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Back Print Name Class Date Skills Worksheet Active Reading Section: Viruses Read the passage below. Then answer the questions that follow. Viruses are pathogens—agents that cause disease. Viruses lack the enzymes necessary for metabolism and have no structures to make proteins. Therefore, viruses must rely on living cells for replication. Before a virus can replicate, it must first infect a living cell. In bacterial viruses, the cycle of viral infection, replication, and cell destruction is called the lytic cycle. After the viral genes have entered the cell, they use the host cell to replicate viral genes and to make viral proteins, such as capsids. The proteins are then assembled with the replicated viral genes to form complete viruses. The host cell is broken open and releases newly made viruses. During an infection, some viruses stay inside the cells but do not make new viruses. Instead of producing virus particles, the viral gene is inserted into the host chromosome and is called a provirus. Whenever the cell divides, the provirus also divides, resulting in two infected host cells. In this cycle, called the lysogenic cycle, the viral genome replicates without destroying the host cell. SKILL: READING EFFECTIVELY Read each question, and write your answer in the space provided. 1. Why must viruses rely on living cells for replication? 2. What relationship exists between viruses and pathogens? 3. What sentence expresses the main idea of the second paragraph? Copyright © by Holt, Rinehart and Winston. All rights reserved. Holt Biology 5 Viruses and Bacteria Back Print Name Class Date Active Reading continued SKILL: INTERPRETING GRAPHICS The figure below shows the lytic and lysogenic cycles. In the spaces provided, describe what is occurring in each numbered part of the figure. Lytic Cycle Lysogenic Cycle Part 1 Part 5 Part 2 Part 4 Part 3 4. Part 1: ______________________________________________________________ 5. Part 2: ______________________________________________________________ 6. Part 3: ______________________________________________________________ 7. Part 4: ______________________________________________________________ 8. Part 5: ______________________________________________________________ In the space provided, write the letter of the phrase that best completes the statement. ______ 9. Viruses cause damage when they a. invade cells. b. replicate inside cells. c. remain inside a host cell. d. Both (a) and (b) Copyright © by Holt, Rinehart and Winston. All rights reserved. Holt Biology 6 Viruses and Bacteria Back Print TEACHER RESOURCE PAGE Answer Key Directed Reading 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. SECTION: VIRUSES 1. Viruses do not grow, do not maintain homeostasis, and do not metabolize. 2. They found that infected tobacco plant 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. sap from which bacteria had been filtered still caused the disease. Purified tobacco mosaic virus could be crystallized, which is a property of chemicals. b d a c In the lytic cycle, the viral genes enter the cell and use the host cell to replicate viral genes and to make viral proteins, such as capsids. Complete viruses are formed, and the cell is broken open and destroyed, releasing the viruses. In the lysogenic cycle, the viral genome replicates without destroying the host cell. A provirus is the viral gene that has been inserted into the host chromosome during the lysogenic cycle. A pathogen is any agent that causes disease. HIV is the name of the virus that causes AIDS. Individuals who have AIDS are unable to defend themselves against infections that do not normally occur in healthy individuals. Certain viruses can cause some types of cancer. Viruses associated with human cancers include hepatitis B (liver cancer), Epstein-Barr virus (Burkitt’s lymphoma), and human papilloma virus (cervical cancer). Both prions and viroids are infectious agents. Prions consist of only proteins, with no nucleic acids. Viroids consist of only nucleic acid, with no proteins. 17. 18. 19. 20. 21. 22. 23. Active Reading SECTION: VIRUSES 1. Viruses do not have enzymes necessary 2. 3. 4. 5. 6. 7. SECTION: BACTERIA 1. 2. 4. 5. B B Gram-negative bacteria and eukaryotes pili and flagella antibiotics photosynthetic photosynthetic chemoautotrophic heterotrophic Bacteria compete for the proteins, fats, carbohydrates, minerals, and vitamins in your body. Borrelia burgdorferi causes Lyme disease. Lyme disease is transmitted by the bite of an infected tick. Dense collections of Streptococcus mutans bacteria in the mouth cause tooth decay. Bacteria harm hosts by metabolizing their hosts or by releasing toxins that are poisonous to eukaryotic cells. penicillin viruses Clostridium oil 8. 9. B B E B to support metabolism and have no structures to make proteins. Viruses are types of pathogens, or agents that cause disease. The cycle of viral infection, replication, and cell destruction is called the lytic cycle. The virus attaches to a cell and injects DNA. Viral DNA enters the lytic cycle or lysogenic cycle. New viruses are made. The cell breaks open and releases viruses. The provirus may enter the lytic cycle. b Copyright © by Holt, Rinehart and Winston. All rights reserved. Holt Biology 60 Viruses and Bacteria