Articles, Blog

Viruses: Molecular Hijackers

December 20, 2019

Hey it’s professor Dave; let’s talk about
viruses. We now have a pretty good understanding of
what’s going on inside a cell, so we are ready to look at different kinds of unicellular
organisms. But before we get to those, let’s take a
look at something a bit simpler, a virus. Contrary to popular belief, viruses are not
alive. They are much smaller and simpler than single-celled
organisms like bacteria, and they do not meet most of the criteria that biologists agree
are required to call something alive. Living organisms must be able to perform metabolism,
making energy from food. Viruses cannot. Life must be able to reproduce out of its
own capacity. Viruses do not. They can be considered biologically inert,
so viruses exist in a kind of gray area in between simple molecules and living organisms. So what’s inside a virus exactly? In truth, a virus is pretty much just genetic
material in a protein casing. There is no membrane, no organelles, not much
of anything we are used to seeing in living creatures. Nevertheless, viruses reproduce by injecting
their genetic material into a host cell, thereby hijacking the cellular machinery of the cell
and forcing it to make copies of the virus instead of what the cell would normally be
doing, sort of like pirates on the high seas capturing a large vessel and taking command. Viruses were discovered in the late 19th century
when examining certain diseases that afflicted plants. It was found that sap from the plant would
transmit the disease even though no bacteria were visible in the sap when examined with
a microscope, and the sap would still transmit the disease even when it was filtered by a
process meant to remove any such bacteria. This meant that the agent responsible for
transmitting the disease must be way smaller than a single bacterium. But this agent could not be cultivated in
test tubes or Petri dishes, so it must also be much simpler than a bacterium. Later, as we became able to examine viruses
with more sophisticated techniques, we began to realize the structure of the virus, which
comes in a number of forms. Some are rod-shaped or helical, like the tobacco
mosaic virus. Some are icosahedral, like an adenovirus. Some have a membranous envelope covered with
spikes, like the influenza virus. And some even look like weird little spiders. This is called a bacteriophage, and it’s
kind of like a rod-shaped and icosahedral virus combined, with some fiber tails. The thing they all have in common is that
they carry their own genetic material, which could be double-stranded or single-stranded,
and either DNA or RNA. This will typically be found as either a single
linear molecule or a circular molecule. The protein shell that encloses the genetic
material is called the capsid, which comes in different shapes for different viruses,
and the capsid is made of smaller subunits called capsomeres. That’s really all there is to the structure
of a virus. So how exactly do they reproduce? As we said earlier, viruses hijack the machinery
of a host cell. This is because they do not have ribosomes
or the other components necessary to express genes, so they need a cell to do it for them. Certain viruses are able to infect certain
kinds of cells, and this is due to the system of recognition that must occur between the
two. In order to get inside the cell, a virus must
be recognized by surface receptors on the cell, so there must be some specificity for
these receptors to that particular virion. For this reason, many viruses are specific
to only a small set of species, or even one individual species, and sometimes even a particular
type of cell found within that individual species. Once this recognition occurs, the virus either
injects its genetic material into the cell if it’s a bacteriophage, or the virus can
be brought inside the cell completely intact through endocytosis. Once inside, the virus disassembles and the
viral DNA gets transcribed and translated by all the parts of the cell that are typically
busy working for the cell itself. Once there are many copies of the viral DNA,
the capsid proteins reassemble and form new viral particles, up to hundreds or even thousands
of them, which then exit the cell. Sometimes this process can damage or destroy
the host cell, so let’s look at the different mechanisms viral replication can utilize for
bacteriophages, as these are the best understood viruses. With the lytic cycle, the host cell is terminated
at the end of the replicative cycle. This happens once many viruses have been generated,
and the cell bursts open, or lyses, releasing them to then go and infect other cells. Given the exponential nature of this process,
just a few successive lytic cycles can destroy an entire bacterial population in a couple
of hours. By contrast, with the lysogenic cycle, the
host cell is not destroyed. This is because rather than usurping the cellular
machinery to exclusively produce viruses, the viral DNA is incorporated into the genome
of the cell. We can refer to this kind of viral DNA as
a prophage. This DNA remains largely silent, and the cell
is able to divide many times, with each daughter cell also containing the prophage. Then some environmental signal may trigger
a switch from the lysogenic mode to the lytic mode, where the prophage returns to the form
of a separate circular DNA molecule, and all of the infected cells could lyse at once. Apart from bacteriophages, other viruses have
envelopes, which allow them to enter and exit the cell by endocytosis and exocytosis without
destroying the cell. So it is of great importance to the virus
that it will be recognized by these surface receptors. Other viruses are considered retroviruses,
because they contain an enzyme called reverse transcriptase, which transcribes an RNA template
into DNA, which is the opposite of normal transcription. There are even smaller infectious agents called
viroids, which are naked circular RNA molecules that disrupt certain regulatory systems in
plants, and prions, which have no genome, but are instead infectious protein particles
that cause other proteins in brain cells to aggregate and bring on disease symptoms, possibly
including Alzheimer’s and Parkinson’s disease. It would seem that no cells are safe from
viruses. But nature is clever, and bacteria are constantly
evolving. Chance mutations in genes that code for surface
receptor proteins may result in receptors that no longer recognize a particular virus,
so it can no longer enter the cell. Viruses in turn mutate at random, and if glycoproteins
on a viral envelope become modified such that they will be recognized by the new receptors,
they will proliferate anew. In this way, bacteria and viruses are engaged
in constant evolutionary flux. The origin of viruses is still disputed, though
it is generally thought that viruses came about shortly after unicellular life first
evolved, and there are a number of anomalous viruses that contain up to several thousand
genes, including some previously found only in cellular genomes. A discussion of the specific diseases caused
by viruses and the strategies we use to combat them will have to wait for a pathology course
at a later time. For now, let’s move on to the biological
structure of the simplest organisms.


