Viruses, Viroids, and Prions PDF

2.2 Viruses, Viroids, and Prions Each fall millions of Canadians line up for an annual flu shot. The flu shot is a vaccine designed to help protect you from the influenza virus and prevent...

2.2 Viruses, Viroids, and Prions Each fall millions of Canadians line up for an annual flu shot. The flu shot is a vaccine designed to help protect you from the influenza virus and prevent you from getting the seasonal flu (Figure 1). In this section you will explore the biology of viruses and other infectious par- ticles. You will examine their role in causing disease as well as how they can be used to treat or prevent disease. viruses Viruses are small, non-living particles. A virus particle consists of genetic material Figure 1 Human influenza viruses cause surrounded by a capsule made of protein, called a capsid. Viruses have no cytoplasm, seasonal flu. It would take 10 million and many are less than 0.1 µm in diameter—hundreds of thousands of viruses could viruses placed side by side to cover a easily fit inside a typical human cell. Viruses cannot grow or reproduce on their own distance of 1 mm. and do not produce or use energy. They do not create waste. You can think of them as virus a small infectious particle packages of genetic instructions that can enter and take control of cells. Their genetic containing genetic material in the form material is a piece of DNA or RNA (ribonucleic acid). Like DNA, RNA can carry infor- of DNA or RNA within a protein capsule mation that provides instructions for synthesizing protein molecules. All viruses are infectious—they are passed from cell to cell and from organism to capsid a protein coat that surrounds organism. After a virus enters a host cell, the viral DNA (or RNA) may begin to take the DNA or RNA of a virus control of the cell. The cell eventually makes copies of the virus. RNA (ribonucleic acid) a nucleic acid found in all cells and some viruses; usually why viruses Are Important carries genetic information that provides Viruses are responsible for many human diseases. Some viral diseases, such as the instructions for synthesizing protein common cold and chicken pox, produce relatively mild symptoms. Others, such as AIDS, cholera, and rabies, are much more serious and can be deadly. Viral diseases are significant not only because they affect individuals, but also because of their ability to spread. Some, such as the influenza virus, are transmitted easily from person to person and can infect millions of people in a relatively short time. A large, epidemic a large-scale outbreak of rapidly spreading outbreak of disease in a particular region is called an epidemic. disease; usually confined to a limited When an epidemic spreads on a global scale, it is called a pandemic. Table 1 lists some geographic region significant viruses and the diseases they cause in humans. pandemic an epidemic that occurs over a A small number of viruses play a role in certain cancers. All cancers involve un- widespread geographic area, often globally controlled cell division caused by mutations in the cells’ DNA. When viruses infect host cells, they sometimes create changes in the host’s DNA that can lead to cancer. The hep- atitis C virus, for example, has been shown to be a major contributor to liver cancer. Viruses cause diseases in wild and domestic animals as well as in humans. Plant viruses destroy millions of tonnes of crops every year, especially cereals, potatoes, sugar beets, and sugar cane. Although some viruses can be harmful, they are important in ecosystems. By causing disease, they control the populations of other organisms. Viruses are also extremely abundant. A single millilitre of ocean water can contain millions of viruses. Table 1 Disease-Causing Viruses DNA viruses Disease hepadnavirus hepatitis B herpesvirus cold sores, genital herpes, chicken pox adenovirus respiratory infections, tumours RNA viruses Disease paramyxovirus measles, mumps, pneumonia, polio, common cold retrovirus HIV/AIDS rhabdovirus rabies 54 Chapter 2 The Prokaryotes, Viruses, and Protists NEL Classification and Phylogeny Viruses challenge the basic categories of living and non-living. They are classified as bacteriophage a virus that infects non-living because they do not have the key characteristics of living cells. However, bacteria viruses do share one important trait with living things: they reproduce. Unlike other living things, however, viruses cannot reproduce without a host cell. The way viruses reproduce makes them very interesting to biologists. Viruses are classified into orders, families, genera, and species. They are classified based on a variety of features including size, shape, and type of genetic material. About 4000 virus species have been classified, but scientists believe that there may be millions. Most viruses can infect only a single host species or a few closely related hosts. A species of virus might infect only one organ system, or a single tissue or cell type in its host. For example, human immunodeficiency virus (HIV) infects only certain immune system cells. However, some viruses can infect many species. For example, the rabies virus can likely infect all species of mammals and birds. Of the roughly 80 known virus families, 21 include viruses that cause disease in humans. Viruses that infect bacterial cells are called bacteriophages, or phages. Most other types of viruses enter the host cell, but phages do not. Instead, they inject their DNA Figure 2 Three bacteriophages attach into the bacterium, and their protein capsule remains outside the