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Retrovirus: Definition, Life Cycle & Example

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  • 0:00 What are Viruses?
  • 0:45 Life Cycle of the Cold Virus
  • 1:45 How is a Retrovirus Different?
  • 3:22 The Most Infamous…
  • 5:16 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.

This lesson will discuss retrovirus infection, survival, and reproduction. First we'll discuss how a regular virus works, then how a retrovirus differs, using HIV as an example.

What are Viruses?

The term 'retro' may provoke images like disco, shag carpeting, or the 1970s depending on your age. However, a retrovirus is not a virus with bad fashion. To really understand a retrovirus, we need to first talk about the average virus life cycle. Then we can discuss what makes a retrovirus different.

Viruses can infect all types of living organisms, from bacteria and yeast to plant and animal cells. Viruses cannot replicate on their own, meaning they have to invade a host cell to complete their life cycle. There are many ways a virus can invade a host, and different viral species have different invasion strategies. Let's use the common cold as an example.

Life Cycle of the Cold Virus

First, a virus particle, or virion, is the form of the virus that is infective. So, when your friend with a cold sneezes on you, their sneeze contains hundreds of virions. When you breathe, the virus is inhaled. It can then stick to cells lining your nose and throat. After it sticks to a cell, it injects its genome into the host cell. The viral genome can be DNA or RNA and single or double stranded.

Once inside the host, the viral genome is turned into more viral proteins and viral genomes. This is actually done by host machinery. In other words, the virus you just caught from your friend enslaves your cells and forces them to make copies of itself. The new viral genome and viral proteins become a new virion which can infect more of your cells. At this point it is important to point out that the virus 'takes over' your cells; it does not 'become part' of your cells.

How is a Retrovirus Different?

A retrovirus is different because it inserts its genome into the host's genome. In this way, the retrovirus becomes part of your cells. The way a retrovirus is able to become part of your cells is the reason it is called a retrovirus. A little protein biology is needed at this point.

Normally, when a cell needs to do something, it makes a protein. The protein is designed to do a specific task. Your cells make hundreds of proteins every day. DNA is the blueprint for making a protein. However, this blueprint can't leave the nucleus. Think of your nucleus as a library with rare cookbooks (DNA stored in chromosomes) that are not allowed to leave. You can make copies of individual protein recipes (called mRNA) by a process called transcription. mRNA leaves the nucleus and is copied into protein by a process called translation.

Your body's protein-making machinery does not know the difference between its own RNA and viral RNA. So it turns both into proteins. DNA being transcribed into RNA which is translated into protein is called the central dogma. For a very long time it was thought that this process only moved in one direction. Then retroviruses were discovered.

Retroviruses actually have a protein called reverse transcriptase that is packaged inside their virions which turns their RNA genome into DNA! This backwards transcription is why we call them retroviruses. In this way, a retrovirus violates the central dogma.

The Most Infamous Retrovirus: HIV

You have probably heard of the human immunodeficiency virus, or HIV, but did you know there are actually at least 13 known retroviruses? We will talk about the HIV life cycle as an example of how retroviruses work.

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