  • Reply Theodore Zuckerman February 23, 2019 at 8:09 am

    "Life must be able to reproduce OUT OF ITS OWN CAPACITY [emphasis mine]." Jeez Loueez what a messy, smelly pile of mangled language. Living things are able to reproduce. Viruses are unable to reproduce all by themselves. They do not have the capacity to reproduce all by themselves; in order to reproduce they must, instead, take control of the cellular functionality of a host organism. Saying they are unable to reproduce "out of their own capacity" is not a concise way to say they don't have the capacity to reproduce all by themselves, without the help of another organism. More than just an awkward phrase, it is a weirdly mangled phrase. If a virus could reproduce "in" its own capacity or "in of its own capacity" what would THAT mean? Would that be the opposite of "out of its own capacity" and would it make any sense? I don't think so.

  • Reply Christian Autajay February 23, 2019 at 10:56 am

    fun fact: Phage means kill, Bacteriophage means Bacteria Kill which means Bacteriophage kills bacteria. Of course, bad bacteria.

  • Reply Alto February 24, 2019 at 9:58 am

    Now that's a good teacher, where can I get one? 🙂

  • Reply Educator Sharmin March 2, 2019 at 3:51 am

    It is really helpful for students!

  • Reply psycronizer March 2, 2019 at 5:23 am

    I would argue that the very existence of such a thing as a virus, with it's total disregard for all the requirements to be called alive, is firm proof that life arose from simple to not so simple organic compounds, purely because of the nature of the compounds themselves, the multitude of ways in which they interact and come together to form ever more complex systems. just one very small piece, or nail, in the coffin for the religious out there….

  • Reply James S. March 2, 2019 at 6:37 am

    In your little graphic demo you're showing a virus seeking out a cell, latching on to it and injecting material or being sucked inside the cell.

    This almost implies some sort of intelligence or your graphic is being metaphorical for the sake of explanation.

    Either way is fine but do you think a virus might display some rudimentary intelligence? I assume the same laws of evolution applies to viruses as to all other life forms on this planet.

  • Reply StarDust 308 March 2, 2019 at 7:35 pm

    Bloody hell, this is much better than anything they've taught me at school. Amazing video.

