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Chances are, in your life, you’ve had the common cold before.
It can cause sneezing and coughing and just makes you feel awful.
The worst thing about the common cold is that you can take medications to help with the
symptoms, but you really can’t treat the actual cold with antibiotics like you would
a strep throat infection or an infected tooth.
Well let’s talk about what causes it: a virus.
The common cold is caused by a virus.
And while all living things are made of 1 or more cells---see the modern cell theory---viruses
are not cells.
Viruses aren’t technically classified as living things since they lack many characteristics
of life, but there is debate in calling them nonliving as well.
Medications like antibiotics attack bacteria like E.coli which are living cells.
Anti-fungal medications can be used on fungi like athlete’s foot which are made of living
Viruses are not bacteria or fungi.
You can’t classify a virus as a prokaryote or a eukaryote either because it’s not a
So what is their structure like?
First of all, viruses are much, much smaller than cells.
You typically are going to need an electron microscope in order to see a virus.
Viruses come in many shapes.
Our favorite one to draw is the bacteriophage!
It sounds very fancy, but it is a type of virus that tends to attack bacteria.
Despite this being our favorite to draw, know that the shape of viruses vary.
One thing all viruses have in common is that they have some type of genetic material.
This genetic material can be in the form of DNA or RNA.
Viruses usually have some kind protein coat, also known as a capsid.
The capsid can protect that DNA or RNA genetic material.
Viruses can also can have other structures.
Some viruses package special enzymes with them.
Some viruses have an outer envelope.
These additions can be useful to the virus in virus replication.
One thing that makes virus replication, or reproduction, unique is that they can’t
replicate without a host.
And it’s their reproducing that tends to be a problem for its host: whether the host
is you --- in the case of a common cold---, an insect, plant, or this bacterium.
Viruses tend to be very specific for the host that they’re going to go infect, but anything
that is alive can be a host, because cells have the machinery viruses need in order to
We’re going to talk about two types of viral replication cycles.
The first one is the lytic cycle.
In the lytic cycle, the virus, remember very selective, attaches to a host cell.
It often binds to a receptor that this cell has which gives it access to dock there.
The virus can then inject either its DNA or RNA, depending on what kind of genetic material
it has, into the cell.
Some types of viruses are actually taken inside the cell themselves.
Now you would think the cell would notice viral DNA or RNA or an entire virus that has
been taken in, but in many cases it does not.
It takes the genetic material from the virus and it starts following the instructions,
which in this case is very bad, because the instructions tell it to makes copies of the
The cell uses its own resources to start building.
It starts making so many copies of the virus that it can cause the cell membrane of the
host to rupture, explode, lyse!
So what happens is now these new viral copies get out of the cell, and they go and infect
This is known as the lytic cycle.
By the way, the lysing of the cell membrane is a very bad thing for the cell.
The cell cannot survive without its cell membrane.
The other cycle is called the lysogenic cycle, and I like to remember this is the longer
word so I like to think of this as the longer, sneaky cycle.
Viruses that go through this cycle tend to do the same thing at the beginning.
They inject their genetic material, but this time, the genetic material stays hidden in
the host’s genetic material.
What happens is that when the host makes new cells, it replicates its own genetic material
and the viral genetic material.
Then those daughter cells make new cells, and they also replicate their own genetic
material and the viral genetic material.
And so on.
That is the lysogenic cycle.
That may not seem like a problem, but it really becomes one if it gets triggered to go into
the lytic cycle from that point, because then all of those cells that had the viral genetic
material can start assembling viruses.
As far as what triggers it to go into the lytic cycle, it can vary.
It could be a chemical trigger for example or a lack of food for the host.
Going back to the common cold, a lot of times it has to run its course.
Your immune system will typically take care of it and medications can help the symptoms
feel not as bad.
But, unfortunately, some viruses are extremely complicated, such as HIV, a virus that can
lead to AIDS.
Like most viruses, it is very specific.
It binds to a CD4 glycoprotein that is found on the surface of specific cells such as Helper
The problem about this particular target is that Helper T cells play an important role
in protecting the body in the immune system.
Since HIV goes after these immune cells, it can make a person that is infected with HIV
vulnerable to other infections.
There are medications that have been developed to help keep HIV from replicating as quickly,
and there continues to be research looking into ways to keep the virus from attaching---because
if the virus can’t attach---it can’t insert its genetic material.
A problem that continues to be a challenge is that many viruses, including HIV, can mutate.
So a treatment developed against a certain virus type may not work on a mutated form.
So with our talk about viruses, you might wonder, “Is there anything positive about
Well, viruses can play a useful role in gene therapy.
Definitely something to explore more.
Also, remember how we mentioned that viruses can go after other organisms like insects
Some of these viruses can target certain types of pest insects.
A virus that targets pest insects could be an alternative to a chemical pesticide.
One such example virus that you may want to explore is the Nuclear Polyhedrosis Virus.
Still, it’s important to consider that whenever talking about any kind of pesticide---chemical
or viral---there could be consequences in the ecosystem.
Well, that’s it for the Amoeba Sisters and we remind you to stay curious!