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18 CHAPTER Viruses and Prokaryotes GETTING READY TO LEARN Preview Key Concepts 18.1 Studying Viruses and Prokaryotes Infections can be caused in several ways. 18.2 Viral Structure and Reproduction Viruses exist in a variety of shapes and sizes. 18.3 Viral Diseases Some viral diseases can be prevented with vaccines. 18.4 Bacteria and Archaea Bacteria and archaea are both single-celled prokaryotes. 18.5 Beneficial Roles of Prokaryotes Prokaryotes perform important functions for organisms and ecosystems. 18.6 Bacterial Diseases and Antibiotics Understanding bacteria is necessary to prevent and treat disease. Review Academic Vocabulary Write the correct word for each definition. archaea homeostasis lipid 1. : prokaryotes belonging to one of the three domains of life 2. : nonnpolar molecule made of carbon, hydrogen, and oxygen 3. : condition of constant internal conditions Preview Biology Vocabulary To see how many key terms you already know from this chapter, choose the word that makes sense in each sentence. antibiotic toxin vaccine 1. A doctor may prescribe a(n) bacterial infection. if you have a 2. You may have received a(n) shot to prevent getting the flu. in the form of a 3. Any poison produced by an organism, including the oil produced by poison ivy, is a(n) . 296 Student text pages 544–545 SECTION Viruses 18.1 Studying and Prokaryotes KEY CONCEPT Infections can be caused in several ways. Viruses, bacteria, viroids, and prions can all cause infection. Any living organism or particle* that can cause an infectious disease is called a pathogen. An infectious disease is an illness that can be passed from one organism to another. Bacteria, viruses, viroids, and prions can all be pathogens. Although many types of bacteria are helpful—and even necessary—for your health, some types of bacteria can make you sick. RELATIVE SIZES OF CELLS AND INFECTIOUS PARTICLES Bacteria, viruses, viroids, and prions can all cause infection. Eukaryotic cells can be microscopic, but they are larger than prokaryotic cells, like bacteria. Prokaryotes are larger than viruses, viroids, and prions. 1 nanometer (nm) = one billionth of a meter 100 nm eukaryotic cells 10,000–100,000 nm viruses 50–200 nm prokaryotic cells 200–10,000 nm viroids 5–150 nm prion 2–10 nm Bacteria These single-celled organisms are prokaryotes. Bacteria are living things—they contain DNA, use nutrients and energy, grow and reproduce, and respond to their environment. Viruses A virus is an infectious particle made only of a piece of DNA or RNA surrounded by a protein coat. Viruses have some characteristics of living things, but not all. For example, they cannot reproduce on their own. They need living cells to help them reproduce and to make proteins. Viroids A viroid is a piece of single-stranded RNA without a protein coat. Viroids cause diseases in plants, and are passed through seeds or pollen. Like a virus, a viroid cannot reproduce without the help of living cells. * ACADEMIC VOCABULARY particle a little piece of something Interactive Reader 297 Prions A prion (PREE-ahn) is an infectious particle made only of proteins. Recall from Chapter 2 that proteins have a three-dimensional shape. Prions cause proteins to fold incorrectly, so they end up with the wrong shape and do not work properly. Prions are different from other pathogens because they have no genetic material, but can still cause disease. Prion diseases are always deadly, because the body has no immune response against a protein. The illness commonly called mad cow disease is caused by prions. Circle the names of four pathogens presented above. 18.1 Mark It Up Vocabulary Check virus pathogen Go back and highlight each sentence that has a vocabulary word in bold. viroid prion Match each clue below to the correct term from the list above. 1. made of protein only 2. made of DNA or RNA and a protein coat 3. causes infectious disease 4. made of RNA only 18.1 The Big Picture 5. Fill in the table below with the following terms: bacteria, viruses, viroids, and prions. PATHOGENS LIVING CELLS 298 McDougal Littell Biology NOT LIVING CELLS This drawing shows the three-dimensional structure of the blood protein hemoglobin. The proper function of a protein is based on its shape. Prions affect the shape of proteins. As a result, the proteins will not work properly. Student text pages 547–551 SECTION Structure 18.2 Viral and Reproduction KEY CONCEPT Viruses exist in a variety of shapes and sizes. Viruses differ in shape and in ways of entering host cells. Not all viruses are the same. They can be