  • Reply CaveGame March 10, 2019 at 10:49 pm

    What if you transmitted a virus with something other than dna like a picture stimulating your brain and transmitting the virus also another creepy idea what if the virus is alive in a way that it controls your brain and uses your body’s hardware to do creepy virus stuff

  • Reply Steve Heywood March 11, 2019 at 3:09 pm

    I wonder why nature created viruses and allows them to mutate ? Is it nature’s way of culling living creatures and keeping them in check ??

  • Reply Emerson D'Vortex March 11, 2019 at 7:39 pm

    From the point of view of the virus , they are quite alive .

  • Reply USA Latino March 13, 2019 at 6:59 am

    Excellent video. Thanks.

  • Reply Cyrus Javier March 13, 2019 at 9:00 am

    This dude out here saving lives

  • Reply Aqua Vlogs! March 17, 2019 at 8:04 pm

    Bacteriophages do not kill humans nor do they cause disease. They only kill bacteria that is harmful, not harmless. This does not apply to white blood cells as they're immune.

  • Reply Cure Cure March 17, 2019 at 11:24 pm

    Hey Dave. I have a couple questions about viruses, can I email you please?

  • Reply Arthur Migliazza March 20, 2019 at 1:34 pm

    Great video Dave!

  • Reply David San Buenaventura March 22, 2019 at 12:21 pm

    Bactariophage is not a virus

  • Reply Pumpkin Pie March 23, 2019 at 1:54 am

    Thank you for explaining this so well!!

  • Reply Niha Rika March 24, 2019 at 12:54 pm

    Very use full vedio

  • Reply Pluto Decronithum March 29, 2019 at 7:09 pm

    If you people actually paid attention in your class , maybe you wouldn't blame your teachers. It's ridiculous how most of the commentators think that schools are useless. While I don't disagree that there are some teachers who aren't that good at teaching , but on a second thought , really? Schools are useless? Schools present information in an organized manner , and I would prefer information in an organized manner compared to videos like this that present information in oversimplified manner. No offence intended, this video is good. I would watch this video for enjoyment cause there's no stress of tests or anything but I would prefer school for a formal education. I would rather go to school to learn microbiology formally than watch out of the blue random videos without any sort of organization, unless they are uploaded in an organized fashion by tutors like that at MIT.

  • Reply Suani Avila March 30, 2019 at 1:19 am

    just bc I was a comment on it now I finish the ads on ur vid

  • Reply Salvo Sk March 30, 2019 at 11:50 pm

    Viruses are such a mistery..a scary mistery..

  • Reply mr's Yasinzai April 4, 2019 at 10:37 am

    Fantastic adorable beautifull amazing sir you understand me in a very very simple method i am very thankfull of you keep it up ❤❤❤

  • Reply Oscar Ramos April 8, 2019 at 1:46 am

    Screw you, viruses ?.

  • Reply Xavier Chan April 11, 2019 at 1:12 am

    Dave certainly seems virile to me. Didn’t I already talk about virtue? As a demon, I’m not virtuous.

  • Reply Ptooey!! April 12, 2019 at 12:44 pm

    heard the intro and instantly subscribed

  • Reply Origami and Cats April 16, 2019 at 5:54 am

    So if I can't have children of my own and must use a surrogate does it mean I'm not alive?

  • Reply Noelia Pasco April 20, 2019 at 8:22 pm

    LmaoooOo that intro

  • Reply Prem Gupta April 22, 2019 at 6:27 am

    Please virus sir don't harm us play a positive role make a doctor's free WORLD

  • Reply Doug G April 22, 2019 at 11:38 am

    How do they have purpose if not alive. Rabies uses the nerves ( to avoid detection) to get to the brain and scramble the signals and get a violent host, then moves to the saliva glands for infection. How on earth does this thing have specific purpose, much less want ( for lack of a better word) to replicate.

