Country: United States
Date: March 2006
What is the mechanism by which a virus actually
infects a cell?
Hi Tahlia, good question.
A virus is basically a protein that carries inside of it some genetic
material, such as DNA or RNA, and some enzymes. Viruses can infect the cells of many
different organisms such as plants, animals or even bacterial cells.
1) To infect a cell, first viruses must recognize certain cells that they
are able to infect and attach to them. They do this by looking for certain
"markers" on the surface of the cell's membrane. For example, HIV viruses
recognize and attach to the CD4 cell surface markers that are found
specifically on the host's helper T cells, macrophages and dendritic cells.
2) To infect the cell, they must then enter the cell. Certain other markers
(called co-receptors) are usually required for entry into the cell.
3) Once inside the cell, the virus sheds it's protein coat and releases it's
enzymes and genetic material into the cell (DNA or RNA, depending on which
type of virus it is).
If it is a DNA virus, it is then free to integrate it's genetic material
into our genetic material using the enzymes it brings with it. If it is a RNA
virus, it uses its enzymes (reverse transcriptase) to make viral DNA from
its RNA, and then it integrates it's viral DNA into our DNA.
4) Once it has integrated into our DNA it may use our genetic machinery
(ribosomes, nucleotides and enzymes) to reproduce itself many times over
(called an active viral infection). Or it may not replicate and it can stay
inactive, integrated in our cell's DNA, but not actively producing new
viruses (called an latent viral infection).
Remember, a virus is simply a small strand of genetic material (DNA or RNA)
with some enzymes + a protein coat. It cannot reproduce on its own, so it needs
to borrow the equipment we use to make copies of our own genetic material, and
it uses our stuff to make copies of itself.
The activity of the virus disables our cells and usually kills them. This
that our cells can no longer do what our bodies need them to do. This makes
sick. As our immune system fights off the viral infection, we feel tired,
achy and may run a fever.
The devastation of HIV is that it kills our major immune cells (helper T's,
macrophages and dendritic cells).
The helper T is the main hub of our immune system, without helper T cells we
cannot mount an appropriate immune response to even small infections, ones
would not even bother us at all if we had healthy helper T's. So when HIV
infections kill enough helper T cells, we call this condition Acquired
Immunodeficiency Syndrome - AIDS. This means that we have an a deficiency
of our immune system that was acquired via HIV infection. Once we have AIDS,
we are susceptible to extremely harmful infections and even the most common of
infections are potentially fatal.
5) Once an active virus has made all of its components using our genetic
machinery, it assembles and is released from the cell. One single cell
usually releases thousands and thousands of new virus particles that are now free to
infect other healthy cells of the host.
So to review. There are 6 basic steps to viral infections.
1 - Recognition
2 - Attachment
3 - Entry
4 - Integration into the hosts DNA
5 - Replication
6 - Release
Also remember, HIV and AIDS are two very different but related
diseases. HIV is
a viral infection. AIDS is syndrome of the immune system. HIV frequently
leads to AIDS. Just because a patient is infected with HIV does not mean
they have or ever will develop AIDS.
Hope this helps and study hard,
Stephen A. Sardino Jr.
MS-II Penn St. COM
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Update: June 2012