Glossary
 
     
Living with HIV/AIDS  






Source: http://www.vpul.upenn.edu/ohe/services/hiv.jpg

HIV is a Virus

HIV is a virus that infects only people and creates a deficiency in their body's immune system. Viruses similar to HIV affect other animals, including cats and monkeys.

A virus is a tiny, relatively simple, non-living organism, usually made up of little more than a few strands of genetic material and a protein shell.

The Human Immunodeficiency Virus belongs to a family of viruses called retroviruses. Unlike regular viruses, which have DNA (dioxyribunucleic acid) as their genetic material, retroviruses have RNA (ribonucleic acid). Retroviruses also contain an enzyme called reverse transcriptase, which allows the retrovirus' genetic material to infiltrate the host cell's genetic material.

In comparison to cells and DNA-based viruses, retroviruses mutate (that is, their genetic code changes) very quickly. When retroviruses mutate, a person's immune system often does not have any defenses against the new version of the retrovirus. As a result, retroviruses are more difficult to fight than normal viruses.

Retroviruses mutate more quickly than other viruses because of the way they reproduce. When cells and normal viruses reproduce, an enzyme "proofreads" the resulting genetic material. The equivalent enzyme involved in retroviruses' reproduction--reverse transcriptase--does not proofread the new genes. As a result, new batches of retroviruses are more likely to have errors, or mutations, in their genes than are new batches of normal viruses or cells.

The Human Immunodeficiency Virus also belongs to a subfamily of retroviruses called lentiviruses. Lenti means "slow," so lentiviruses are retroviruses that have a long delay between the time they initially infect a person and the time the person starts to show serious symptoms (1).


Although deadly to the cell it attacks, a single human immunodeficiency virus (or viral particle) is much smaller than a human cell. HIV particles have a diameter of only 1/10,000 of a millimeter, compared to the average human cell size of 1/10 of a millimeter. HIV particles are also much simpler in structure than human cells. HIV particles are made up of the following parts:

The Viral Envelope

The outer coat of the virus is called the viral envelope, or lipid membrane. The viral envelope is composed of two layers of fat molecules ("lipid" means fat). HIV gets its outer envelope from its host. As newly formed HIV breaks through a host cell's surface in a process called "budding," they wrap themselves in fat molecules from its outer membrane (3).

 

The Spikes

The complex proteins that protrude through the surface of the viral envelope are frequently called spikes. These spikes are HIV's landing gear, attaching the virus to a host cell and fusing the two together. Each HIV has an average of 72 spikes. Each spike is made up of two parts: a stem and a cap. Each stem consists of three glycoprotein 41 (gp41) molecules, and each cap consists of three glycoprotein 120 (gp120) molecules. The stem anchors the spike to the viral envelope.

 

The Capsid and its Contents: RNA and Reverse Transcriptase

Within the envelope of a mature HIV particle is a bullet-shaped core, called the capsid. The capsid surrounds two single strands of HIV's genetic material, RNA (ribonucleic acid). Each strand of RNA has a copy of the virus's genes. These genes contain the information that HIV uses to make new virus particles. HIV has only nine genes, in comparison to human cells, which have an average of 30,000 - 50,000 genes. The capsid also houses two molecules of HIV reverse transcriptase. Reverse transcriptase is an enzyme that allows the HIV's RNA to change into DNA, so that it can pass into the host cell's nucleus, commandeer the host cell, and to begin reproducing itself. (3)

References:
1. Anderson, K. (Ed.) Mosby’s Medical, Nursing, & Allied Health Dictionary, 4th Edition. St. Louis, MO: Mosby – Year Book, Inc., 1994.

2. Cohen, P.T. Clinical overview of HIV disease. HIV InSite Knowledge Base Chapter. San Francisco, CA: Center for HIV Information, University of California San Francisco, 1998. Retrieved on February 2, 2004 from http://www.hivinsite.com/InSite.jsp?page=kb-03&doc=kb-03-01-01.

3. Institutes of Allergy and Infectious Diseases (NIAID). How HIV Causes AIDS. National Institutes of Allergy and Infectious Diseases, National Institutes of Health, U.S. Department of Health and Human Services, October 2001. Retrieved on January 14, 2004 from http://www.niaid.nih.gov/factsheets/howhiv.htm.

© Sociometrics Corporation, 2004