The Life Cycle of a Virus: How Viruses Live, Attack & Replicate

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  • 0:05 Viral Lifecycle
  • 0:40 Attachment and Penetration
  • 2:39 Uncoating,…
  • 6:44 Maturation and Release
  • 9:18 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Find out how viruses infect and replicate as we explore the important steps of the virus life cycle, including attachment, uncoating, penetration, replication, and release, as well as the lysogenic and lytic cycles.

Viral Life Cycle

Let's take a little trip into the future. Our mission will be to find out the secrets of a mad scientist's lab and how it is that he is able to create little clones of himself at the push of a button. The little clones are running wild and wreaking havoc all over our future city, so it is of utmost importance we figure out how he produces clones so quickly. Thankfully, since we are ardent students of virology, we can figure out how the mad scientist does what he does by comparing his work to the life cycle of viruses.

Attachment and Penetration

In order to figure out the madman's secrets, we need to climb into his lab. Thankfully, we have some masks to help protect our identity and a grappling hook specifically designed for this building. Similarly, when viruses want to attack a living host or cell, they use a very specific protein on the outer surface of its capsid or envelope that will allow its attachment to the surface of the host cell.

This specificity is really important as the virus doesn't want to attach to just anything that comes its way because it may have no use for it. Hence, having a specific hook that will only attach to its specific target's surface receptors will improve its chances of survival. This process, the process of viral capsid or envelope proteins attaching to the receptors on a target cell, is termed, not surprisingly, attachment. The specific set of cells or entities upon which a virus can successfully attach to is known as its host range.

Next, we can use the grappling hook we threw onto the roof of the building to climb up and use a stolen key to a door on the roof in order to penetrate inside the lab. Similarly, viruses undergo penetration, the process of entering a host, through several mechanisms. Some viruses inject their genomes into the cell by using a needle and syringe-type mechanism. Other viruses will actually trick the cell into engulfing the virus into the cell, something called endocytosis. Further still, enveloped viruses will undergo a process called membrane fusion whereby their envelope will fuse with the envelope of the cell. This will allow the virus to enter into the cell without any impediment.

Uncoating, Replication, and the Life Cycles

Okay, now that we're inside the lab, there's no point in keeping those masks on or those coats on. We can remove them to reveal who we are since no one from the outside can see us. Viruses, upon penetration into the cell, will also uncoat themselves by removing their viral capsid and thereby unmask the nucleic acid inside. This entire process is called uncoating and again is a process whereupon the viral capsid is removed in order to release the viral nucleic acid into the host.

Now that we're unmasked and the viral nucleic acid is released into the host, we can begin to replicate the little madman's clones to see how he's been creating all of these havoc-wreakers all along. It turns out that the viral nucleic acid, be it DNA or RNA, will basically take over the host once inside. There are many processes by which this occurs, and this will be covered in later lessons.

In general, what you need to grasp now is that the viral genome will take over the host cell in order to copy its own nucleic acid many times over and use the organelles and metabolic processes of the cell to create and assemble the proteins for its capsid. This entire process is called replication, which is, again, a process whereby a virus uses its host to copy its genome, generate and assemble a protein capsid, and thereby reproduce itself.

So, what our mad scientist basically did is copy his own genome many times over and use a bunch of equipment in the lab to create and assemble little clones of himself. Sometimes, the viral genome will actually integrate itself into the host's chromosomes. When the viral genome is integrated into, and replicates along with, the host's genome, we call this viral genome a provirus. A provirus is a term specific to viruses that infect animal cells. Likewise, when a bacteriophage's genome integrates into, and replicates along with, the host's genome, we call this viral genome a prophage. Remember that a bacteriophage is a term specific to viruses that infect bacteria.

In either case, you can liken it to our mad scientist attaching part of his DNA to a file on his computer's hard drive. Whenever any other user copies that same file, they will inadvertently copy the viral genome as well, allowing it to become more numerous every time someone else copies that file. Very clever - very clever indeed.

When a prophage is passively replicated along with its host's genome, we term it the lysogenic cycle. In this passive cycle, no viruses are produced, and the viral genome is simply copied if the host replicates. This cycle is more colloquially known as a latent infection, and its virus is called a temperate virus. This is when the virus lies dormant until triggered. An animal virus cannot undergo a true lysogenic cycle like the bacteriophage can, but slightly similar latent infections do occur.

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