next.jpgmore.jpgback.jpgProviruses v ERVshelp.jpgJoin56x56.jpghome.jpg The stretch of DNA that a retrovirus integrates into the host DNA is called a provirus.
It has a distinct structure, oriented to doing it's job of producing virus particles that can reinfect other cells.
Here again is the link to see how retroviruses operate.more.jpg

But there are other stretches of DNA in the cells of all creatures. They look just like proviruses, but there are some important differences.
These are the stretches of DNA we call endogenous retroviruses, or ERVs for short.

The following table sets out the differences between proviruses and ERVs.
Retroviral proviruses
Structured for producing viruses
Same structure *
Viral codon bias
Viral codon bias
Makes a cell produce viruses
Does not make a cell produce viruses
Inserted in the DNA by a retrovirus
Not inserted by a retrovirus
Varied insertion site
Always found at the same site
Proviruses target certain types of cells
ERVs are found in every cell
Not every cell of that type is infected
ERVs are found in every cell
Provirus content varies from one insertion to another
Does not vary
Individuals have only few or no types of proviruses
Individuals have hundreds of thousands of them
Only some individuals are infected
Often, the same ERVs can be found in every individual
Usually only present in one species
Often found in whole groups of species
So retroviruses can explain proviruses. What is it that can explain all these ERVs, exactly the same in every cell of the body?
Click on next.jpg to find out.