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Lysogenic Cycle of a Virus: Definition & Steps

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  • 0:00 What Is the Lysogenic Cycle?
  • 0:52 Why Do Viruses Invade Cells?
  • 1:49 Exposure
  • 2:14 Attachment and Entry
  • 3:09 Replication and Release
  • 4:33 Lesson Summary
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Lesson Transcript
Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

Once a virus infects one of your cells it begins to replicate. Some viruses make hundreds of copies of themselves and then escape their host. Lysogenic viruses choose a different path. They can actually insert their genetic material into your DNA.

What Is the Lysogenic Cycle?

Viruses have two ways to reproduce. Both of them involve invading a cell called the host cell and reprogramming it to make copies of the virus, which are then released back into the host body. One of these reproductive cycles, the lytic cycle, involves taking over a cell and filling it with thousands to millions of copies of the virus. This results in the death of the host cell because the increasing pressure in the cell causes it to lysis, or burst open. The other replication method, the lysogenic cycle, results in only a few virus copies being released at a time, but the host cell stays alive. That means the virus can survive and replicate in a host for many years.

Why Do Viruses Invade Cells?

In both the lytic and the lysogenic cycles, the virus is dependent on the host replication machinery to make new virions, or individual copies of the virus. That's because a virus is a simple structure. It consist only of a genome, the set of DNA that determines the structure of the virus, and a few structural proteins. These components don't give viruses the ability to reproduce on their own, so they have to take over cell machinery, which can reproduce for them.

The main difference between the lytic and lysogenic cycles is the location of the viral genome. That is, where in the cell the virus deposits its genetic material. In the lytic cycle the genome remains in the cytoplasm. From there, it takes control of the cell's machinery and begins producing viruses. In the lysogenic cycle, the genome enters the nucleus, the command center of the cell, and inserts into the host genetic material.

Exposure

A virus' reproductive cycle begins by finding a host cell. This involves entering a host body, an action which is called exposure. There are several different ways a host can be exposed to a virus. The virus can find its way in through a cut or a scrape, it can be injected (such as by a mosquito bite), or it can enter the body via a mucus membrane like the mouth or nostrils.

Attachment and Entry

Once a virus is inside a host body, it finds a cell to attack. The first step is attachment. That's when the virus physically attaches to the host. Viral proteins interact with host surface proteins during entry and the virus will attach to the protein receptors on the cell wall. These receptors will only allow certain types of protein to attach. That means each virus can only connect to certain types of cell.

Once it's attached, the virus creates a hole in the cell's membrane and injects its genome into the host. In the lytic cycle, the virus rapidly takes over the host machinery and forces it to make millions of new viruses. The lysogenic cycle is more discrete, though. The virus genome enters the host nucleus and becomes part of the host genome, sticking its own DNA onto that of the cell.

Replication and Release

When a virus' genetic material has been inserted into a cell's genome, we say the cell has be lysogenized. Unlike a virus using the lytic cycle, the lysogenic-cycle virus doesn't have control of the host's replication machinery. Instead, it's replicated whenever the host needs to produce proteins that are next to the viral genome. Thus, a virus inserts its genome into genes that are crucial for host survival. Every time the host cell produces proteins based on those genes, it also produces the material for new virions.

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