cell (Figure 2). to the outer surface of a bacterium. You Phages have been the subject of intense research. Much of our early understanding can see the strands of DNA that the about the structure and function of viruses came from this research. phages are injecting into the cell. The Origin of viruses Several different hypotheses have been proposed to explain the origin of viruses. One possibility is that viruses originated as small infectious cells that over time lost their cytoplasm and their ability to reproduce outside a living cell. Some biologists suspect that viruses originated as “escaped” fragments of DNA or RNA molecules that once formed part of living cells. A recent hypothesis suggests that viruses are ancient, and that virus-like particles existed even before the first cells. Characteristics Viruses vary in structure, but they all consist of an RNA or DNA molecule surrounded by a capsid. Some common virus shapes are shown in Figure 3. In addition to the capsid, some viruses are surrounded by an envelope. The envelope is created when a virus leaves a host cell and part of the host cell membrane wraps around the virus (Figure 4). membrane viral RNA proteins viral DNA viral particle capsid capsid (a) tobacco mosaic virus (b) adenovirus head capsid viral RNA envelope viral DNA enveloped host cell sheath cytoplasm tail capsid (c) HIV (d) bacteriophage Figure 3 Viruses consist of a molecule of RNA or DNA surrounded by a capsid. (a) and (b) The Figure 4 When some viruses leave their capsid takes various geometric shapes. (c) Some viruses, such as HIV, also have an envelope made host cell, a membrane envelope forms from the membrane of a host cell. (d) Phages have a complex head and tail structure. around them. NEL 2.2 Viruses, Viroids, and Prions 55 C02-F08-OB11USB  Infectious Cycles Viruses do not carry out life functions like living cells do. They become active only when their genetic material has entered and taken control of a living cell. The process by which a virus infects a host cell, replicates, and destroys the host cell is called an infectious cycle. Figure 5 shows two common infectious cycles using the lambda phage as an example. 2 The viral DNA forms a 1 The virus binds to the loop. The cell may then 3 In the lysogenic cycle surface of the host cell enter the lytic or the viral DNA gets and inserts its DNA into lysogenic cycle. inserted into the the cell’s cytoplasm. bacterial chromosomes. 9 Lysis occurs as the host cell bursts. 4 lytic cycle lysogenic cycle Each time the bacterium 8 divides, the viral DNA is replicated along with the In the lytic cycle, the bacterial DNA. viral DNA instructs the cell to make and assemble new viral DNA and capsids. 5 7 6 When the viral DNA becomes active, it separates from the bacterial chromosome and enters the lytic cycle. Figure 5 Phage infections can include both lytic and lysogenic cycles. lysis the rupturing of a cell; can occur In the lytic cycle, when assembly is complete, lysis occurs as the host cell ruptures, when newly made viruses are released or bursts, releasing about 100 to 200 new viruses into the host cell’s surroundings. The from a host cell host cell is then destroyed. This entire lytic cycle can take less than one hour. lysogeny a state of dormancy in which In the lysogenic cycle, the viral DNA can stay in a dormant state, called lysogeny, viral DNA may remain within a host cell’s for many years. The bacterium continues to grow and divide normally, but each time chromosome for many cell cycle generations it divides it makes a copy of the virus DNA that was inserted within its own chro- mosome. When triggered by a change within the cell’s environment, the viral DNA becomes active, separates from the bacterial chromosome, and enters the lytic cycle. Science Biology 11 On rare occasions, when the viral DNA separates from the bacterial chromosome ISBN: ISBN # 0176504311 (Step 6 in Figure 5), a small piece of the bacterial DNA may separate from the FN C02-F09-OB11USB chromosome and become incorporated into the viral DNA (this process is not illus- COtransduction a typeDave McKay of gene Illustration trated in Figure 5). When this happens, the newly released viruses carry this piece of transfer in which a virus transfers dhm DNA from one bacterial DNA and may insert it into different bacteria when they infect other cells. bacterium Pass to Fourth Pass 10 10 06 This is a form of gene transfer called transduction. another Approved Not Approved 56 Chapter 2 The Prokaryotes, Viruses, and Protists NEL  The infectious cycles of animal viruses follow a pattern similar to that of bacteriophages, except that the virus’s capsid enters the cell along with the viral DNA. Some viruses do not Table 2 Viral Transmission cause lysis. Some animal viruses enter a dormant phase, similar to the lysogenic cycle for bacteriophages, in which the viral DNA is incorporated into the cell’s chromosomes. Method of Sometimes the whole virus stays in the cell’s cytoplasm in a dormant state. For Disease transmission example, the herpes viruses that infect humans remain dormant in the cytoplasm of rabies bite by infected some body cells for the person’s entire life. At times, particularly during periods of mammal stress, the virus becomes active in some cells. The viruses are replicated and destroy the cells as they are released. When this occurs in large numbers of cells, noticeable HIV/AIDS exchange of ulcers, or cold sores, form. The viruses then infect other cells and may once again go body fluids dormant. In this way, the person stays permanently infected with the