Subtitles on! Click CC at bottom right to turn off. You can find us on Twitter (@AmoebaSisters) and Facebook! 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. Why? 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 cells. Viruses are not bacteria or fungi. You can’t classify a virus as a prokaryote
or a eukaryote either because it’s not a cell. 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. Greatly. 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 replicate. 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 virus. 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 other cells. 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 T cells. 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 viruses?” 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 for example? 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!
