Chances are, in your life, you’ve had the common cold before.
It can cause sneezing and coughing
and can make 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:
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 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,
or an insect, plant, or this bacterium.
还是昆虫 植物 亦或是这种细菌
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.
在裂解循环中 病毒非常有选择性地 附着在宿主细胞上
It often binds to a receptor that this cell has
which gives it access to attach
The virus can 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 make 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 the virus gets triggered to
go into the lytic cycle from that point,
because then all those cells that had
the viral geneticmaterial 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
so that they don’t feel not as bad.
some viruses are extremely complicated,
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.
比如 Helper T细胞
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.
许多病毒 包括艾滋病毒 是会变异的
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, some 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
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 pesticides,
尽管如此 重要的是 每当谈到任何一种杀虫剂时
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!
Chances are, in your life, you’ve had the common cold before.