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.
Regardless, eventually something may actually trigger the latent prophage to begin producing viral progeny. This means the prophage will actually detach its genome from the host genome and begin to replicate as previously described. This is called the lytic cycle and is a process of viral replication that leads to formation of viral progeny and the potential lysis, or destruction, of the host cell.
Viruses that are in the lytic cycle are termed virulent as opposed to temperate. Animal viruses undergo the lytic cycle as well.
Maturation and Release
Some of these little clones (the viruses) produced by the lytic cycle will undergo maturation before release, and others may undergo maturation after their release.
Regardless, the release of viral progeny out of its host cell is a process whereby viruses are expelled out of their host cell via exocytosis, apoptosis, cell bursting, or budding.As I just noted, this viral shedding, or release, may occur through something called budding. This is when the virus essentially leaves the cell and takes a piece of the host cell’s membrane with it. This is especially useful, logically, for enveloped viruses. However, it’s not so useful for the host cell because a lot of viruses leave the cell and take part of the cell membrane with it, causing the cell to die.
Imagine little things leaving your body and taking a piece of your skin as they leave. Eventually you’d have no skin left and simply wouldn’t be able to survive!Another way the viruses are released is through apoptosis. Like viral budding, apoptosis ends up killing the cell because by definition, apoptosis is programmed cell death. Hence, when the cell dies, the virus will be released or taken up by clean-up crews consisting of cells called macrophages. These macrophages will then spread all over the body, taking viruses with them, thereby spreading the virus to other parts of the body!Finally, other types of viruses can leave through a process called exocytosis. This type of viral shedding, or release, occurs without the need for host cell death.
Basically, the host cell will transport the viruses up to the membrane surface in little bubble-like structures called vacuoles, which will then open up into the outside world, thereby releasing the viruses.The three methods of release I just described are all part of the lytic cycle I previously mentioned. Basically, the lysogenic cycle is a passive form of replicating a virus’s genome without the production of any viruses, whereas the lytic cycle occurs when the virus uses the host cell to produce viral progeny, which many times results in host cell death. Something to keep in mind is that some viruses will enter into the lysogenic phase prior to the lytic cycle, while others will skip to the lytic cycle right away.
And right away, it’s also time for our lesson review. There was a lot to learn in this lesson about the viral life cycle.
Recall that the process of viral capsid or envelope proteins attaching to receptors on a target cell is termed attachment. Viruses then undergo penetration, the process of entering a host through one of several mechanisms. Upon penetration into the cell, a process whereupon the viral capsid is removed in order to release the viral nucleic acid into the host will occur. We call this action uncoating. After uncoating, replication must occur.
This is when a virus uses its host to copy its genome, generate and assemble a protein capsid, and thereby reproduces itself.When a prophage is passively replicated along with its host’s genome, we term this the lysogenic cycle. Conversely, the lytic cycle is a process of viral replication that leads to formation of viral progeny and the potential lysis, or destruction, of the host cell. Regardless, the release of viral progeny out of its host cell is a process whereby viruses are expelled out of their host cell via exocytosis, apoptosis, or budding.
Upon completing this lesson, you will be able to:
- Describe the attachment and penetration processes of viruses
- Explain what happens once a virus has penetrated a cell
- Identify the three methods of release in the virus life cycle
- Differentiate between the lysogenic and lytic cycles