  • Reply The No.1 Guy April 24, 2019 at 2:39 am

    the one question I have is HOW do the viruses destroy? once in the cell, they've insured their replication. and obviously it's the cell destruction that generates the unique symptoms felt by a virus on a much larger scale. but, what occurs once inside an individual cell? how does it blow it apart or break it down like that?

  • Reply Prabhleen Reen April 24, 2019 at 8:34 am

    the way the bacteriophage stuck in the cell was so gross and uncomfortable ewwwww

  • Reply gxd000 April 26, 2019 at 7:27 am

    who else has the flu???

  • Reply kl; fghf April 29, 2019 at 11:29 pm


  • Reply Spenser T May 2, 2019 at 6:22 pm

    Why do viruses want to infect or get more complex? What's in it for them? It is such a weird idea that they get into a shell.

  • Reply Psychachu [ official ] May 8, 2019 at 12:01 pm

    "Viruses don't reproduce." (Five seconds later) "Viruses reproduce by…"

  • Reply THANATOS FANATICOS May 9, 2019 at 3:14 am

    I'm single celled , of course.

  • Reply Dana Lynn May 22, 2019 at 3:14 am

    Now if only we could have a virus that would attack specific cancer cells. Must be research going on regarding this.

  • Reply Nemo D123 May 23, 2019 at 5:33 pm

    Hi Dave, as origins of viruses is disputed, can it be created by living cells itself as biological offence/defense mechanism against other weaker cells? So to speak viruses are created and modified by living cells as control mechanism of population of living organisms (Nature). Lets say it started back in history when only single cell organisms were present. Every cell need food, energy to live and reproduce. Too many cells means less food = death. I see viruses as "mines" released by cells and floating around , waiting to infiltrate-corrupt-reproduce in other cells. As you said viruses attack certain type of organisms. Which suggest cells can produce virus only to attack other same type cell. Which is correct, because one type cell cannot know dna of other type cell. At 6:52 you mentioned that viruses wait inside cells until "some environmental signal" occur. Which could be when the cell get weaker, because of less food/energy. So virus strike as it was designed to do so and allow stronger cell to survive shortage of food. Mutation of viruses you mentioned at 8:31 technically has nothing to do with virus itself. Virus cannot mutate on it's own or carry necessary information to make mutation in the cell. Only cells who survive will adjust its defense to prevent further attacks. And same time it will release new updated viruses to attack other cells with new defenses. And so on and so on. I'm not professor, just random guy with little bit of thought.

  • Reply Donovan Collado May 24, 2019 at 7:14 am

    phages are good not bad

  • Reply Mo no May 27, 2019 at 10:54 pm

    How to make cells to reject to make copies of the viruses?

  • Reply john hanrahan June 2, 2019 at 1:42 pm

    238 viruses gave this a thumbs down.

  • Reply Andrei P June 6, 2019 at 12:35 am

    The Bacteriophage looks alive and well to me..

  • Reply Daniel Freeman June 10, 2019 at 12:59 am

    If this guy is a professor, why does he look like the guy who delivers my pot?

  • Reply Cat Mom June 13, 2019 at 1:54 am

    1:25 will forever haunt my dreams. ??

  • Reply Sjs78 00 June 23, 2019 at 8:07 am

    They are perfectly designed for what it looks like they do, attach /anchor, little bulb up top holding poison/virus /pathogen, with a barrel and hypodermic needle tip at the end! Perfect death applicator, and micro size!!

  • Reply unknown June 29, 2019 at 7:19 pm

    Works Just like the kindergarten injectors on Steven universe

  • Reply westfield90 June 30, 2019 at 4:30 am

    I’m not sure what is the evolutionary selection of why viruses even evolved and why they continue to evolve. Is it just nature’s way to control runaway population growth?

  • Reply Gee purrs July 5, 2019 at 7:13 pm

    SO why the hell aren't we making nanites to go in and to attack these viruses?

  • Reply oman sharma July 6, 2019 at 3:33 pm

    Virus is border line of living and non living ???

