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By the end of this section, you will be able to:
  • Describe prions and their basic properties
  • Define viroids and their targets of infection

Prions and viroids are pathogens (agents with the ability to cause disease) that have simpler structures than viruses but, in the case of prions, still can produce deadly diseases.

Prions

Prions , so-called because they are proteinaceous, are infectious particles—smaller than viruses—that contain no nucleic acids (neither DNA nor RNA). Historically, the idea of an infectious agent that did not use nucleic acids was considered impossible, but pioneering work by Nobel Prize-winning biologist Stanley Prusiner has convinced the majority of biologists that such agents do indeed exist.

Fatal neurodegenerative diseases, such as kuru in humans and bovine spongiform encephalopathy (BSE) in cattle (commonly known as “mad cow disease”) were shown to be transmitted by prions. The disease was spread by the consumption of meat, nervous tissue, or internal organs between members of the same species. Kuru, native to humans in Papua New Guinea, was spread from human to human via ritualistic cannibalism. BSE, originally detected in the United Kingdom, was spread between cattle by the practice of including cattle nervous tissue in feed for other cattle. Individuals with kuru and BSE show symptoms of loss of motor control and unusual behaviors, such as uncontrolled bursts of laughter with kuru, followed by death. Kuru was controlled by inducing the population to abandon its ritualistic cannibalism.

On the other hand, BSE was initially thought to only affect cattle. Cattle dying of the disease were shown to have developed lesions or “holes” in the brain, causing the brain tissue to resemble a sponge. Later on in the outbreak, however, it was shown that a similar encephalopathy in humans known as variant Creutzfeldt-Jakob disease (CJD) could be acquired from eating beef from animals with BSE, sparking bans by various countries on the importation of British beef and causing considerable economic damage to the British beef industry ( [link] ). BSE still exists in various areas, and although a rare disease, individuals that acquire CJD are difficult to treat. The disease can be spread from human to human by blood, so many countries have banned blood donation from regions associated with BSE.

The cause of spongiform encephalopathies, such as kuru and BSE, is an infectious structural variant of a normal cellular protein called PrP (prion protein). It is this variant that constitutes the prion particle. PrP exists in two forms, PrP c    , the normal form of the protein, and PrP sc    , the infectious form. Once introduced into the body, the PrP sc contained within the prion binds to PrP c and converts it to PrP sc . This leads to an exponential increase of the PrP sc protein, which aggregates. PrP sc is folded abnormally, and the resulting conformation (shape) is directly responsible for the lesions seen in the brains of infected cattle. Thus, although not without some detractors among scientists, the prion seems likely to be an entirely new form of infectious agent, the first one found whose transmission is not reliant upon genes made of DNA or RNA.

Part a illustrates how normal prion protein (PRP) is converted into the disease-causing form (PRP). PRPsc may spontaneously form in brain tissue, may be introduced when a mutant form of the protein misfolds, or may introduced into the brain tissue by inoculation. The misfolded protein causes normal PRP already present in the brain to misfold. A chain reaction occurs, leading to a large amount of misfolded protein.
(a) Endogenous normal prion protein (PrP c ) is converted into the disease-causing form (PrP sc ) when it encounters this variant form of the protein. PrP sc may arise spontaneously in brain tissue, especially if a mutant form of the protein is present, or it may occur via the spread of misfolded prions consumed in food into brain tissue. (b) This prion-infected brain tissue, visualized using light microscopy, shows the vacuoles that give it a spongy texture, typical of transmissible spongiform encephalopathies. (credit b: modification of work by Dr. Al Jenny, USDA APHIS; scale-bar data from Matt Russell)

Viroids

Viroids are plant pathogens: small, single-stranded, circular RNA particles that are much simpler than a virus. They do not have a capsid or outer envelope, but like viruses can reproduce only within a host cell. Viroids do not, however, manufacture any proteins, and they only produce a single, specific RNA molecule. Human diseases caused by viroids have yet to be identified.

Viroids are known to infect plants ( [link] ) and are responsible for crop failures and the loss of millions of dollars in agricultural revenue each year. Some of the plants they infect include potatoes, cucumbers, tomatoes, chrysanthemums, avocados, and coconut palms.

The photo shows shriveled, cracked potatoes.
These potatoes have been infected by the potato spindle tuber viroid (PSTV), which is typically spread when infected knives are used to cut healthy potatoes, which are then planted. (credit: Pamela Roberts, University of Florida Institute of Food and Agricultural Sciences, USDA ARS)

Career connection

Virologist

Virology is the study of viruses, and a virologist is an individual trained in this discipline. Training in virology can lead to many different career paths. Virologists are actively involved in academic research and teaching in colleges and medical schools. Some virologists treat patients or are involved in the generation and production of vaccines. They might participate in epidemiologic studies ( [link] ) or become science writers, to name just a few possible careers.

