What happens to the host cell's machinery when a virus injects its genetic information? For instance, what happens to the cell's own genetic machinery such as the chromosomes and the DNA coiled in it? Why do enzymes take orders or instructions from the foreign DNA and not from the DNA coiled in the chromosomes?
-A curious adult from India
August 7, 2008
When a virus injects its DNA into a cell, the cell's DNA is still there. The virus just tricks the cell into ignoring the cell's own DNA. And makes the cell only pay attention to the virus' DNA.
A virus needs to do this because it travels light. So light in fact that it can't make new virus on its own. This is why it needs to hijack a living cell and force it to make virus.
Viruses Have Few Genes
Viruses cause many diseases such as colds, the flu, and AIDS. Pretty nasty for something that isn't even considered alive!
A living thing needs to have lots of different genes to make new copies of itself. One study says at least 300 are needed.
Viruses have many fewer genes than this though. They usually have just enough genes to make a protective coat around their instructions. In addition, they also have a few genes to trick a cell into reading the virus' instructions.
To understand how viruses trick cells, we need to understand how cells work. And in particular, how cells read the instructions found in genes.
How a Cell's Instructions are Read
The instructions for making and running a cell are found in the genes of its DNA. These instructions are written in a simple four letter language consisting of A, G, C, and T.
Cells read a gene's instructions by making an RNA copy of it. RNA is made up of almost the same letters: A, U, G, and C. One big difference between DNA and RNA is how each is made.
Each is made with a special protein called a polymerase. New DNA is made with a DNA polymerase. RNA is made from DNA with a protein called RNA polymerase.
The RNA polymerase is very good at stringing together an RNA letter for each DNA letter it reads. But it is not very good at finding the start of a gene. For this, it needs the help of other proteins.
One very important protein it teams up with is called TFIID. TFIID's main job is to find the start of genes.
Once the cell turns a gene into RNA, the cell then reads the RNA and makes a protein. Each protein does a special job -- they are the workhorses of a cell.
Tricks of the Viral Trade
Viruses stop many parts of this process from DNA to RNA to protein. The trick is that the virus can stop these steps for the cell but its own instructions are free to be followed. So with its own instructions blocked, the cell follows the virus'.
One way a cell can do this is by making the cell only see the virus' genes. A great example of this approach is from poliovirus.
When poliovirus enters a cell, it unleashes a protein weapon called 3C. One thing that 3C does is chop up a cell's TFIID.
Remember without TFIID, RNA polymerase can't find where the genes are. This means no genes from the cell can be made into RNA and protein.
But the virus doesn't use TFIID for its genes. The trick is that the cell's genes are made of DNA, but the virus's are made of RNA! That means that everything the virus does just needs to go from RNA to protein. And since the cell can't make much RNA, most of the RNA around is the virus'. The virus takes over!
This is just one example of how a virus can take over a cell. The amazing thing is that some viruses can take control of a whole animal.
One example of this is the rabies virus. When it's in an animal or a human, it makes them violent and want to bite. (The Egyptians first wrote about this 3000 years ago.) This behavior helps the virus enter a new host. The virus goes from the saliva of one host into the blood of a new host through a bite.
Although it sounds like viruses are only bad, scientists are finding neat ways to use them to help us. When some viruses infect us they insert their DNA into our DNA. The DNA they insert is normally bad. But scientists are changing these viruses so that they can insert good DNA into humans. This is called gene therapy.
Using a virus, a working copy of a gene could be put into a patient. This would be great for someone with only bad copies of a gene. For example having two bad copies of the CFTR gene causes cystic fibrosis. If a virus could insert a good copy of the CFTR gene into someone with cystic fibrosis they could be cured!
George Wang, Stanford University