  • Reply وتوبوا إلى الله جميعا أيه المؤمنون لعلكم تفلحون July 9, 2019 at 11:47 am

    the nature is clever! what a stupid thing to say

  • Reply Sarah Rose July 12, 2019 at 2:58 am

    Lol I love this and I’m only 12

  • Reply Sultan AlKhulaifi July 16, 2019 at 7:09 am

    Thank you for making these videos, keep them coming.

  • Reply Stephen Gillie July 16, 2019 at 4:26 pm

    It's a little confusing to explain viruses without first explaining normal intracellular communication, for instance white blood cells exchanging organelles with muscle cells in the human body. These are done by wrapping the transport items in a similar lipid layer as creates a cell wall. These have chemical markers on the outside, so the cell will know it's okay to accept.

    From here, it's a short leap to see viruses and virii as this same process, but as hostile. Instead of friendly cells shipping good packages to you, enemy cells are shipping bad packages to you. Viruses are to cells as anthrax in the mail is to humans.

  • Reply SD July 22, 2019 at 10:31 am

    I wish human cells too could evolve like bacteria.

  • Reply Renuka Thiru July 27, 2019 at 2:47 pm

    Excellent way of imparting the knowledge

  • Reply Renuka Thiru July 27, 2019 at 2:47 pm

    Simply superb

  • Reply A cookie July 29, 2019 at 7:43 am

    Good thing bacterial phage doesn’t attack human cells ?

  • Reply Chateau Leon July 29, 2019 at 6:17 pm


  • Reply Myles Gallagher August 9, 2019 at 6:36 am

    Viruses are not alive?? You would state as fact something that is is still debated by scientist's?

  • Reply Andrew harbit August 10, 2019 at 5:01 am

    Ok so with the idea that viruses require other biological machines to preform there replication, it's fair to question how did they come about?

  • Reply that wes main that doesn't even have wes August 14, 2019 at 1:01 am

    I can't believe you ripped of enter the gungeon with the gundromida strain, bruh.

  • Reply CatNinja BG August 14, 2019 at 10:05 am

    What if the virus was a left over dna

    materia (from a dead cell) that was still
    active, so it made a shell but the gene for female reseptors was missing to stop the making of male reseptors.

  • Reply Jep Maningo August 15, 2019 at 2:20 pm

    Could you please explain more the HIV? Thank you so much. I'm shocked by how virus can hijack a cell…

  • Reply Mischel Estores August 15, 2019 at 11:30 pm

    You're so cool prof Dave! Hehe

  • Reply Frank and Dana Snyder August 23, 2019 at 3:02 am

    But viruses do reproduce…..

  • Reply Hitesh Patel August 23, 2019 at 3:27 am


  • Reply Jamie Carter September 1, 2019 at 3:25 pm

    its sad you cut more information out than the script writer was even qualified to do so leaving so many holes in this video. why do this?

  • Reply teflontelefon September 10, 2019 at 8:20 pm

    "They're just RNA in a protein" — Cue the animation of a complex creature with legs hooking to a cell, actively injecting. This did not help in understanding how it works 🙁

  • Reply Audrey Blake September 11, 2019 at 4:47 pm

    great video thanks!

  • Reply Gary Klafta September 19, 2019 at 9:25 pm

    i'll buy you a store – bought hair cut and shave .

  • Reply vitaly goji September 22, 2019 at 6:02 pm

    Afte looking at all this , this guy still says "evolution". How can you be such an idiot to believe that DNA happens accidentally. Dawin religion is the strangest of all religions

  • Reply Culvea Solvere September 25, 2019 at 8:29 am

    I love Prof Dave's chest filling his shirt nicely!

  • Reply Dolly Dupee October 10, 2019 at 6:14 pm

    It needs it to mutate

  • Reply MV N October 11, 2019 at 7:41 pm

    Looks like aliens sprayed them virus here on earth.

  • Reply Michael Hartman October 12, 2019 at 8:52 am

    Viruses strike me as parasites. It is like listening to arguments whether Pluto is a planet, dwarf planet, or comet on some arbitrary rules. I realize the fuzzy line, but they are not simple chemical molecules. That virus that injects its DNA is complex, and had to evolve.

