The Deadly Cycle
Depending upon students' grade and maturity levels, the essay, ”The Deadly Cycle," may be used as teacher background information or as a student reading assignment. It is especially effective when read aloud.
An HIV virus particle is far too small to be seen with an ordinary light microscope. More than one hundred times smaller than the white blood cells they invade, HIV virus particles look like miniature cells—but they are not cells. Rather, HIV particles, like all viruses, are best described as containers of genetic material.
The HIV particle is surrounded by an envelope of cell membrane material, taken from the cell from which it emerged. Inside, the HIV virus contains enough genetic material (in the form of RNA molecules) to direct a host cell to make new virus copies. Viruses cannot live, grow and reproduce on their own. Instead, they must invade the cells of living organisms and force those cells to produce more viruses. This is how viruses cause disease. The term, “virus particle” (or “virion”) usually refers to the infectious version of the virus, as it exists outside a host cell.
The surface of an HIV particle typically has between 14 to 73 small projections, referred to as glycoprotein spikes. Glycoproteins (gp) are protein molecules with carbohydrates incorporated into their structure. They are represented by concentric circles on the outside of the paper model used in the previous activity. Two different glycoproteins, gp120 and gp41, comprise each spike on an HIV particle. The numbers, 120 and 41, refer to each protein’s molecular weight (an indicator of a molecule’s size). The gp120 glycoproteins allow the HIV virus particles to attach to, or “dock” with certain kinds of white blood cells.
HIV cannot survive for long outside the body, and only can be transmitted to another person through body fluids from someone who already has the infection. Once inside the body, HIV particles enter the blood stream and make contact with leukocytes, or white blood cells, the body’s chief defenders against infectious diseases. There are five different kinds of leukocytes. However, HIV most often attacks one kind, called a CD4+ cell. CD4+ cells get their name from a particular protein, called CD4, found on the outside cell surface (in other words, these cells are “positive” for the presence of a CD4 protein). CD4+ cells sometimes are referred to as T-cells.
HIV particles—specifically the exterior glycoprotein spikes—attach to CD4 molecules on the surface of CD4+ cells. This connection is similar to that between a lock and key. Once attached, the virus particle fuses with the cell membrane and releases its contents into the cell. After this stage in the infection process, the HIV particle and white blood cell together can begin to reproduce more HIV particles.
Inside the fatty envelope of an HIV particle is a bullet-shaped core, called the capsid. Made of proteins, the capsid holds the virus’s genetic material and triggering enzymes. HIV’s genetic material consists of two single-stranded RNA molecules (or ribonucleic acid). The viral RNA strands contain just nine genes, compared to the 20,000 or 25,000 genes in humans. Once HIV RNA is inserted into a cell, an enzyme called reverse transcriptase transcribes, or changes the RNA strands into double-stranded DNA. The viral DNA then integrates with the host DNA in one chromosome within the cell’s nucleus. From this point, the virus may remain inactive for many years. Eventually, though, the viral DNA is activated and the cell begins replicating the parts required to make new HIV virus particles—by the hundreds of thousands. In essence, HIV hijacks the cell’s functions and turns the cell into a kind of virus factory. Raw materials inside the cell are reworked into new strands of RNA, proteins, and enzymes, which gather just inside the cell wall. Then, the new HIV virus particles bud from the wall of the host cell into the bloodstream.
The HIV replication process eventually overwhelms the host cell until it dies. New HIV particles, millions of them, pass through the blood stream to attach and insert themselves into other leukocytes and begin the replication process again. Over time, the number of white blood cells declines to the point where they can no longer provide protection. Other components of the immune system, such as the lymph nodes, also are affected, and the host body becomes less and less able to defend itself against diseases. A person infected with HIV is diagnosed with AIDS when he or she has one or more serious illnesses associated with HIV, such as pneumonia or tuberculosis, and has dangerously low numbers of infection-fighting white blood cells.
Keywords: AIDS | blood | capsid | disease | DNA | epidemic | epidemiologist | epidemiology | HIV/AIDS | illness | immune system | infection | microbe | microbiology | microscope | pandemic | pathogen | replication cycle | retrovirus | RNA | SEM | T-cell | TEM | viral assembly | virion | virus | white blood cell | HIV
- Vogt, G., and Moreno, N. (2012) The Science of HIV/AIDS Teacher’s Guide. Baylor College of Medicine: Houston. ISBN: 978-1-888997-62-0
- Scanning electron microscope image courtesy of the National Institute of Allergy and Infectious Diseases, NIH.
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Grant Number: 5R25RR018605