simple or complex in structure, and they have different ways of getting into host cells. Host cells are cells that are infected by a virus. Viruses can only reproduce after they have infected host cells. The Structure of Viruses Viruses are made up of two main parts: a piece of genetic material, and a protein covering, or coat, called a capsid. Unlike eukaryotes and prokaryotes, the genetic material of viruses may be either DNA or RNA. The shape of a virus plays an important role in how it works. The proteins on the surface of a viral capsid match a particular host—like a key fits a lock. Each type of virus can infect only certain hosts. Some viruses can infect several species. Others can infect only a single species. VIRAL SHAPES The different proteins that make up a viral capsid give viruses a variety of shapes. capsid genetic material (DNA or RNA) surface proteins Some viruses are shaped like a ball. surface proteins surface capsid proteins genetic material (DNA or RNA) capsid genetic material (DNA or RNA) Some viruses have a long twisted or coiled shape. Some viruses are many-sided. Interactive Reader 299 Viruses that Infect Bacteria Bacteriophages (bak-TEER-ee-uh-FAYJ-ihz) are a group of viruses that infect bacteria. Bacteriophages are often called “phages” for short. Phages attach to the outside of a bacterium, and inject their DNA into the cell. Viruses that Infect Eukaryotes Viruses that infect eukaryotes enter a host cell differently than phages enter host bacterial cells. For example, viruses that infect eukaryotes may get taken into a cell through endocytosis. Once inside the eukaryotic cell, the virus finds the nucleus, where the host cell’s DNA is located. capsid DNA This part of the phage attaches to a bacterium and pierces the cell wall. The viral DNA then gets injected into the bacterium. Bacteriophage What are the two main structural parts of a virus? Viruses cause two types of infections. Once inside a host cell, there are two basic pathways of infection that are similar for all viruses. A virus may end up destroying the host cell, or it may add its genetic material to the host cell without destroying it. These two pathways are shown for bacteriophages on the next page. Lytic Infection Visual Connection A lytic infection (LIHT-ihk) results in the host cell bursting open and releasing new viral offspring into the host’s system. First, the virus enters the host cell. Then, the viral DNA directs the host cell to copy the viral DNA and produce more capsids. The capsids and viral DNA come together to make new virus particles. The host cell is destroyed and the viral offspring are released. They can then infect other cells. See bacteriophages attack a bacterium: student text, pg. 549 Lysogenic Infection A lysogenic infection (LY-suh-JEHN-ihk) results in the viral DNA combining with the host cell’s DNA. The phage DNA inserted into the host cell’s DNA is called a prophage. When the host cell goes through mitosis, the prophage gets copied and passed on to daughter cells along with the host cell’s DNA. The prophage can remain a permanent part of the host cell’s DNA. Or, it can enter the lytic cycle and produce new viruses. 300 Endocytosis is a process in which a substance is surrounded by the cell membrane, and then released into the cell. McDougal Littell Biology LYTIC INFECTION A lytic infection results in the lysis, or breaking apart, of the host cell and release of new viral particles. A lysogenic infection does not destroy the host cell. host bacterium The bacteriophage attaches and injects its DNA into a host bacterium. The viral DNA forms a circle. The host bacterium breaks apart, or lyses. Bacteriophages are able to infect new host cells. The viral DNA directs the host cell to produce new viral parts. The parts assemble into new bacteriophages. The virus may enter the lysogenic cycle, in which the host cell is not destroyed. LYSOGENIC INFECTION The viral DNA is called a prophage when it combines with the host cell’s DNA. The prophage may leave the host’s DNA and enter the lytic cycle. Many cell divisions produce a colony of bacteria infected with prophage. Although the prophage is not active, it is copied along with the host cell’s DNA. An infected cell may stay in the lysogenic cycle for a long time. A trigger, such as stress, can push the virus into the lytic cycle. The virus will then use the cell to produce new viruses. In which cycle of infection are new viral particles released— lytic or lysogenic? Interactive Reader 301 18.2 Vocabulary Check capsid bacteriophage lytic infection lysogenic infection prophage Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. Circle the correct term from each pair to complete the sentences below. 