and tricks the cell into making more viral particles.
But the virus itself doesn't have children.
It forces another living organism to create more viruses, which is something we'll discuss
in part two of this series.
Think of a virus as a computer code.
You can have a computer code for turning lights on and off, you can even have a computer code
for launching a rocket into space.
But the code itself is just a set of instructions.
If you look at just a computer code, it's useless without a host.
It needs a computer to read the code to act on the code, to turn the lights on and of
course to shoot that rocket into space.
Let's take the music example.
So you're trying to copy music from a friend's computer the old fashioned way.
The music in mp3 format has to be put onto something like a usb stick.
The usb is not the music, instead, the music that you copied is now code that lives in
that usb stick.
It can survive outside a computer, but it's not music.
You need to take that usb insert it into another computer and then you can play the music.
Makes sense so far?
If yes, quickly give this video a thumbs up and then let's talk viral science.
Viruses are obligate intracellular parasites.
That means they're parasites, things that infect with an obligation to live inside a
cell.
If they're not in a cell, they can be destroyed.
To be clear, they can exist outside the cell.
In fact, a virus living outside of a cell is called a variant.
But unlike your music, living in a usb, it can't exist for months or years or even decades
in your office or a backpack.
If a virus is outside the cell too long or exposed to chemicals, heat or other conditions,
it is subject to die.
Or should we say subject to destruction because if it's not alive, can't really die.
There are many different types of viruses specific and the type of organisms they can
infect.
Bacteriophage or Bacteriophage is a virus that specifically infects bacteria.
There are also specific viruses that infect plants, animals and humans which are technically
considered animals.
Some common viruses that probably come to mind include influenza, the flu or rhinovirus,
which is the common cold.
The zoonotic virus is an interesting one coming from the word zoo.
Zoonotic virus are animal viruses that can infect people.
I bring this up because the corona viruses are zoonotic viruses.
Remember the SARS coronavirus from 2002 that was transmitted from a civet cat to humans
Covid 19 is another coronavirus believed to have originated in bats, transmitted to the
pangolin and from the pangolin to humans.
Every virus contains a viral genome and the protein code called the capsid.
Some viruses also contain additional proteins that act as the machinery to help in the takeover.
And some viruses also contain a viral envelope, a lipid bilayer just like the cell or the
endoplasmic reticulum that surrounds the protein capsid.
Back to our music usb example.
The viral genome would be the code for your music, the code for your songs, the protein
code or the capsid would be the USP stick itself.
The additional proteins that may or may not be there are the additional pieces of code
that tell the new computer what to do with this information, how to play the music.
And finally, to envision the envelope, think of those funky usb covers that look like teddy
bears or other animals.
The viral genome can be made up of four different types of nucleic acids.
There are DNA viruses that can consist of double stranded or single stranded DNA.
And the RNA or retroviruses can also exist as double stranded or single stranded RNA.
As a quick reminder, DNA is found in the cell coding for all of the genes that we need.
DNA stands for deoxyribonucleic acid.
RNA stands for ribonucleic acid.
It doesn't have the deoxy, it's not missing that oxygen.
And typically we see T -RNA, m-RNA and r-RNA which are involved in helping to create the
proteins that are coded for by the DNA.
Double stranded DNA is just a double helix of A,T,C,G and a single strand will be the
same thing but without having a partner strength.
Same thing for RNA.
A double strand of RNA will be A,U,C,G, but it will have a partner to form that helix.
Single strand that RNA is just a single strand of A,U,C,G.
The nucleic acids live in a protective protein code called the capsid.
The capsid is made up of individual proteins called capsomeres, where meres tells us that
its units.
So we'll have a very small number of proteins that just repeat in a crazy pattern to give
us the entire protein codes.
The protein code can take on two common shapes.
The first one is helical.
Just think of it as a helix or a tube with a hollow center and the nucleic acids live
on the inside.
The length of the tube is determined by the length of the viral genome so it can be short
like a disc or really, really long.
More common shape is the icosahedron, which you can think of as a rounded cube with 20
faces and that's what I tried to draw here.
And finally, we have the viral envelope, which is seen on a lot of viruses that infect humans
and animals.
This is a lipid bilayer around the protein coat, which is very similar to what you'll
see in a cell membrane or membrane organelles inside the cell.
In fact, these membranes are made from the cells that the virus infects.
What's important about the viral envelope is that it may contain proteins and glycolipids,
carbohydrates attached to the membrane that participate in cell signaling and is how a
lot of these viruses can identify and attach to the cells if they're going to infect.
How does having this structure allow the virus to infect the cell and ultimately hijack the
cell to replicate?
Well, that's exactly what we're going to discuss in the next video where we look at viral reproduction,
including the Lytic and Lysogenic stages.
But first, make sure to give this video a thumbs up and subscribe to my channel so you
don't miss out on any new videos that I post and then visit leah4sci.com/virus.