virus. influenza, airborne and by Viruses are spread, or transmitted, in many ways. Some spread through the air or common cold, contact by direct physical contact with an infected individual. Others are spread by biting chicken pox insects or enter the body through injuries. Table 2 lists some viruses and the ways measles, mumps direct contact they are transmitted. vaccinations and Human Health Vaccines are mixtures that contain weakened forms or parts of a dangerous virus. When these altered viruses are injected into an individual’s body, they trigger a response by the immune system without causing an infection. This exposure creates a form of chemical “memory” that allows the immune system to react quickly if the individual ever comes in contact with the real virus. Vaccination programs have dramatically reduced human suffering and saved countless lives. In countries with modern healthcare systems, many serious diseases have been nearly eliminated. Smallpox was once a dreaded disease, but it has been completely eradicated (Figure 6). In 2006, a vaccine was created for several strains of the human papillomavirus (HPV). HPV is spread through sexual contact and is responsible for more than 70 % of all cancers of the cervix, a part of the female reproductive system. The vaccine is considered more than 99 % effective at preventing the spread of the virus. Unfortunately, it is not always possible to develop effective vaccines. For some diseases, such as AIDS, the structure of the virus and characteristics of the infection are obstacles to vaccine development. For other diseases, such as influenza, the virus Figure 6 The last recorded case of is constantly changing, so a vaccine that works against a form of the disease in one smallpox was in 1977. year is unlikely to be as effective the next year. Research This viral Diseases and the wHO SKILLS Skills: Researching, Analyzing, Evaluating, Communicating HANDBOOK A2.1, A5.1 The World Health Organization (WHO) monitors and controls outbreaks of infectious diseases, such as SARS, malaria, tuberculosis, swine flu, and AIDS. Each year, they predict which viral diseases are most likely to become a serious concern. They then recommend the mass production of a vaccine for those strains. In this investigation, you will examine the role of the WHO and research a viral disease of your choice. 1. Visit the website of the World Health Organization. 2. Choose a viral disease from the WHO website and research its cause, symptoms, prevention, and treatment (if any). 3. List and describe any disease outbreaks currently being reported. A. Summarize your findings, including the WHO recommendations about your selected disease. T/I C B. List and summarize the current status of any other viral disease outbreaks reported by the WHO. T/I C go t o N ELsoN sc iEN c E NEL 2.2 Viruses, Viroids, and Prions 57  Putting viruses to work gene therapy a method of treating Although all viruses cause disease, they can be beneficial. As mentioned before, disease in which genes are introduced viruses play an important role in ecosystems. Of particular interest to humans is the into cells to replace, supplement, or repair role viruses play in lowering the numbers of harmful bacteria. a defective gene Recently scientists have been exploring the use of viruses in genetic engineering and in gene therapy—the treatment of diseases using genes. As you have learned, viral new viral viruses can enter specific cells, and some can insert their own DNA into the chromo- DNA gene DNA somes of the cells they infect. Scientists can therefore use viruses to deliver drugs or target cell genes to targeted cells (Figure 7). They place drugs inside virus capsules or replace the viral DNA with DNA they want to insert into a host cell. This technology is still modifed DNA relatively new, but it is being used effectively in some applications and holds great injected into promise in others. Table 3 lists some possible uses of viruses in biotechnology. vector Table 3 Applications of Technologies That Use Viruses virus enters cell Technology Application or possible application using a virus capsule to This method may be used to deliver drugs to targeted cells in deliver a drug the body, for example, to deliver toxic chemotherapy drugs to cancerous tumour cells. using a virus to insert a new This method may be used to insert corrective genes into virus delivers gene copy of a gene individuals who suffer from a genetic disorder. to cell nucleus using a virus to insert This method can be used to create genetically modified organisms. a gene taken from one It is widely used in the genetic engineering of plants. Figure 7 Viruses can be used to deliver species into another species genes and drugs to targeted cells. The use of viruses in medicine has technological problems, serious risks, and C02-F10-OB11USB ethical concerns. Early attempts to treat people with virus therapies have had only limited success and have directly caused at least one death. The most recent trials, however, have been more promising. viroids and Prions viroid a very small infectious piece Viroids are small, infectious pieces of RNA that were first discovered in 1971. Viroids of RNA responsible for some serious are smaller than any virus and do not have a capsid. They also differ from viruses in diseases in plants that their RNA does not code for any proteins. Viroids are plant pathogens that can quickly destroy entire fields of citrus, potatoes, tomatoes, coconut palms, and other crop plants. In one case, a viroid outbreak killed more than 10 million coconut palms in the