1. Viral DNA combined with the host cell’s DNA is called a prophage / bacteriophage. 2. A virus is made of two main parts, genetic material and a bacteriophage / capsid. 3. In a lysogenic infection / lytic infection the viral DNA becomes part of the host cell’s DNA and the host cell is not destroyed. 4. In a lysogenic infection / lytic infection the host cell bursts, releasing new viral offspring into the host’s system. 18.2 The Big Picture 5. What do viruses need to reproduce? 6. What is a main difference between the lytic and lysogenic cycles of infection? 302 McDougal Littell Biology Student text pages 552–554 SECTION 18.3 Viral Diseases KEY CONCEPT Some viral diseases can be prevented with vaccines. Viruses cause many infectious diseases. Your body has defenses against viruses. Skin is your body’s first defense. A virus cannot get through your skin unless there is an opening such as a cut or a scrape. A virus can also enter your body through other openings, such as the mouth, nose, genital areas, eyes, and ears. Some viral infections are deadly and others are barely noticeable. Several human illnesses caused by viruses are listed below. The Common Cold More than 200 viruses cause the illness that we call the common cold. It is hard to find a cure for the common cold because it is caused by so many different viruses that mutate rapidly. Influenza The influenza virus is often called the flu for short. The flu spreads quickly. There are often flu epidemics, or rapid outbreaks that affect many people. Subtypes of the flu can infect birds, horses, pigs, whales, and seals. Currently, only three subtypes infect humans. The flu can be controlled with a vaccine (vak-SEEN), a substance that gets the body’s immune system to prepare a response against a virus. Flu viruses mutate rapidly, so a new vaccine must be made every year. SARS SARS (severe acute respiratory disease) has symptoms similar to the flu, including fever and coughing. SARS first appeared in Asia in 2002. By the following summer it had spread to other countries. Replication 9C6 9C6 Transcription HIV HIV (human immunodeficiency virus) is a type of virus called a retrovirus. Retro- means “backward.” A retrovirus copies its genetic material “backwards.” Usually, DNA is used to make a copy of RNA, but a retrovirus contains RNA that gets used to make DNA. The DNA then becomes part of the host cell’s DNA. GC6 GC6 Translation cjXaZjh XnideaVhb EgdiZ^c EgdiZ^c Usually, DNA is used to make RNA. But retroviruses can use RNA to make DNA. Interactive Reader 303 HIV can remain part of the host cell’s DNA for many years with no symptoms. When the virus becomes active, it makes more viral particles. The host cells burst, and the new viruses can infect other cells. This stage of the disease destroys the host’s white blood cells. White blood cells are a very important part of the human immune response. The loss of white blood cells leads to AIDS, or acquired immune deficiency syndrome. Once a person’s white blood cells are destroyed, she or he may be unable to fight off the common microorganisms that humans come into contact with every day. HIV mutates very quickly, which makes it a challenge to treat. However, there are medicines that can help to slow the spread of the virus once a person is infected. Name three infectious diseases that are caused by viruses. VIRAL DISEASE VIRAL INFECTION SYMPTOMS OF DISEASE TRANSMISSION OF DISEASE U.S. VACCINE RECOMMENDATION Chickenpox rash, itchy skin, fever, fatigue* contact with rash, droplet inhalation for children between 12 and 18 months Hepatitis A yellow skin, fatigue, abdominal pain contact with contaminated feces for people traveling to infected locations and protection during outbreaks Mumps painful swelling in salivary glands, fever droplet inhalation for children between 12 and 15 months and again at 4 to 6 years Rabies anxiety, paralysis, fear of water bite from infected animal for veterinarians and biologists in contact with wildlife West Nile fever, headache, body ache bite from infected mosquito no available vaccine Vaccines are made from weakened pathogens. In the United States, most children are vaccinated against diseases such as measles, mumps, and chicken pox. Every year, millions of people are vaccinated against influenza. * ACADEMIC VOCABULARY fatigue a feeling of tiredness 304 McDougal Littell Biology A vaccine contains the weakened virus or parts of the virus that it is supposed to protect against. A vaccine triggers the body’s immune system response. Because the vaccine contains a weakened virus, it does not cause illness. But if the virus enters the body again, the body will be prepared to start an immune response before the virus can cause damage. What main substance would an influenza vaccine contain? 