Leah here from leah4sci.com/mcat and in this video, we're going to look at the virus replication
cycle, including the lytic and lysogenic phases.
The replication cycle is also called the life cycle.
Even though viruses are technically not alive, as we discussed in video 1 which you can find
on my website link below or go to leasci.com/virus.
Because viruses are not alive, they don't replicate through cell division.
Instead, viruses require a living host for replication.
Viruses have two phases or replication cycles.
The first is the lytic cycle, which comes from the word lytic or lyse, which means to
break or to burst.
This is the destructive and deadly cycle.
Then we have the Lysogenic phase, which is the dormant phase.
In this case, we still have the word lyse, but instead of a lysing the cell where lysing
the genes.
So let's break it down.
See what I did there.
Depending on your book or reference viral replication can have five or six stages.
It's really just a question of how they break it up.
But the general theme goes something like this attachment, viral entry, replication,
assembly and release.
Let's take a look at what happens when the influenza virus attacks a human cell.
The viral particle moves through the system looking for a receptor.
This is the first step attachment of the virus to the cell when it finds one that it can
recognize, it attaches to the receptor which triggers the cell to take in the virus through
receptor mediated and those endocytosis.
The particle is going to start by pushing and on the cell membrane.
The virus is going to push and push, taking the membrane with it to the point where it
creates a new membrane around itself.
The virus is now inside the cell with its own lipid bilayer, the same type of bilayer
that sits around the cell.
The viruses can't just attack any cell at random.
They have to find one that has a receptor they recognize.
If they can't attach, they can't gain entry.
Going back to the usb music example from video one, if you have a USB stick you can't put
it into an HDMI port.
Trust me, I've tried.
Once the virus is attached, we get viral entry with a goal of getting the virus into the
cell.
Viruses ienter bacteria in a slightly different way.
If a bacteriophage latches onto the receptors on a bacteria, instead of taking the entire
virus into the cell like the influenza, the virus creates a hole.
It creates a channel into the cell so that the capsid, the protein coat remains on the
outside.
But the viral genome, DNA or RNA is able to be injected directly into the cell.
The viral genome is now ready for replication in the bacteria.
The influenza virus needs a little more help.
Remember, we still have the viral coat and the membrane that's around the entire virus.
In order to expose the viral genome, the viral membrane has to break and the viral genetics
are injected into the nucleus, which is where we do DNA replication in the eukaryotic cell.
And then chaos hits.
I want you to think of the viral genome and the proteins that come in to help as armed
guards taking over an ice cream factory.
They point the gun at all the workers and they say, we know you're used to making delicious
flavors like chocolate, strawberry and vanilla.
But from now on, we're making garlic licorice, ice cream, and we're going to put out a hundred
times as much ice cream.
That means no breaks.
Everybody is forced to work overtime.
We're not stopping until we're done.
When the virus takes over the cell machinery, it tricks the cell into making copies of the
virus.
If the viral genome was DNA, the cell will copy that DNA so that all the new viral particles
will have an identical set of DNA.
But then some of that DNA is also going to be transcribed into RNA and the RNA will be
translated into proteins to reassemble the new viral particles.
This would include any helper proteins that get packaged with a viral genome as well as
the proteins or the capsomeres that make up that viral coat.
RNA viruses can act similar to DNA in that we make RNA copies, but also use some of the
RNA as mRNA for the new proteins.
But there's another type of RNA virus called the retrovirus that completely violates the
central dogma of DNA to RNA to protein, because the retrovirus is an RNA virus that is copied
to form DNA and that DNA will be copied to have more DNA, which can then be transcribed
to RNA, which can then be transcribed to new proteins.
The enzyme responsible for creating DNA from RNA is called reverse transcriptase because
it's transcribing backwards.
We're used to seeing DNA to RNA, but this codes from RNA to DNA.
A common retrovirus example is HIV.
The human immunodeficiency virus is an RNA retrovirus.
Once the cell has made thousands and thousands of copies of the DNA, RNA and proteins.
We get to the next step assembly.
Once the cell has made hundreds of copies of the viral DNA, the viral RNA and the proteins,
simple viruses will start to self-assemble, more complex viruses will have their proteins
helping assemble them.
But all we're doing here is packaging up the DNA or the RNA surrounded by a capsid and
anything else that specific virus requires.
The final stage is release or egress.
This exit stage is what classifies the Lytic cycle with so many viral copies taking over
the cell.
The cell is completely overwhelmed.
The viruses start to push out on the cell membrane, trying to exit and in doing so with
all the viruses trying to escape every which way the cell ultimately Lyses or bursts.
