HIV is a virus. A virus is a tiny, relatively simple, nonliving organism, usually made up of little more than a few strands of genetic material and a protein shell. 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.
HIV belongs to a family of viruses called retroviruses. Unlike regular viruses, which have DNA (deoxyribonucleic acid) as their genetic material, retroviruses have RNA (ribonucleic acid). Retroviruses also contain an enzyme called reverse transcriptase, which allows the retrovirus's genetic material to imitate 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 (Anderson, 1992).
Discussion question: HIV is classified as a retrovirus and a lentivirus. How do these classifications make HIV differ from regular viruses, and what implications do they have for HIV disease testing, diagnosis, progression, and treatment?
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 structure of HIV-1 (National Institute of Allergy and Infectious Diseases [NIAID], 2001).
The outer coat of the virus is called the viral envelope or lipid membrane (see figure above). 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 particles break through a host cell's surface in a process called "budding," they wrap themselves in fat molecules from the host's outer membrane (NIAID, 2001).
The complex proteins that protrude through the surface of the viral envelope are frequently called spikes (see figure above). 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.
Within the viral envelope of a mature HIV particle is a bullet-shaped core called the capsid (see figure above). The capsid surrounds two single strands of HIV's single-strand genetic material, ribonucleic acid (RNA). 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 double-strand deoxyribonucleic acid (DNA), so that it can pass into the host cell's nucleus, commandeer the host cell, and begin reproducing itself (NIAID, 2001).
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