18.3 Vocabulary Check epidemic vaccine retrovirus Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. Write a short definition for each vocabulary term. 1. epidemic: 2. vaccine: 3. retrovirus 18.3 The Big Picture 4. What is the body’s first defense against infection? 5. How does a vaccine protect a person against infection? 6. Why is a person with AIDS more easily infected by pathogens than a person without AIDS? Interactive Reader 305 SECTION 18.4 Bacteria and Archaea Student text pages @E;@8E8JK8E;8I;J 555–558 B.8.3 KEY CONCEPT Bacteria and archaea are both single-celled prokaryotes. Prokaryotes are widespread on Earth. Bacteria and archaea live just about everywhere on Earth—even inside rocks and in polar ice caps. A drop of your saliva may contain 40 million bacterial cells. A gram of soil may contain 5 billion bacterial cells from over 10,000 types of bacteria. Prokaryotes are an important part of the communities in which they live. Prokaryotes can be divided into groups based on their need for oxygen. • An obligate anaerobe (AHB-lih-giht AN-uh-ROHB) cannot survive in the presence of oxygen. • An obligate aerobe (AHB-lih-giht AIR-ohb) needs oxygen to survive. • A facultative aerobe (FAK-uhl-tay-tihv AIR-ohb) can survive whether or not oxygen is present. What is the difference between an obligate anaerobe and an obligate aerobe? Bacteria and archaea are structurally similar but have different molecular characteristics. All prokaryotes either belong to the domain Archaea or the domain Bacteria. The two groups of organisms have similar appearances but have many differences in their genetics and biochemistry. Structural Comparison • Cell shape Archaea come in many shapes. Bacteria have three common forms: rod-shaped, spiral-shaped, and spherical-shaped. • No organelles Prokaryotes have no nucleus or other membranebound organelles. • DNA Prokaryotic DNA is in the shape of a loop, or a circle, and is located in the cytoplasm. • Plasmids Prokaryotes can also have plasmids. A plasmid is a small piece of DNA that replicates separately from the main chromosome. • Movement Many bacteria and archaea use flagella to move. A flagellum (fluh-JEHL-uhm) is a long structure outside of the cell that is used to move. The flagella of bacteria and archaea look similar but they are structurally different. • Pili Many prokaryotes have structures called pili, which are shorter and thinner than flagella. Pili help prokaryotes to stick to surfaces. 306 McDougal Littell Biology VOCABULARY Something that is obligatory is something that must happen. Something that is facultative is something that may happen but does not have to happen. PROKARYOTE STRUCTURE pili flagellum chromosome plasmid cell wall plasma membrane Molecular Comparison Archaea and bacteria are very different on a molecular level. Molecular evidence suggests that archaea have at least as much in common with eukaryotes as they do with bacteria. The cell walls and cell membranes of archaea and bacteria have very different chemical components. For example, archaea have a type of lipid in their membranes that is not found in any other living organism. What one way archaea in whichand archaea anddiffer? bacteria differ? What is one wayisin which bacteria JK8E;8I;J :?<:B Bacteria have various strategies for survival. You might use a thoughtful plan, or a strategy, for playing a game. In science, a strategy also means an adaptation that improves a species’ chance for survival. The exchange of genetic material through conjugation and the formation of endospores are two bacterial strategies for survival. Bacteria reproduce asexually. However, through a process called conjugation (KAHN-juh-GAY-shuhn), two or more bacterial cells can exchange DNA. This process allows bacteria to adapt quickly to different environmental conditions. VISUAL VOCAB In conjugation, genetic material transfers between prokaryotes, producing genetic variation. DNA passes through a structure called a conjugation bridge which connects the cells. conjugation bridge Interactive Reader 307 When conditions are very bad for growth and survival, some bacteria can produce an endospore. An endospore is a specialized cell with a thick, protective wall that can survive drying out, temperature changes, and even disinfectants. Endospores can last for hundreds of years. Why is conjugation considered a survival strategy? 