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Course: Health and medicine > Unit 13
Lesson 1: Bacteria and viruses- Overview of Archaea, Protista, and Bacteria
- Bacterial characteristics - Gram staining
- Antibiotics
- Antibiotics: An overview
- What is antibiotic resistance?
- Bacterial meningitis
- Virus structure and classification
- Viral replication: lytic vs lysogenic
- Retroviruses
- Subviral particles: viroids and prions
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Viral replication: lytic vs lysogenic
Viral replication has two cycles: lytic and lysogenic. In the lytic cycle, viruses quickly take over the host cell, make many copies, break the cell, and infect other cells. In the lysogenic cycle, viruses sneak into the host's DNA, stay hidden, and wait. Later, they become active, make copies, and infect other cells.
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Can't the hydrolytic enzymes found in some cells break down the makeup of some invading viruses? Isn't that part of their functions?(8 votes)
Yes It is possible for a cell to defend itself against viruses. It does it in 2 main ways. The first way it does this is that the lysosomes digest the virus after it gets into the cell and before and after it has replicated. The 2nd major way is RNAi which mean RNA interference. This is a process where cytoplasmic proteins cut up the RNA and if it is dsRNA to separate the 2 strands. It then degrades it into nucleotides that the cell can use for its own RNA. A Similar thing could theoretically(or maybe it does actually happen) happen with DNA and that would be called DNAi and would essentially be the same process but with DNA instead.(14 votes)
do both of the cycles end with lysis or is there a different way that it can exit?(7 votes)- Some viruses instead of lysing the cell instead bud off of the cell making it have less membrane. This it can easily fix by taking in more triglycerides and turning them into phospholipids(Which is why sickness can lead to weight loss). And some like the flu virus don't lyse or bud off but instead use their protein coats to guide themselves through the lipid bilayer(7 votes)
I wonder which method of replication is doing more harm to the cell/body?(4 votes)
You mean between lysogenic and lytic? Technically speaking, the lysogenic cycle is just an in-between step leading to the lytic cycle; So both will eventually kill the cell. But, if you were to compare the lysogenic cycle (before becoming lytic) and a lytic cycle, then you can conclude the following: the lysogenic is NOT harmful AT ALL, but the lytic cycle is entirely harmful, as it is murdering the host while it is alive.(8 votes)
What makes a host cell? Can any cell be a host cell? Like.. what's a host cell for COVID? any cells in the lungs??(3 votes)- Host cell: living cell invaded by or capable of being invaded by an infectious agent.
OR
A cell that is infected by a virus or another type of microorganism.
Purpose of host cell :
A virus must attach to a living cell, be taken inside, manufacture its proteins and copy its genome, and find a way to escape the cell so that the virus can infect other cells.
Viruses can infect only certain species of hosts and only certain cells within that host.
Can every cell be host cell :
No every cell can't be host cell.
Cells that a virus may use to replicate are called permissive.
For most viruses, the molecular basis for this specificity is that a particular surface molecule known as the viral receptor must be found on the host cell surface for the virus to attach.
Host cell for covid:
There are two proteins that help the SARS-CoV-2 virus enter human cells. It is found that subsets of cells in the lung, the nasal passages, and the intestine that express RNA for both of these proteins much more than other cells.
The viral "spike" protein binds to a receptor on human cells known as angiotensin-converting enzyme 2 (ACE2). Another human protein, an enzyme called TMPRSS2, helps to activate the coronavirus spike protein, to allow for cell entry.(8 votes)
- please explain in detail that what decides whether a lytic cycle or a lysogenic cycle will take place?(4 votes)
- This is a typical example of decision making at the subcellular level, as viruses with identical genomes infecting isogenic cells can either become lytic or lysogenic.