  • Reply Marco the dragon October 19, 2019 at 7:12 pm

    Can anyone tell me virus and virus are same?

  • Reply Tom Zeman October 22, 2019 at 7:23 pm

    Random mutations of cells are hijacked by viruses that evolve into disease & cancer. Darwins oxymoron kills! Tottaly pathological Charlie

  • Reply CozmicPlayz Minecraft October 24, 2019 at 4:05 am

    Aka Bacteriaphage

  • Reply Vinod Kumar Choppara October 24, 2019 at 6:52 am

    What make your think viruses should have exactly what us living organisms have, they might be doing something else special, we don’t see it everyday 24/7

  • Reply D!lldan October 26, 2019 at 8:27 pm

    Viruses aren’t living so they only exist to replicate and kill

  • Reply Redmi5 Plus October 29, 2019 at 4:38 am

    Before at school i though that virus is living biology is not good hehehe

    Thx professor, so virus is a such molecular machine not a living creature. Machines which create another typical identical machines…

  • Reply Redmi5 Plus October 29, 2019 at 4:46 am

    Computer virus not computer bacteria …hmmm…now i got it ?

  • Reply David Williston November 2, 2019 at 10:25 pm

    Doesn't respond to stimuli. Then hijacks cell by injecting DNA. Um ok. It seems to have responded to the stimuli of being attached to the cell membrane of the host, by injecting DNA into the host….

  • Reply Алмазік Алмазіков November 3, 2019 at 8:09 pm


  • Reply ronch550 November 11, 2019 at 2:02 am

    I think phages are viruses that just wanna bang bacteria.

  • Reply Josh Marcus November 15, 2019 at 7:02 pm

    The virus you are talking about that infects bacteria at the beginning is a particular virus called the t4 bacteriophage. They are the only known virus that has that head like structure, and they are among the only viruses that attack bacteria in the way you mentioned. They are also the deadliest thing in existence.

  • Reply bubbala November 24, 2019 at 8:33 am

    I believe it will be found that viruses cause most ailments,from cancer to diabetes

  • Reply Alex T November 24, 2019 at 11:57 pm

    Definition of a living cell is obviously antiquated.

  • Reply Lisa Taylor November 25, 2019 at 7:26 pm

    Virus is dead and alive

  • Reply First Son of Man November 28, 2019 at 3:15 am

    Viruses are Communism.
    They seize the means of production.

  • Reply Mitchell Byerly November 28, 2019 at 3:50 am

    Why do viruses just decide that they want to reproduce when they aren't living? I've always wondered why a virus would want to attack a living cell when the virus itself isn't living!

  • Reply Jay A M November 29, 2019 at 3:26 am

    "A piece of bad news wrapped in protein", Peter Medawar.

  • Reply Jacob Wellington November 30, 2019 at 1:09 am


  • Reply Pendulous Testicularis December 1, 2019 at 3:11 am

    Religion is a mind virus.

  • Reply Ruben Rajkavi December 1, 2019 at 6:59 pm

    If the viruses are injecting their genetic material into other cells and thus multiplying, is it not a way of reproduction? Something like a parasite… ?

  • Reply Singularity Approved December 5, 2019 at 12:47 pm

    Well, I didn't think I was going to find evidence of God in this video…but what the hell.

  • Reply Titanium Hearts Loft December 9, 2019 at 1:14 pm

    I truly hate viruses

  • Reply Justinas Beinorius December 10, 2019 at 10:44 am

    Thanks, great explanation of viruses.

  • Reply Rude Uno Card December 14, 2019 at 8:37 pm

    Its slowly getting Boring

  • Reply John C Gibson December 16, 2019 at 12:47 am

    I find it hard for my brain to wrap around to think of viruses actively hijacking cells. It is easier for me to think of virus as metal shaving scraps in a machine shop. High precision machines in the workshop that produces the shaving wastes go haywire when they ingest the shavings themselves. Gears grinding to a halt, motors exploding from excess heat produces even more shrapnel.

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