Philippines, devastating this important agricultural crop. Scientists do not know how viroids cause disease. Recent research indicates that the viroid may inter- fere with the normal formation and functioning of RNA within the host cell. prion an abnormally shaped infectious Prions, or proteinaceous infectious particles, cause a number of rare diseases in protein responsible for some brain mammals. Prions are abnormally shaped proteins found in the brain and nervous tis- diseases of mammals, including humans sues of infected animals. When those tissues are eaten by another animal, the prions enter that animal’s bloodstream and go to its brain. In the infected animal’s brain, the prions interact with normally shaped proteins, causing those proteins to change shape and become abnormal and infectious. The brains of affected animals are full of spongy holes. Prion diseases made headlines around the world in the late 1980s when farmers in the United Kingdom reported a new disease spreading among their cattle. The disease, called bovine spongiform encephalopathy (BSE), or “mad cow disease,” is estimated to have infected over 900 000 cattle in the United Kingdom. Many of those infected cattle entered the human food chain before developing symptoms. Tragically, some people who ate the contaminated meat developed a new human disease, known as variant Creutzfeldt-Jakob disease (CJD). Between 1996, when variant CJD was first described, and 2007, there were 208 cases in 11 countries. The vast majority of these cases were in the United Kingdom. 58 Chapter 2 The Prokaryotes, Viruses, and Protists NEL  2.2 Summary Viruses are tiny, nonliving particles that infect cells and cause diseases. Viruses consist of genetic material in the form of either DNA or RNA surrounded by a capsid. After a virus or its genetic material enters a host cell, it takes control of the cell in order to reproduce itself. Phages are viruses that infect bacteria. They undergo lytic or lysogenic cycles. Some viruses stay dormant in their host cells for many years. Important human viral diseases include HIV/AIDS, influenza, measles, mumps, chicken pox, and hepatitis. Vaccinations have been extremely successful in reducing the incidence of many serious viral diseases. Viruses are being used as tools for inserting drugs or DNA into cells. Viroids are small, infectious pieces of RNA that cause diseases in plants. Prions are abnormal infectious proteins that cause disease in mammals. 2.2 Questions 1. Why are viruses considered to be non-living? K/U invasive species. Do online research to find an example of a 2. What characteristic do viruses share with all living things? K/U virus used to control rabbit populations in Australia. K/U T/I (a) When and why did rabbits become a problem in Australia? 3. Which viral diseases are quite common and associated with (b) Why and how were viruses used to control the rabbits? the winter season? K/U (c) How successful was the viral pest control? 4. Create labelled sketches of (d) What are some possible drawbacks of using viruses as (a) a virus surrounded by an envelope pest control? Have any examples of these drawbacks (b) a bacteriophage been observed? (c) the lytic cycle of a bacteriophage K/U C 11. Kuru is a human prion disease discovered among some 5. How is the behaviour of a bacteriophage different from that indigenous peoples of New Guinea. Research how this of a virus that infects an animal cell? K/U disease is transmitted and how long it takes to develop. 6. Explain the relationship between a virus’s dormant period in T/I a cell and the appearance of cold sores. K/U 12. Use the Internet and other sources to find out what routine 7. Give examples of viral diseases that are spread by vaccinations are currently recommended by the Ontario (a) the bite of an animal Ministry of Health. T/I (b) the exchange of bodily fluids 13. Conduct research to find out what vaccinations are (c) direct contact or through the air K/U A recommended for travel to a tropical country of your choice. 8. Smallpox viruses can replicate only inside a human cell. Human T/I influenza viruses can replicate in human cells and in the cells 14. Dogs and cats are susceptible to a number of serious viral of pigs and some other animals. How might this difference diseases. Check with your local veterinary clinic or go influence the success of vaccination programs? K/U A online to see what vaccinations are recommended for these 9. The human influenza virus H1N1—also referred to as the pets. Report your findings to the class in a format of your 2009 swine flu—was declared a pandemic by the World choice. T/I C Health Organization. Use the Internet and other sources to 15. Mumps is a potentially serious viral disease. It can cause answer the following questions: deafness, sterility, meningitis, and encephalitis. Do online (a) What criteria does the WHO use to designate a disease research to answer the following questions: outbreak as a “pandemic”? (a) Why is there renewed concern about this disease when (b) How many deaths are thought to have resulted from there is already a vaccination program in place? this pandemic? (b) What does “herd immunity” mean? How does it help (c) How many countries have reported cases of H1N1? control the spread of disease? (d) How did Canada respond to this outbreak? T/I (c) Why is getting the disease after puberty of particular 10. Viruses control populations of organisms by causing disease. concern to males? T/I Humans have also used viral diseases to control pests and go to nels on s c i en c e NEL 2.2 Viruses, Viroids, and Prions   59