Think of this as the ice cream factory workers starting to collapse from exhaustion and starvation
because they're working so hard and they're just completely drowning in disgusting ice
cream.
Once the cell bursts and the viral particles or the virions are released, they go out looking
for another host.
Some of the particles will find other cells within that same organism that have receptors
to infect those cells, but other particles can try to escape the organism and get released
into the environment where they find another living host.
The human influenza virus can undergo the first Lytic phase in just six hours.
That means when a cell gets attacked by the virus, the virus goes into the cell, goes
through all the stages of entry, replication and ultimately lysis.
This happens in just a matter of six hours.
When a person is infected with the influenza, one viral particle enters their system and
attacks a cell, the body still doesn't know.
Six hours later, we have a couple of hundred cells infected.
They start copying the virus.
Immune system might be starting to figure it out.
By the next day you have hundreds of thousands of cells being infected and now you also have
the immune system starting to kick in.
But for the entire time that the virus is replicating through those initial thousands
of cells, you don't know because you don't yet have symptoms.
The incubation period is the time from when a virus infects an organism, until the virus
makes so many copies that the organism starts to react.With something like influenza, this
could be one to four days.
That means two days after catching an influenza viral particle, you still don't know that
you're sick, but you may have enough viruses, for example, in your throat where if you cough
it up and somebody else picks it up, they can get sick before you even know that you're
sick.
On the other extreme, we have something like the Coronavirus, which has an incubation period
of up to 14 days.
That means a person can be replicating the corona virus in their system and spreading
it to other people without knowing.
When the system hits a critical mass, then the symptoms begin between the damage caused
by the cells lysing.
And the additional damage caused by your immune system attacking those cells, then you start
to feel sick.
The Lysogenic cycle is the dormant phase and this is typically not what we think of when
we picture a viral infection.
A common example of this is the herpes simplex virus.
Many people have the virus without any symptoms.
They can have it for months or even years.
And then something happens and they erupt with cold sores and painful blisters.
How does this happen?
Why is herpes silent and then suddenly causes blisters only to go silent again for months
or years before another breakout?
The silent phase is the Lysogenic phase of the herpes virus, but then it can go into
the Lytic phase to cause an eruption before going back into the Lysogenic phase, the Lysogenic
phase is called the dormant phase.
But this doesn't mean it's not a problem because during the dormant phase what happens is instead
of making copies of the viral genome and the protein and reassembling and bursting the
cell, the viral genome finds a comfy spot within the cells' own DNA and just parks itself
there.
The viral DNA enters the cell.
It enters the nucleus and finds a nice comfy spot in the DNA where it cuts the DNA and
just sticks itself right in.
And the cell has no idea.
To picture this, I want you to imagine taking a book and ripping it right down the middle.
Then adding a few pages of pure gibberish into the book and binding it back up and then
gluing it back together.
With a virus safely embedded in the DNA, it can stay that way for a long time and the
cell has no idea.
Every time the cell undergoes mitosis, it copies everything inside the cell, including
the viral DNA.
Imagine taking that book with a gibberish inside and making hundreds and thousands of
copies.
Every new book now has a copy of that gibberish.
Does it really matter, though?
So what if we have some garbage code embedded in our DNA?
Well, actually, this can lead to a number of problems.
So you have an important gene within the cell that codes for something related to survival
and the virus inserts itself into the sequence, mutating that gene.
When the cell tries to transcribe that gene, it doesn't know what to do with it.
And it definitely doesn't know how to make the correct proteins and enzymes that the
gene used to code for.
The silent copies can remain dormant in the cell for a very long time until a trigger
suddenly has them going from the silent Lysogenic phase into the Lytic phase.
Now the viral DNA starts coding for the viral proteins and making copies of the viral genome.
And suddenly all of these cells with harmless DNA start to burst, spreading viral particles
everywhere.
The trigger that makes the virus go from the Lysogenic to the Lytic cycle can be anything
extreme that makes the virus feel uncomfortable.
This can range from a chemical stimulus temperature variation, physical, mental or emotional stress,
starvation, and anything else that makes the virus feel.
If it stays in the cell, it won't survive long.
But if it quickly makes copies and escapes, it can hopefully find another cell to live
in and continue its viral life cycle.
Sometimes the virus will get embedded into the DNA of a cell and then it will lose its
viral ability.
Over the generations of cell replication, the virus can mutate or somehow forget how
to be a virus and just live as junk DNA inside the cell.
In fact, if you look at the theory behind introns, one of the explanations is junk DNA
that came from viruses.
They don't quote for anything human.
But they no longer code for a virus either.
Now that you have a good understanding of viruses, let's talk about the Corona virus
specifically, which you can find on my Web site using the link below or going to leah4sci.com/virus
.