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Name: Tahlia
Status: Student
Grade:  9-12
Location: CA
Country: United States
Date: March 2006


Question:
What is the mechanism by which a virus actually infects a cell?



Replies:
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 means that our cells can no longer do what our bodies need them to do. This makes us 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 that 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 that 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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