Bacteria and Viruses

Transcription

1 CHAPTER 13 LESSON 3 Bacteria and Viruses What are viruses? Key Concepts What are viruses? How do viruses affect human health? What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 5. Viruses are the smallest living organisms. 6. Viruses can replicate only inside an organism. Think-Pair-Share Work with a partner. As you read the text, discuss what you learn about bacteria for each topic. Then compare viruses with bacteria. 1. Name two shapes that viruses can have. Characteristics of Viruses Have you heard of chicken pox, mumps, measles, or polio? You might have had shots to protect you from these diseases. You might have also received a shot to protect you from influenza, which is called the flu. What do these diseases have in common? They are caused by different viruses. A virus is a strand of DNA or RNA surrounded by a layer of protein that can infect and replicate in a host cell. If you have ever had a cold, you have been infected by a virus. A virus does not have a cell wall, a nucleus, or any other organelles present in cells. Viruses are 20 to 100 times smaller than most bacteria. Viruses can have different shapes, as shown below. Cylinder Crystal Sphere Bacteriophage 230 Bacteria and Viruses Reading Essentials

2 Dead or Alive? Scientists do not consider viruses to be alive because they do not have all the characteristics of a living organism. Recall that living things are organized, respond to stimuli, use energy, grow, and reproduce. Viruses cannot do any of these things. A virus can make copies of itself in a process called replication. Viruses and Organisms Viruses must use organisms to carry out their processes. Viruses have no organelles, so they cannot take in nutrients or use energy. A virus also uses the cellular parts of another organism to copy itself. Viruses must be inside a cell to replicate. The living cell that a virus infects is called a host cell. When a virus enters a cell, it can be either active or latent. The inactive stage of a virus is called the latent stage. At this stage, the virus does not replicate into more viruses. Replication A virus cannot infect every kind of cell. A virus can only attach to a host cell with specific molecules on its cell wall or cell membrane. The diagram below shows what happens when a virus infects a host cell. After the virus attaches to the host cell, its DNA or RNA enters the host cell. Once inside, the virus does one of two things. It either becomes latent, or it starts to replicate. After a virus becomes active and replicates, the virus destroys the host cell. Copies of the virus are then released into the host organism, where they can infect other cells. When latent viruses become active again, they take control of the host cells and replicate. 4 1 A virus attaches to a host cell. It then inserts its genetic material. Virus DNA New viruses are released as the host cell bursts and is destroyed. Viruses find other cells to infect. Host cell Host cell DNA 3 2 Once the virus s genetic material is in the host cell, the virus can become latent or active. Virus DNA Host DNA Becomes active When active, the virus s genetic material takes over the host cell and directs it to make more viruses. C Becomes latent The virus s genetic material removes itself and becomes active. Key Concept Check 2. Explain Are viruses alive? Why or why not? Make a folded book from a sheet of paper to organize your notes on viral replication. 3. Describe What occurs when a virus becomes latent? A Viral Replication The virus s genetic material combines with the host s genetic material. B The host cell continues to function and reproduce normally, making copies of the virus s genetic material as well as its own. Reading Essentials Bacteria and Viruses 231

3 REVIEW VOCABULARY mutation (noun) a change in genetic material 4. Describe How does mutation enable viruses to continue causing disease? Mutations When viruses replicate, their DNA or RNA often mutates, or changes. These mutations are helpful to the viruses. The mutations make it possible for viruses to adjust to changes in their host cells. For example, the molecules on the outside of the host cells might change so that the viruses can no longer attach to the cell. Mutations in the viruses produce new ways for the viruses to attach to host cells. Viruses change quickly. Curing or preventing viral diseases can be difficult because the viruses can mutate again. Viral Diseases You have learned that viruses cause human diseases such as chicken pox, influenza, HIV (human immunodeficiency virus), and the common cold. Viruses also infect animals and plants. Most viruses attack and destroy specific cells. This cell destruction causes the symptoms of the disease. Some viruses cause symptoms soon after infection. As shown below, flu viruses begin to replicate immediately. Flu symptoms, such as a runny nose and scratchy throat, appear within 2 3 days. 5. Explain Why is HIV said to be a latent virus? 6. Locate Where do flu viruses replicate? Other viruses might not cause symptoms for years. These viruses are sometimes called latent viruses. Latent viruses continue replicating without damaging the host cell. HIV is one example of a latent virus that might not cause immediate symptoms. This virus infects white blood cells, which are part of the body s immune system. At first, the infected cells function normally, so the person does not seem sick. If the virus becomes active, it destroys cells in the immune system. The person will not be able to fight other infections easily. People infected with latent viruses might not know for many years that they have been infected. 1 Flu virus particles in the air enter the body through the nose or mouth when a person inhales. 3 2 Viruses travel to the lungs, where they begin infecting cells. The viruses enter lung cells and begin replicating. Flu virus Lung cell The new viruses enter the bloodstream and travel to other parts of the body. They continue to infect cells all over the body. The infected person experiences flu symptoms and begins to feel sick. 232 Bacteria and Viruses Reading Essentials