18.4 Vocabulary Check obligate anaerobe obligate aerobe facultative aerobe plasmid flagellum conjugation endospore Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 1. Which type of bacterium is likely to live at the bottom of a lake, where there is no oxygen? 2. Look back at the figure of prokaryote structure on page 307. - Draw an arrow that points to the flagellum. - Circle a plasmid. 3. Name one process and one structure that are bacterial strategies for survival. 18.4 The Big Picture 4. Where can bacteria and archaea live? 5. Bacteria and archaea look very similar. In what main way do they differ? 308 McDougal Littell Biology Student text pages @E;@8E8JK8E;8I;J 559–561 SECTION Roles 18.5 Beneficial of Prokaryotes B.4.4 KEY CONCEPT Prokaryotes perform important functions for organisms and ecosystems. Prokaryotes provide nutrients to humans and other animals. Bacteria are important parts of animal digestive systems. Both the bacteria and the host animal benefit from this relationship. The bacteria benefit from a stable home and source of food. The host animal benefits because the bacteria help break down foods and produce vitamins and other nutrients. Prokaryotes are also important to human nutrition because they are used to make many common foods. Cheese, sour cream, yogurt, butter, pickles, soy sauce, sauerkraut, vinegar, and many other foods all depend on fermentation by prokaryotes to produce their flavors. You may have heard about food poisoning caused by bacteria. Although some types of bacteria in food can be harmful, eating food produced by bacteria is not dangerous—as long as it’s the right kind of bacteria! Many types of foods are made using bacteria. Which of the foods listed above have you tried? Prokaryotes play important roles in ecosystems. You cannot see prokaryotes with your bare eyes, so it may be easy to forget about them. But they play very important roles in every ecosystem. For example, some prokaryotes produce oxygen through photosynthesis. Others help cycle carbon, nitrogen, hydrogen, and phosphorous through the environment. One example of prokaryotes’ roles in nutrient cycles is shown in this drawing. Bacteria in the soil help to cycle nitrogen through the environment. NITROGEN CYCLE Much of the nitrogen cycle occurs underground, where bacteria transform nitrogen into a form that can be used by plants. nitrogen in atmosphere animals plants denitrifying bacteria decomposers nitrogen-fixing bacteria nitrifying bacteria Interactive Reader 309 Prokaryotes can break down many natural substances. They can also break down many substances made by humans. Because of this ability, scientists use prokaryotes in a process called bioremediation. Bioremediation (BY-oh-rih-MEE-dee-AY-shuhn) is a process that uses living things to break down pollutants. For example, some types of bacteria can digest oil. These bacteria are used to help clean up oil spills and other industrial accidents. What is one of example of a pollutant thatcan bacteria can What is one example a pollutant that bacteria help to JK8E;8I;J :?<:B help to break down? break down? 18.5 Vocabulary Check bioremediation Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 1. Remediation means “fixing a problem.” The prefix bio- means “life.” Explain how these word parts contribute to the meaning of bioremediation. 18.5 The Big Picture 2. What are two ways that prokaryotes provide nutrients to humans? 3. What is one important role that prokaryotes play in ecosystems? 310 McDougal Littell Biology Student text pages 563–565 SECTION Diseases 18.6 Bacterial and Antibiotics KEY CONCEPT Understanding bacteria is necessary to prevent and treat disease. Some bacteria cause disease. Some bacteria can cause disease in plants and animals. Bacteria can cause illness in two basic ways: 1. Bacteria can attack the cells in one of the body’s tissues. For example, the disease tuberculosis, sometimes called TB, is caused by bacteria that invade the lungs and use the tissue for nutrients. 