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Viral replication: lytic vs lysogenic | Cells | MCAT | Khan Academy

December 31, 2019

– [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.


  • Reply Escellon haruka March 31, 2015 at 11:15 pm

    Wow! Really helpful video. Cleared up confusion between the cycles.

  • Reply Amal Magdi May 26, 2015 at 2:42 pm

    That's actually a great video. I have a test tomorrow and I'm totally going to nail whatever's related to viral infections. THANKS

  • Reply Sebastian Schwank August 27, 2015 at 9:15 pm

    The Virus integrates into the DNA after activating.

  • Reply Underdog Rising September 15, 2015 at 10:49 pm

    Great video. I was learning about Bacterial Phages today and stumbled on it.

  • Reply Yadi y October 10, 2015 at 8:47 am

    Thank u so much!i have a test tomorrow and i had no idea how lysogenic cycle work…but now I completely understand it

  • Reply Hanna Kozak November 26, 2015 at 8:24 am

    So awesome! Great video, thank you very much ๐Ÿ™‚

  • Reply 330janderson December 5, 2015 at 8:03 pm


  • Reply Troy Hackett February 6, 2016 at 11:00 pm

    Thanks ๐Ÿ˜€

  • Reply TheCassholeJo February 18, 2016 at 8:57 am

    You used terms that made this so much easier to understand. Thank you so much for putting this process in simple terms!

  • Reply mimy haifa June 13, 2016 at 8:05 pm

    Thank you ! You really help me ๐Ÿ™‚

  • Reply JediMasterChief August 7, 2016 at 9:08 pm

    you are the best source for understanding stuff….thank you!!!!!!!!!!!!!!!!!!!!!!!!!!!! ๐Ÿ˜‰

  • Reply Ana Cabo August 8, 2016 at 11:36 pm

    The best I've seen

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    thanks from my heart <3

  • Reply bro yo October 3, 2016 at 1:06 am

    much better teacher than that idiot nesbitt

  • Reply Empress Lioness October 10, 2016 at 5:29 pm

    thank you for the video.ย  simple explanation

  • Reply doctor abdirahim October 14, 2016 at 2:35 pm

    thank you dr!

  • Reply Glen DeLoid November 6, 2016 at 11:03 pm

    So simple and useless. Until you teach for USMLE, you're just meaningless noise. I don't think you have the chops to do this right.

  • Reply Mihir Patel November 10, 2016 at 6:14 pm

    I'm pretty sure, since this is bacteria, it is definitely a prophage!

  • Reply Sian Brookes February 8, 2017 at 9:11 pm

    thank you ๐Ÿ™‚

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    The best I've seen

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  • Reply Prof : sunil Sharma. April 26, 2017 at 10:28 am

    not all the points r clearly explain

  • Reply arpana bansal May 4, 2017 at 4:12 pm


  • Reply Mike June 10, 2017 at 1:27 am

    Firstly, thank you for the video – very informative.

    My only feedback is if you can please stop saying "Soooo…" .. Its just very distracting, and if you could possibly clear up the writing as its a bit messy.. I'm just being nit picky but some what expected more from such a large organization like KhanAcademy (that being said it is free education so I can't exactly complain!)

  • Reply Cai Cai July 3, 2017 at 10:43 am

    really helpful and easy to understand ?

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    who ever you are, I love you.

  • Reply yea boi October 24, 2017 at 11:21 am

    0:50 how do they "trick the cells" I don't understand

  • Reply Moonlight Hadeer October 27, 2017 at 6:53 pm

    Thank you

  • Reply Natalia D November 10, 2017 at 2:52 am

    Great video! However, I think it should be clearly stated the that lysogenic cycle and the creation of prophages is something distinctive to bacteriophages (as far as I understand). I ran into this misunderstanding when I first came across this topic and thought that eukaryotic cell viruses could use the lysogenic cycle to create prophages. I would have continued assuming that after this video. If I'm still mis-conceptualizing this, someone please correct me!

  • Reply saures1999 January 19, 2018 at 1:46 am

    Thank you.

  • Reply Elisa Ortiz January 21, 2018 at 2:19 am

    I'm eating chicken

  • Reply Zainab Naqvi January 28, 2018 at 7:35 pm


  • Reply Lauren James February 9, 2018 at 2:21 am

    Get inside…. XD

  • Reply Josh S. March 5, 2018 at 5:09 am

    This is a great explanation. This is the first time I've seen this explained in a way that is clear and concise.

  • Reply Zainab Ahmad April 21, 2018 at 2:31 pm

    Is the protein coat called a capsid

  • Reply Carlos Gutierrez May 4, 2018 at 9:39 pm

    Awesome video ????????โค๏ธโค๏ธโค๏ธ

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  • Reply Pelumi Odubunmi October 14, 2018 at 11:24 am

    The virus can't march to meet other cells, they come across them by chance, viruses don't have organelles for movement

  • Reply Ricky Farrugia October 21, 2018 at 9:19 pm

    I'm sick as a dog. Last night I had to sit up to sleep. I've got some kind of replicating virus. It's active on a 24 hour period

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    Saved my life

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    Thanks Ellen Page

  • Reply Anthony Garcia April 18, 2019 at 8:00 pm


  • Reply Nate Baum May 3, 2019 at 3:43 am

    Learned way more in this short video than in 10 hours of biology classes

  • Reply agha jan June 22, 2019 at 4:15 pm

    Like from Quetta??

  • Reply Edris Alkozi July 5, 2019 at 4:51 pm

    Lysogenic cycle can happens inside human cell instead of bacteria as host ?

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  • Reply twb Js November 12, 2019 at 5:09 am


  • Reply nikon12x42 November 17, 2019 at 4:32 am

    how would a "patient virus" know that there were no other hosts around – lol.

  • Reply Nahom Okubaldet December 3, 2019 at 12:24 am

    im used to sals voice when explaining

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