Welcome back to BOGObiology! In this video we'll discuss
viruses. We'll go over what they are, how they replicate,
how we defend ourselves against them, and of course discuss the novel coronavirus.
Before we get started, I want to acknowledge that, at least here in the
US, things have definitely deteriorated over
the three weeks that it hass taken me to make this video.
We're now up to 100,000 known cases of coronavirus, probably
significantly more unknown cases. I hope that you're all safe out
there, and that you're practicing social distancing
and that you're washing your hands with soap. We'll learn more about why
that's important later on. Okay, here we go.
Computer viruses and malware are likely to be pretty familiar to you.
These programs infect one machine and hijack its inner workings, often to steal
useful data like banking information. Then the program begins replicating
itself it then sends those copies out to other machines.
They quickly spread from host to host and can alter the way a computer or
other electronic device functions. Sometimes the malicious code stays
hidden inside the computer until something triggers it to activate,
at other times the virus destroys the machine permanently.
Viruses in nature are actually quite similar to computer viruses.
Viruses are tiny infectious agents that are only capable of replicating
inside a host cell. Without a host, the virus is essentially inert.
They're also extremely small, usually far smaller than bacteria.
Viruses come in many shapes, but the basic structure consists of genetic
material (either dna or rna) surrounded by a
protein coat. This table shows several common viral
shapes along with a few examples. Helical viruses include the tobacco
mosaic virus, which was one of the first to be studied extensively because its
effects were so easy to observe. Polyhedral viruses include polio,
hepatitis A B and C and adenoviruses. Some polyhedral
viruses such as hepatitis C have what's known as an "envelope"
surrounding its exterior. Spherical viruses are round and can
sometimes have protein spikes protruding outwards.
These include HIV, influenza and the novel coronavirus SARS-CoV-2.
Complex viruses are those that don't fit into any of the other categories, and
they look kind of like a lunar lander. A great example is the T4 bacteriophage.
The protein shell that surrounds the viral genome is called a capsid.
The specialized exterior of each virus allows it to invade a host cell.
Some are even disguised by envelopes made of host cell phospholipids and
membrane proteins. This is similar to how malware often
relies on an appealing exterior to trick people into putting malicious code onto
their machines. When you add an appealing digital "capsid",
far more people are fooled. As with computer viruses, most viruses in
nature can only affect a finite range of hosts.
This is because exterior proteins on the virus's surface have to interact with
other proteins on the surface of the host cell in the style of a lock and key.
Some are so specific that they can only infect a single species,
but others are capable of jumping between species. As of the making of this
video in mid-March of 2020, scientists believe that SARS-CoV-2, which
causes the respiratory condition known as COVID-19,
has somehow jumped from bats to humans. We believe the virus was able to do this
because bats and humans have similar receptor proteins on the surface of our
lung cells. In humans, the virus appears to attach to
the ACE2 receptor. Usually this protein is involved in
regulating blood pressure. By the time you're watching this,
hopefully we will know more about this virus, where it came from, and how to stop
it. For now, please practice social
distancing, and wash your hands well with soap and warm water for at least 20
seconds. Coronaviruses are surrounded by a
delicate lipid membrane that soap destroys.
Viruses lack the equipment necessary to manufacture proteins or to reproduce on
their own. Instead, they hijack a cell and use its
organelles, enzymes and other equipment in order to do so.
A common debate in biology is whether or not viruses are alive.
Viruses definitely have many characteristics that make them similar
to living beings. We often say that living things exhibit
growth, excretion, reproduction, movement, sensation,
respiration, and nutrition. Viruses don't have all of
these characteristics. So, in short, viruses are not alive but
...it's complicated. Viruses reproduce extremely rapidly, so
their populations can grow in a very short amount of time.
Hence the term "going viral". Initially, their growth tends to be exponential.
