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Chances are in your life, you’ve had the common cold before. The cold is extremely
common. You know, it causes the sneezing and the coughing and just makes you feel awful.
The worst thing about the cold is that you can take medications to help with the symptoms,
but you really can’t treat the actual cold. You can’t really get rid of what causes
it. Why is that?
The reason for this is because most colds are caused by viruses. We’ve talked before
about antibiotics; they only work on bacteria. Viruses are definitely NOT bacteria. So antibiotics
have no effect on viruses. In fact, viruses aren’t even cells at all. You can’t say
that viruses are a prokaryote or a eukaryote. It’s not a cell. It doesn’t have a cell
membrane like all cells have; it doesn’t have a lot of cell components. It’s a non-living
structure. In fact, viruses can’t even reproduce on their own. So, viruses aren’t even alive,
which makes them very difficult to treat. Let’s talk about what they actually have.
What IS their structure?
One thing all viruses have in common is that they have some type of genetic material. This
does not mean they can reproduce on their own, but they are going to use their genetic
material and force their host to reproduce themselves. This genetic material, for some viruses, is in the form
of DNA. For other viruses, it’s RNA. Viruses also usually have some kind protein coat,
also known as a capsid. Capsid is a vocab word to think of that can protect that DNA or RNA
genetic material. Another thing to mention is that viruses are very, very small; they
are MUCH smaller than bacteria. You typically are going to need an electron microscope in
order to see a virus. It’s very, very small.
Viruses can also contain other structures. Some of them package special enzymes with
them. Some viruses have an outer envelope. There are different varieties of what viruses
can have, but they main thing is that they have genetic material and they typically have
some kind of protein coat, also known as a capsid. Now, viruses are very, very specific.
So you’ll want to think of them as picky. Usually, viruses have specific types of cells
that they go after.
So let’s talk about viral reproduction, because we did mention that they can’t reproduce
on their own and their very, very specific for the host that they’re going to go infect.
So the first cycle of the two we’re going to talk about is the lytic cycle. In the lytic
cycle, the virus (remember they're very selective) attaches to a host cell, and it’s going
to inject either its DNA or RNA, depending on what kind of genetic material it has, into
the cell. You would think the cell would notice this, but in many cases it does not. It takes
that genetic material and it starts following the instructions, which in this case is very
bad. In this case it makes copies of the virus. It starts making so many copies of the virus
that usually it’s going to 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 very bad. 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. It’s definitely not a good thing for a cell.
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. A lot of times when we’re
talking about this cycle, we’re talking about a special kind of virus called a bacteriophage.
It sounds very fancy, but what it means is that it is a virus that goes after bacteria.
In many cases, when we’re talking about a lysogenic virus, this is what we’re talking
about. These types of viruses that go through this cycle, they do kind of the same thing
at the beginning. They inject their genetic material, but this time, the genetic material
stays hidden in the host DNA. What happens is that when the host replicates it also replicates
the viral DNA. Eventually, what can happen is that it can be triggered to go into the
lytic cycle and then the virus can start being assembled and lyse out of the cell. In the
meantime, whenever it’s during the lysogenic cycle, it’s kind of hidden in there. It’s
kind of a sneaky kind of thing. As far as what triggers as what triggers it to go into
the lytic cycle, it can be a variety of things. Scientists are still doing a lot of studies
about this. Sometimes we notice they are environmental triggers like UV radiation or chemical triggers,
but it can cause the virus to suddenly switch into the lytic cycle.
So going back to the cold, which again is typically caused by a virus, a lot of times
it has to run its course. Your immune system will take care of it. Some viruses are extremely
complicated, like HIV, a virus that can lead to AIDS. The host that
it goes after are immune cells. By going after immune cells, it makes you very vulnerable
to other infections. In fact, for someone who has HIV, the common cold could be very detrimental
to their health. It’s something that scientists are really researching to try to
treat. It’s difficult, too, because HIV is a virus that tends to mutate quickly and
so if the medications are very selective against one specific type of virus and the virus mutates,
it makes it difficult to treat.
So with our talk today about viruses, you might wonder, “Is there anything positive
about viruses?” Well, viruses do play a major role right now in gene therapy. It’s
a very big field right now that is growing with research. Viruses do play a big role
with that. Also, scientists are looking for ways to manipulate or engineer viruses; for
example, there is a type of virus that goes after insects. It’s called the Nuclear Polyhedrosis
virus. I highly encourage you to look it up. This virus is very selective; it goes after
certain insects. However, if it could be targeted to a certain type of insects, the types that
eat your crops, then it could be a great form of a pesticide. The only problem is that whenever
you add some kind of pesticide, whether chemical or viral, there can be major consequences
in the ecosystem. When we get to ecology, we will talk about that. Viruses are a major
component of scientific research right now. That’s it for the Amoeba Sisters, and we
remind you to stay curious.