if you want to find more details about cellular decision making, please refer to this paper:
"Cellular Decision Making and Biological Noise: From Microbes to Mammals"(4 votes)
- why dna of bacteriophage superimpose over dna of bacteria instead both have same chemical composition(4 votes)
at3:28it is explained that the virus DNA is repressed, which shows that the virus is capable of manipulating the host's reaction to its own DNA (the host not realizing it has a different DNA). through evolution the viruses that were better able to superimpose their DNA over the bacteria DNA, survived. this causes the virus to have this ability(3 votes)
2:00how do they self-assemble? wouldn't that violate the second law of thermodynamics?(5 votes)
In reference to the last scenario: if the cell wants to repair its genome, it has to be because it recognizes there is a "problem" and wants to fix it. But that ends up excising out the viral genome, thereby making the virus active. This is surprising to me because I would imagine that the cell would have an offensive mechanism to deal with the viral genome. As far as we understand, why is this not the case? Are there any other mechanisms that the organism uses to "fix" this? If we were to make medicines for this etc. then what part of the process would we have to target to fight the dormant --> active virus?(4 votes)- Do all viruses have the option of choosing the lytic or lysogenic cycle, or do most viruses go with the lytic cycle? I've heard the lytic and lysogenic cycles being applied only to bacteriophages and wasn't sure if the option to choose either path exists for other viruses as well.(3 votes)
- Why would the virus even go through the lysogenic cycle when in the end it would just have to go through the lytic cycle and kill the host?(2 votes)
- Sometimes conditions are not optimal for the virus at the time, and the virus can also replicate much more rapidly if it "waits" for the cell to replicate and reproduce. For instance, if a virus immediately kills the host cell, it's just a few viruses released against the host's defenses. Chances are, the host can fight it off. If the virus waits until the cell has replicated, let's say 100 times, now the proviruses can immediately multiply into the hundreds (I'm just making numbers up but you get the point about the rapid rate at which they can each replicate). The host's immune system likely will have a harder time fighting that off.(3 votes)
3:28Video transcript
- [Voiceover] So, let's talk
about viral replication. Since we know that viruses are made of only proteins and one
type of nucleic acid, which means they have no organelles to make copies of themselves with, they have to get inside other cells in order to use their
ATP and their organelles to make copies of themselves. So first step, get inside that cell. And I'm just going to mention
a few things for review. So remember, there are three
ways of getting into a cell. One of the ways of getting
in is that bacterial phages, they literally just inject
their genetic material inside of the cell. Whereas every other type of virus uses one of two ways. If they're non-enveloped they
trick the cell's receptors into letting them in. Or if they're enveloped they can either use that receptor trick, or they directly fuse with the membrane. And once that genetic material
is inside of the cell, the virus has hacked in, and
it's ready to do a takeover. But here's where it makes a choice. It's kind of either impatient and ready to get going to copy itself, or it sits back and says, "You know what? "I'd rather sit and wait until I'm ready "to take over." So let's just draw these
two possibilities out to see what happens. And with the first choice, the impatient virus goes ahead and takes over the cell's machinery. Their ATP, ribosomes,
nucleic acids, amino acids, to start making copies of
the virus's genetic material, which again, can be RNA or DNA. And also, the virus's proteins to make their protein coat. And these will self-assemble. That means they just come
together on their own to form fully functional viruses. And because it keeps making
more and more and more of these, it will eventually force the
cell to lyse, or break open, and once it breaks open, all of these viruses are
released into the environment. And if there are other cells nearby, then this army can start
marching out to infect, to hack into those nearby cells, and create more armies. So this makes a lot of sense if you have a lot of hosts around, and your goal is to just
create the biggest army in the fastest way possible. And so that's the impatient virus. So what about the other option? The other option is
where the virus decides it's just going to sneak
in and hitch a ride. It thinks that the bacteria seems to be doing fine on its own, maybe there aren't other hosts nearby, so there's no reason to kill off the host, because lysing it would kill the host, so we don't want that. We're just gonna keep the host alive. And in order to sneak
in and let the bacteria do its thing while it's waiting, it's going to combine with the
host's genetic information, so that the host really
can't tell that it's there. It's basically quietly sitting there because it's repressed. There are repressor genes on this virus. So it's not expressed,
it's not transcribed. So it's not doing anything. And this is called a provirus, or you might also hear the word prophage. And again, because it's
not doing anything, this is called a dormant or latent phase. So the bacteria just does its own thing, it continues replicating, and of course the virus is already there. So it will continue to replicate. So it will replicate
when the bacteria does, because it's part of its genome. But about one in 10,000
times that this happens, or if something happens like the bacteria is exposed to UV light or something. Well, in any case, there's something that weakens that repressor
gene that we talked about on that virus. So it's no longer quiet. And the host's genome,
like most other genomes, will start wanting to repair itself. That means it will cut out
to try to repair its genome, it will excise out part of its DNA that just happens to be the virus. And now, the virus is active. It's ready to make copies
of itself, lyse the cell, and get its army out into
infecting other cells. So the official terms
for the impatient method is the lytic cycle. And the hitch a ride method
is called the lysogenic cycle. So now, you should have a good idea of what the lytic and lysogenic cycles are in viral replication.