We're all familiar with the basics of viruses: These particles infect living cells and basically wreak havoc throughout the body. But viruses aren't the only villains around causing chaos in living things. Other infectious agents called viroids and prions — which are also tiny but powerful — can take down both plant life and entire animals.
How are viroids and prions the same as viruses? And how are they different? All three — viruses, viroids, and prions — are acellular particles. Acellular particles are not alive, which means:
- They're not made of cells.
- They don't transform energy.
- They can't reproduce on their own.
- They can only be seen with an electron microscope.
- They don't grow or divide.
Acellular particles can hang around forever — sitting on a countertop or on a doorknob, doing nothing and causing no harm. However, viruses, viroids and prions are infectious agents. Once they enter a suitable cell, be it animal, plant or bacterial, they take over the cell's metabolic agents to increase their own numbers [source: Paustian].
You can almost think of them as hijackers. The only goal for viruses, viroids and prions is reproduction, and the only way for them to achieve that goal is to take over host cells. Once they take over, they use those cells to alter normal functioning and make new virus particles. These particles can cause disease in plants, animals and humans — and they've even changed the history of life on Earth by changing the DNA of various organisms.
Now let's look at how the three are different, starting with viruses.
A virus is a package of genetic material. This genetic material is either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). This little package is carried in a shell called the capsid. Some viruses have an extra envelope covering the capsid. While covered in its capsid, a virus is in an extracellular state. This means the virus hasn't invaded a host cell and is pretty much just hanging around doing nothing.
However, once a virus invades a host cell, it becomes intracellular, and that's when the action starts. A virus can infect a cell several different ways — through bodily fluids (such as saliva or blood), air (sneezing or coughing) or a mosquito bite. The virus then begins its attack by triggering the cell to let it in and take control. The virus starts replicating and overriding the cell's normal functioning and, in some cases, inserts its own genetic material into the cell's DNA. The cell actually does all the work — the virus just calls the shots. The virus becomes a commander and starts sending out more infectious troops into the body.
Even though the body has natural defenses against viruses, many viral infections replicate so rapidly that our immune system simply can't keep up. Antibiotics are useless against viruses, although immunizations work. Common examples of viruses include:
- Chicken pox
- Common cold
- West Nile Virus
So how do viroids and prions operate differently?
Viroids and Prions
A viroid (an infectious RNA molecule) is similar to a virus but not quite the same thing. It's smaller than a virus and has no capsid. A viroid is a coiled, "naked" RNA molecule that can affect a cell. Although RNA (unlike DNA) is single-stranded, the RNA in a viroid coils around itself to become double-stranded for strength. Its claim to fame is its ability to infect plant life. Although it has no protein of its own, a viroid's RNA affects a plant's ability to produce essential proteins.
Discovered by scientists in the 1970s, viroids affect crops like tomatoes, potatoes and fruit trees. They don't cause human disease, but they can cause crop failure, which means a huge loss of revenue for the agriculture industry each year [source: Microbe World].
Viroids typically spread via seeds or pollen. An infected plant might rub up against an uninfected neighbor, an insect could feed on different plants and spread the viroid around, or farm tools might distribute the infection throughout an entire field.
Prions (infectious protein particles) have neither DNA nor RNA to transmit infection. Not much is known about them, but we do know that a prion is an abnormal or mutated form of a usually harmless protein. When this normal protein becomes a prion, it becomes dangerous and can cause several mostly fatal neurodegenerative brain diseases. The most well-known disease caused by prions is mad cow disease (the human form is Creutzfeldt-Jakob disease).
Once prions enter the brain, they force the normal cellular proteins to begin folding into abnormal shapes. This destroys the neurons and eventually leads the brain to become riddled with holes. This spongelike brain consistency is where the medical name for mad cow disease comes from — bovine spongiform encephalopathy (BSE). Prions also cause the condition "scrapie," a degenerative disease affecting the nervous system, in sheep and goats.
Prions can cause a few human diseases, although they're extremely rare. One disease, kuru, is associated with cannibalism. Some scientists believe that prions may also have a role in Alzheimer's disease [source: Microbe World]. There is no treatment for any prion-caused disease.
You cannot destroy prions by sterilization or by cooking (although incineration works). Prions are resistant to heat, radiation and chemicals, so they are almost unstoppable. The best way to avoid spreading prions in humans and animals is to prevent brain and spinal cord tissue from getting into the food supply or onto medical equipment. Our research even turned up a novel called "The Zombie Autopsies" that imagines a zombie apocalypse caused by prions released by scientists [source: Stone]. The takeaway here is that although prions are small, they are mighty.
To wrap up, viruses, viroids, and prions all create havoc if they enter your body (or a plant or animal). They all have the same goal — taking over a living cell the way a pirate might take over a ship — but the way they achieve that goal is slightly different for each.
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- Bauman, Robert. "Microbiology with Diseases by Taxonomy." Pearson Education. 1995. (Sept. 16, 2014) http://wpscms.pearsoncmg.com/pls_1256139408_bauman_microbio_3/190/48837/12502332.cw/index.html
- BSEinfo. "Scrapie." 2014. (Sept. 16, 2014) http://www.bseinfo.org/scrapie.aspx
- Hoefnagels, Mariëlle. "Biology: Concepts and Investigations." McGraw-Hill. 2011. (Sept. 16, 2014) https://www.inkling.com/read/biology-marielle-hoefnagels-2nd/chapter-15/15-6-viroids-and-prions-are
- Microbe World. "Viruses — and Some Virus-like Agents." 2006. (Sept. 16, 2014) http://archives.microbeworld.org/microbes/virus/
- Paustian, Timothy. "Through the Microscope." 2014. (Nov. 29, 2014) http://www.microbiologytext.com/5th_ed/book/displayarticle/aid/181