4 Treating and Preventing Viral Diseases Viruses are always changing, so it can be hard to treat viral diseases. Antibiotics work only against bacteria, not against viruses. Antiviral medicines can be used to treat some viral diseases or prevent infection. These medicines keep the virus from entering cells or stop the virus from replicating. Like bacteria, viruses can change quickly and become resistant to medicines. One of the best ways to prevent a viral infection is to limit contact with an infected human or animal. The most important way is to practice good hygiene, such as washing your hands. Immunity Has anyone you know ever had chicken pox? Did the person get it only once? Most people who become infected with chicken pox develop an immunity to the disease. This is an example of acquired immunity. Acquired Immunity When a virus infects a person s body, the body begins to make proteins called antibodies. An antibody is a protein that can attach to a pathogen and make it useless. The figure below shows how antibodies fight infection. They bind to viruses and other pathogens, keeping them from attaching to a host cell. Antibodies also target viruses and signal the body to destroy them. These antibodies can multiply quickly if the same pathogen enters the body again. Virus Antibodies Host cell 7. Specify If someone in your family gets a cold, how could you limit contact with him or her? 8. State What prevents a person from getting chicken pox more than once? (Circle the correct answer.) a. mutation b. replication c. acquired immunity 9. Explain How does the antibody prevent the virus from attaching to the host cell? Host cell Reading Essentials Bacteria and Viruses 233

5 10. Define What is the condition called when a pregnant woman passes along her antibodies to her unborn baby? Natural Immunity A mother can pass antibodies on to her unborn baby. This condition is called natural immunity. Like acquired immunity, natural immunity helps the body fight infection. Vaccines You have read that people can avoid getting viral diseases through natural immunity or acquired immunity. Another way to prevent viral diseases is through vaccination. A vaccine is a mixture containing material from one or more deactivated pathogens, such as viruses. A vaccine is given in a vaccination. Polio United States, Interpreting a Graph 11. Explain There were no new cases of polio in the United States during what period? (Circle the correct answer.) a to 2005 b to 2003 c to 1999 Cases *Unvaccinated U.S. residents Year * * Summarize Why don t you get the flu when you get a flu shot? You might get a vaccine for a viral disease. When you do, it causes your body to make antibodies. The antibodies are produced in a way that is similar to what happens when you are infected by the virus. However, because the pathogen in the virus is not active, you might have mild symptoms or none at all. After being vaccinated, you will not get sick if you are exposed to that particular pathogen. The graph above shows how successful a vaccine made from a deactivated poliovirus was against the viral disease polio. Vaccines can prevent diseases in animals as well as humans. Pets and some farm animals are vaccinated against rabies. This protects the animals from the disease. Humans are then protected from getting rabies from their animals. 234 Bacteria and Viruses Reading Essentials

6 Gene Transfer Normal genetic material + Cold virus Normal genetic material inserted into cold virus 13. Locate Circle the specific cell that receives the normal genetic information. Nucleus Nasal cell Normal genetic material Lung Research with Viruses Scientists study viruses. They look for new ways to treat and prevent viral diseases in humans, animals, and plants. They also study the link between viruses and cancer. Viruses can cause changes in a host s DNA or RNA that result in tumors or abnormal growth. Because viruses can change quickly, scientists must always be working on new ways to treat and prevent viral diseases. Did you know that viruses also can be helpful? They can be used to treat genetic disorders and cancer in a process called gene transfer. In this process, shown above, scientists use viruses to insert normal genetic information into a specific cell. Someday, gene transfer might be used to treat genetic disorders that are caused by one gene. Two of these disorders are cystic fibrosis and hemophilia. Key Concept Check 14. Identify Name two ways in which viruses can affect human health. Reading Essentials Bacteria and Viruses 235

7 Mini Glossary antibody: protein that attaches to a pathogen and makes it useless vaccine: a mixture containing material from one or more deactivated pathogens, such as viruses virus: a strand of DNA or RNA surrounded by a layer of protein that can infect and replicate in a host cell 1. Review the terms and their definitions in the Mini Glossary. Write two or three sentences that explain how viruses and vaccines interact. 2. Fill in the table below with Vaccine, Acquired immunity, and Natural immunity to identify ways to prevent viral infections. Prevention Process Definition the ability to fight an infection because of antibodies in our bodies from a previous infection 3. State three things you learned about bacteria and viruses. What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? the ability to fight an infection because of antibodies passed on by your mother before you were born the ability to fight an infection because of the injection of deactivated pathogens that cause the body to make antibodies ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 236 Bacteria and Viruses Reading Essentials