Lytic Cycle of a Virus: Definition & Steps

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  • 0:00 What is a Lytic Cycle?
  • 1:25 Step One: Exposure
  • 2:00 Step Two: Attachment and Entry
  • 2:27 Step Three: Replication
  • 3:20 Step Four: Lysis
  • 4:04 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.

Viruses infect our cells, replicate, and then need to leave. One of the ways they choose to leave is by destroying the host cell. They do this by cutting (lysing) their way out. This is called the lytic cycle of a virus and is the subject of this lesson.

What Is a Lytic Cycle?

Like many living things, viruses live to reproduce. They find a host cell, replicate themselves, and leave the host cell with all of their new copies. One method of reproducing and fleeing a cell is the lytic cycle.

In the lytic cycle, the virus reproduces thousands to millions of times in just a few hours, then weakens the cell wall enough that the cell will lyse, or burst open, setting the army of new viruses free. As you might have guessed, this results in the death of the infected cell.

Why Lyse?

Why use such a destructive method of reproduction? Because viruses are very simple, consisting of only a genome -- the set of DNA that makes the virus -- and a few proteins. They can't reproduce using their own devices and so are completely reliant on host machinery to produce more copies of the viral genome.

They're also dependent on host machinery to make viral proteins. The cell has all the components a virus needs to make proteins and copy itself; that's why cells are such attractive hosts. In the lytic cycle, the viral genome finds its source material in the cytoplasm, the jellylike substance that fills most of the cell.

In a different type of life cycle, called the lysogenic cycle, the viral genome enters the nucleus and inserts itself into the host genome, keeping the host cell alive.

Step One: Exposure

For a virus to reproduce, it must infect a host cell. To do this it must first come in contact with the host organism and enter its system. This part of the viral life cycle is called exposure, and it's one step of the lytic cycle.

Viruses can enter a host in several ways -- for example, through a cut in the skin, a mosquito bite which injects it into the bloodstream, or by direct contact with mucous membranes like the stomach, nostrils, or genitals. That means you can get a virus by eating contaminated food, by breathing in certain types which travel through the air, or through sexual contact.

Step Two: Attachment and Entry

Once a virus has gained entry to its host, it can access target host cells. The first step is attachment, which is when the viral particle physically attaches to the host. Often, the virus will use its proteins to attach to a specific receptor in the cell it attacks. This also means that many viruses can only attack certain kinds of cells. Once it's attached, the virus creates a hole in the host cell's membrane and injects its genome into the cell.

Step Three: Replication

A host cell that has been infected by a virus is said to be susceptible to the virus.

Susceptibility only means that the virus has gotten its genetic material inside the cell; it doesn't mean that the virus can reproduce and survive inside. Some cells will have interior conditions that prevent the virus from reproducing; if the virus has found one of these, it's the end of the road.

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