However, they ultimately run out of new hosts to infect and their growth levels
off. While the details vary depending on the
type of virus, many viruses reproduce using a similar pattern. The first step
is attachment where the virus locates a host cell and binds to a surface protein
on its exterior. The next step is penetration. The virus
either injects its DNA or it fuses its envelope with the cell membrane
and it enters via endocytosis. The third step is biosynthesis
enzymes in the host cell transcribe the viral genome into viral RNA,
and also make copies of the viral genome for later assembly.
Then the host's ribosomes use that viral RNA to make
more capsid proteins. Step four is maturation.
Here, the viral genomes and viral capsids self-assemble into new viruses.
The final step is lysis. Here, the host cell bursts and releases many new
viruses into the environment, each of which has the potential to infec still
more host cells. This basic model of replication is known as the Lytic Cycle,
and it culminates in the death of the host cell.
Phages that reproduce only using the lytic cycle are called
virulent phages. However, many viruses tend to reproduce using a combination of
the Lytic Cycle and another strategy known as the Lysogenic Cycle.
The Lysogenic Cycle involves the incorporation of viral nucleic acids
into the host cell's genome. This new piece of viral genetic material
is called a prophage. The process doesn't immediately kill the
host, rather the virus can quietly lie in wait until the ideal moment.
The gene isn't expressed and it does not code for new phages, so the infected
organism can sometimes go years without showing any symptoms.
Despite this, whenever the infected host cells divide by mitosis
its daughter cells will also include that chunk of viral genome.
All of these infected cells will be ready to begin churning out viruses at a
moment's notice, sort of like a viral sleeper cell.
Under the right conditions, usually some sort of cellular stress, the
viral genetic material is excised from the host's DNA
and the cell begins to manufacture viruses. Then the cell finally lyses.
So, how can we fight back against viruses? The simplest and most cost-effective
method is to avoid contracting a virus in the first place.
Wash your hands with soap and water before eating or touching your face,
and try to cover coughs and sneezes with a tissue or your elbow instead of your
hand to avoid spreading germs to others.
Vaccines are also another extremely effective way to prevent virus
infections. As a general rule, vaccines expose an
organism to specific components known as antigens,
which activates the immune system. Our bodies then mount an immune response
including the production of specialized antibodies.
If we're exposed to the same pathogen later on, the immune system is able to
target the virus very quickly. There are many types of modern vaccines
but the most common include attenuated, inactivated, subunit and toxicoid
vaccines. Attenuated vaccines use a weakened form
of the virus. The varicella vaccine for chickenpox is
a great example. Other vaccines use dead or inactive
forms of a virus. The modern polio vaccine is made from an
inactivated form of the poliovirus. Subunit vaccines are made from specific
pieces of the virus or pathogen that will trigger an immune response.
Examples of subunit vaccines include the inoculations for hepatitis b
and pertussis, also known as whooping cough. Many viruses and bacteria produce
harmful chemicals as they multiply in the body.
Tetanus and Diphtheria are caused by bacteria, not viruses,
but the toxins they produce can be deadly. The vaccines for these diseases
use a tiny dose of a weakened form of the toxic chemical
which offers future protection. If someone has contracted a virus,
anti-viral medications can also be very useful.
These drugs work by interfering with viruses to stop them from reproducing.
One very common example of an antiviral drug is the drug
Oseltamivir, aka Tamiflu. This medication disables the enzyme
neuraminidase which is needed for influenza viral replication.
Usually this enzyme allows the completed new viruses to detach from the host cell.
When this enzyme is inactivated, the cells can no longer release the viruses
and they can't reproduce. There are also a number of drugs on the
market that help to fight the progression of HIV/AIDS.
Many patients take nucleoside or nucleotide reverse transcriptase
inhibitors or "NRTIs". These medications specifically block the
activity of crucial viral enzymes and stop the HIV virus from replicating.
That pretty much wraps up our discussion on viruses, and I hope you found this
video useful. Remember to like, comment, and subscribe
to help others find this video, and in the meantime please stay safe and
remember to WASH. YOUR. HANDS. Bye!