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.
HIV targets cells of the immune system. Like other viruses, the HIV virion fuses with the host cell and injects its genome. Reverse transcriptase turns the HIV RNA into HIV DNA.
Another HIV protein then inserts the HIV DNA into the host’s DNA. This protein is called integrase, and you can remember the name by thinking that it’s a protein that ‘integrates’ HIV DNA into host DNA.Once integrated into the host genome, HIV DNA is transcribed into mRNA just like host DNA.
But remember our central dogma discussion? In your cells, you make copies of individual protein recipes. Copying HIV DNA makes an mRNA with all of the HIV proteins strung together. The host machinery copies the viral mRNA faithfully and a string of linked HIV proteins is produced. These proteins are not functional until they are cut apart, so HIV has one more protein that it brings along in each virion.
This protein is protease. The ‘prote’ part should remind you of protein and ‘ase’ indicates an enzyme. So, protease is a protein-cutting enzyme.Now HIV has mature proteins.
The new virion leaves the host cell by budding. Essentially, the virion exits the host cell and takes part of the host cell membrane with it, like a coat. The host cell survives this process and continues to make more HIV.
Viruses invade host cells and turn them into virus-making machinery. Usually, the host cell is killed in this process. Retroviruses do not kill the host cell at first because they can insert their genome into the host genome. This process is called reverse transcription and is done by the viral protein reverse transcriptase.
In the case of HIV, viral protein integrase then inserts the HIV DNA into host DNA. Host proteins make copies of the viral DNA. However, there are many HIV proteins linked together. This is a problem because these proteins are not functional until they are cut apart by protease. Reverse transcriptase, integrase, and protease are packaged with the HIV RNA genome into a virion. The HIV virion exits the host by budding, taking a bit of the host membrane with it.
The host survives however, and continues to make new HIV.
Lesson at a Glance
Like a virus, retroviruses cannot replicate on their own, meaning they have to invade a host cell to complete their life cycle. Unlike a virus, a retrovirus inserts its genome into the host’s genome. In this way, the retrovirus becomes part of your cells and violates the central dogma.
Upon completing this lesson, you should be able to:
- Compare and contrast a virus and a retrovirus
- Explain why retroviruses are called as such
- Describe the life cycle of HIV