2. Bacteria can make poisons, called toxins, that can be carried in the blood to other parts of the body. A toxin is a poison released by an organism. For example, the most common source of food poisoning is from toxins released by a type of bacteria. COMMON BACTERIAL INFECTIONS INFECTION BACTERIUM SYMPTOMS CAUSES Acne Propionibacterium chronic cysts, blackheads increased oil production in skin Anthrax Bacillus anthracis fever, trouble breathing inhaling endospores Lyme disease Borrelia burgdorferi rash, aching, fever, swelling of joints bite from infected tick Tetanus Clostridium tetani severe muscle spasms, fever, lockjaw wound contaminated with soil Tooth decay Streptococcus mutans tooth cavities large populations of bacteria in mouth Look at the table above. Which of these common bacterial infections have you heard of? Antibiotics are used to fight bacterial disease. Antibiotics are chemicals that kill or slow the growth of bacteria. Many antibiotics work by stopping bacteria from making cell walls. Antibiotics do not affect animal cells because animal cells do not have cell walls. Similarly, antibiotics do not affect viruses, because viruses do not have cell walls. Interactive Reader 311 Antibiotics are very important medical treatments. However, they should only be used when necessary. Recall that bacteria are a very important part of your digestive system. These “good” bacteria are also affected by antibiotics. The overuse of antibiotics can kill the bacteria in your digestive system, resulting in illness. How do many antibiotics prevent the growth of bacteria? Bacteria can evolve resistance to antibiotics. Antibiotics can be life-saving medicines. However, the inappropriate and incomplete use of antibiotics ANTIBIOTIC RESISTANCE has resulted in strains of bacteria that are not A bacterium carries affected by antibiotics. In other words, these strains genes for antibiotic of bacteria are resistant to antibiotics. Antibioticresistance on a plasmid. resistant bacteria are a major public health issue. Resistance occurs as a result of natural selection. A copy of the plasmid is transferred through conjugation. Any bacteria that are resistant will survive and reproduce. Resistance can then be passed on to Resistance is quickly offspring and spread to other bacteria. Three spread through many bacteria and factors that have contributed to resistance are their offspring. described below. • Overuse Anytime antibiotics are used, there is a chance to select for resistant bacteria. The unnecessary use of antibiotics—such as when bacteria are not the cause of an illness— increases the possibility that resistant bacteria will become more common. • Underuse Antibiotics may make a person feel better after just one or two days. But the whole prescription of antibiotics must be taken. Otherwise, it is possible that only the weakest bacteria will be killed, and the ones with some resistance may survive. • Misuse A large portion of the antibiotics used in the United States are fed to livestock, such as cows, to increase their growth. Antibiotics may therefore be added to the food of healthy animals. The misuse of antibiotics in agriculture can lead to antibiotic resistance of bacteria in the animals’ food. 312 McDougal Littell Biology In summary, anytime antibiotics are used when they are not necessary, or when they are used improperly, it increases the chances for antibiotic-resistant bacteria to become more common. An illness caused by antibiotic-resistant bacteria is difficult or impossible to treat because the bacteria do not respond to antibiotics. Explain the role of natural selection in the production of antibiotic-resistant bacteria. 18.6 toxins Vocabulary Check antibiotics Fill in the blanks with the correct term from the list above. Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 1. Some bacterial infections can be treated with . 2. The produced by some bacteria cause diseases in plants and animals. 18.6 The Big Picture 3. Give one example of a disease, illness, or infection caused by bacteria and describe its symptoms. 4. Can colds be treated with an antibiotic? Explain your response. 5. Imagine that your friend has a ten-day prescription of antibiotics for an illness. Your friend feels better after only two days and wants to stop taking the medicine. Explain why your friend should complete the full prescription of antibiotics. Interactive Reader 313 Chapter 18 Review 1. What are four types of pathogens? 2. Is a virus a living organism? Explain your response. 3. List one way that bacteria are “helpful” to humans and one important role they play in ecosystems. B.4.4 4. What do retroviruses do that is different from other viruses? a. use DNA to make RNA b. use RNA to make DNA c. use DNA to make DNA d. use RNA to make RNA 5. What biological process is involved in the formation of strains of bacteria that are resistant to antibiotics? 6. All prokaryotes belong to one of two domains: or . B.8.3 7. How does a vaccine protect against illness? 8. Label the cell wall, a chromosome, a plasmid, and the flagellum on this diagram of a typical prokaryote. 314 McDougal Littell Biology B.8.5