This photo shows a scientist in a field, measuring the length of an egg.
This virologist is engaged in fieldwork, sampling eggs from this nest for avian influenza. (credit: Don Becker, USGS EROS, U.S. Fish and Wildlife Service)

If you think you may be interested in a career in virology, find a mentor in the field. Many large medical centers have departments of virology, and smaller hospitals usually have virology labs within their microbiology departments. Volunteer in a virology lab for a semester or work in one over the summer. Discussing the profession and getting a first-hand look at the work will help you decide whether a career in virology is right for you. The American Society of Virology’s website is a good resource for information regarding training and careers in virology.

Section summary

Prions are infectious agents that consist of protein, but no DNA or RNA, and seem to produce their deadly effects by duplicating their shapes and accumulating in tissues. They are thought to contribute to several progressive brain disorders, including mad cow disease and Creutzfeldt-Jakob disease. Viroids are single-stranded RNA pathogens that infect plants. Their presence can have a severe impact on the agriculture industry.

Questions & Answers

what is ecology
volex Reply
ecology is the study of living organisms in interaction to the environment
Mwansa
show the diagram of sponge
Favour Reply
explain the processes found in a carbon cycle
Elvas Reply
photosynthesis;this is the process by which grean plants make there own food by using carbon,oxygen, sunlight in the precess of chlorophyll combusion ;this is the process by which fossil fuels are converted in form of a gas of carbon dioxide by an enzyme called a foillitrrion respiration;a proces
Mwansa
Respiration;this is a process by which living organism release energy from food there eat
Mwansa
decoposition;this process is carried by dead matters or decomposes such as bacteria and fungi where dead matters decay buried for thousand of years excretions the remove of toxic metabolism waste product from the body through excretory or gain such as anus and skin etc
Mwansa
also tarnspiratiom where by a plant losses water inform of water vapour into the at mosphere by rhisombium ezyme
Mwansa
so not enzyme but bacteria
Mwansa
sorry not enzyme but bacteria
Mwansa
what is a base?
Alice Reply
base can be define as the anatomy of structure
Ogundipe
what are somatic cells
KAKU Reply
Somatic cells responsible for making the body
Ahsan
vegetative cells, they divide by mitosis.
Ishitha
what is angiosperms
Papillon Reply
what is the function of bile
Papillon
what is neurologist
Benjamin Reply
A neurologist is a brain doctor,a physician who diagnose and treat all categories of conditions and disease involving the central and peripheral nervous system eg.covering, blood vessels and all effector tissue such as muscle
Favour
what is a cell
God Reply
is a basic unit of living organism
Mwansa
or a basic building blocks of all living things
Mwansa
explain cell biology
Grace Reply
cell biology is a biology branch which study about function,size,definition ,....of cells.
fitsum
what are endotherms
angelina Reply
pls can someone elaborate more on biomagnification
angelina
Hey,what?
Peter
Biomagnification, also known as bioamplification or biological magnification, is any concentration of a toxin, such as pesticides, in the tissues of tolerant organisms at successively higher levels in a food chain.
chance
An endotherm (from Greek ἔνδον endon "within" and θέρμη thermē "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat set free by its internal bodily functions instead of relying almost purely on ambient heat.
chance
these are substances that absorbs heat energy
Alice
What diffusion
Oseres Reply
it is the movement of water molecules from the region of higher concentretion to a region of lower concentration
Lukas
is the movement of anything from region of higher concentration to a region of lower concentration.
chance
is a movemnt of H2O molecule from a region of higher concentration to the legion of lower Concentration.
Benjamin
diffusion is the movement of particles from a region of higher concentration to a region of lower concentration resulting into an even distribution
Alice
is the movement of water molecules,ions from the region of higher concentration to the region of lower concentration in a semi permeable membrane.
Favour
pls I don't know how to read very well so how can I understand all this
Mercy Reply
But you wrote all this ur self so please continue till you become perfect thanks
Favour
What's succession
Talabi Reply
an acid is a substance that produces hydrogen ions in an aqueous solution
angelina Reply
how? give a reason to support ur answer
SSEJJO
Types of acids and there definitions
SSEJJO
what is acid
asaa Reply
An acid is a substance which gives Hydrogen ions in aqueous solutions.
Ahsan
food poison
asaa
an acid is a chemical substance that produces or gives out hydrogen ions as the only positively charged ion when dissolved in water
Alice
what is salt?
Basit
a sait is a substance formed when hydrogen ions in an acid are partially or fully replaced by a metal or ammonium ions
Alice
When an acid and base react chemically, a salt is form.
Ahsan
what is nervous system
kumkum
nervous system is rensposible of homeostasis
Mwansa
a sait is a compound which tests beta
Mwansa

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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