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Table of Contents

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  1. Viruses Chapter 24 Table of Contents Section 1 Viral Structure and Replication Section 2 Viral Diseases

  2. Section 1 Viral Structure and Replication Chapter 24 Objectives • Summarize the discovery of viruses. • Describe why viruses are not considered living organisms. • Describe the basic structure of viruses. • Compare the lytic and lysogenic cycles of virus replication. • Summarize the origin of viruses.

  3. What is happening in the photograph? Newly assembled HIV viruses are leaving the T cell. How did the viruses, pictured leaving the cell, enter the cell? The genetic material from a single HIV particle entered the cell and was responsible for the production of all the viruses pictured.

  4. Section 1 Viral Structure and Replication Chapter 24 Discovery of Viruses • Researchers in the late 1800s discovered that something smaller than bacteria could cause disease. • In 1935, Wendell Stanley demonstrated that viruses were not cells when he crystallized TMV, the virus that causes tobacco mosaic disease in tobacco and tomato plants. • Virus

  5. Section 1 Viral Structure and Replication Chapter 24 Characteristics of Viruses • What are the characteristics of living organisms? • Cellular organization, reproduction, metabolism, homeostasis, heredity, responsiveness, and growth and development. • Viruses do not have all of the characteristics of life and are therefore not considered to be living.

  6. Section 1 Viral Structure and Replication Chapter 24 Characteristics of Viruses, continued • Viral Size and Structure • Viruses are nonliving particles containing DNA or RNA and are surrounded by a protein coat called a capsid. • Some viruses also have an envelope that is derived from a host cell’s nuclear membrane or cell membrane. • Parts of a Virus

  7. Section 1 Viral Structure and Replication Chapter 24 Characteristics of Viruses, continued • Classification of Viruses • Viruses can be classified based on whether they have RNA or DNA, whether the RNA or DNA is single or double stranded and circular or linear, by capsid shape, and whether or not they have an envelope.

  8. Review • What have we learned about the size and structure of viruses? • Very small • Various sizes and shapes (helical, polyhedral, enveloped) • How are viruses classified? • RNA or DNA • Single or double stranded • Linear or circular • Capsid characteristics • Presence or absence of envelope

  9. Section 1 Viral Structure and Replication Chapter 24 Viral Replication, continued • Replication in DNA Viruses • DNA viruses can enter host cells and directly produce RNA, or they can insert into a host’s chromosome, where they are transcribed to RNA along with the host’s DNA.

  10. Section 1 Viral Structure and Replication Chapter 24 Viral Replication, continued • Replication in RNA Viruses • The RNA genome of some RNA viruses can be directly translated to make viral proteins. • Retroviruses use reverse transcriptase and RNA as a template to make DNA, which is then used to produce viral RNA and proteins. • HIV, murine leukemia virus, feline immunodeficiency virus.

  11. Reverse Transcriptase • Why is the enzyme contained in retroviruses called reverse transcriptase? • Catalyzes the transcription of DNA from RNA

  12. Section 1 Viral Structure and Replication Chapter 24 Viral Replication • Replication in Viruses That Infect Prokaryotes • Bacteriophages are viruses that infect bacteria.

  13. Section 1 Viral Structure and Replication Chapter 24 Viral Replication, continued • Lytic Cycle • Viruses can follow a lytic cycle, making new viral particles immediately. • Occurs when virus is active • Lytic Cycle

  14. Section 1 Viral Structure and Replication Chapter 24 Viral Replication, continued • Lysogenic Cycle • Viruses can follow a lysogenic cycle, becoming part of the host genome and making new particles later. • Not active • Lysogenic Cycle

  15. Section 1 Viral Structure and Replication Chapter 24 The Lytic and Lysogenic Cycles

  16. Section 1 Viral Structure and Replication Chapter 24 Viral Replication, continued • Viruses: Tools for Biotechnology • Viruses are important tools for biotechnology.

  17. Review • How do viruses reproduce? • Directly produce RNA • Insert into host DNA • Lytic or lysogenic cycles

  18. Section 1 Viral Structure and Replication Chapter 24 The Origin of Viruses • Most scientists think viruses originated from fragments of host-cell nucleic-acid material.

  19. Section 2 Viral Diseases Chapter 24 Objectives • Name several vectors of viral diseases. • Identify four viral diseases that result in serious human illnesses. • Discuss the relationship between viruses and cancer. • Name three examples of emerging viral diseases. • Compare the effectiveness of vaccination, vector control, and drug therapy in fighting viruses. • Contrast viroids, prions, and viruses.

  20. Section 2 Viral Diseases Chapter 24 Vectors of Viral Diseases • Vectors, or hosts, of viral diseases include humans, animals, and insects.

  21. Section 2 Viral Diseases Chapter 24 Human Viral Diseases • Viruses cause many human diseases, including the common cold, flu, hepatitis, rabies, chickenpox, certain types of cancer, and AIDS.

  22. Section 2 Viral Diseases Chapter 24 Human Viral Diseases, continued • Chickenpox and Shingles • Chickenpox and shingles are caused by the same varicella-zoster herpesvirus.

  23. Section 2 Viral Diseases Chapter 24 Human Viral Diseases, continued • Viral Hepatitis • Hepatitis, or inflammation of the liver, can be caused by at least five viruses. • Hepatitis A and hepatitis E can be spread by fecally contaminated food and water. • Hepatitis B, C, and D are spread by sexual contact, by contact with infected blood and serum,and by the use of contaminated needles.

  24. Section 2 Viral Diseases Chapter 24 Human Viral Diseases, continued • Acquired Immune Deficiency Syndrome (AIDS) • The human immunodeficiency virus (HIV) is an RNA virus spread by sexual contact, by contact with infected body fluids, and from mother to fetus. • HIV targets macrophages and thus damages the body’s immune system. The disease called acquired immune deficiency syndrome (AIDS) results.

  25. Section 2 Viral Diseases Chapter 24 AIDS (Acquired Immunodeficiency Syndrome)

  26. Section 2 Viral Diseases Chapter 24 Human Viral Diseases, continued • Viruses and Cancer • Some viruses contain oncogenes that can cause cancer, while other viruses convert proto-oncogenes, which usually control cell growth, to oncogenes.

  27. Section 2 Viral Diseases Chapter 24 Emerging Viral Diseases • Emerging viruses usually infect animals isolated in nature but can jump to humans when contact occurs in the environment.

  28. Section 2 Viral Diseases Chapter 24 Prevention and Treatment • Vaccinations • A vaccine contains a harmless version of a virus, bacterium, or a toxin that causes an immune response when introduced to the body. • Vaccines have helped to greatly reduce certain viral diseases.

  29. Section 2 Viral Diseases Chapter 24 Prevention and Treatment, continued • Vector Control • Control efforts, including killing mosquitoes and other vectors and quarantining ill patients, have helped reduce the spread of certain viral diseases.

  30. Section 2 Viral Diseases Chapter 24 Prevention and Treatment, continued • Drug Therapy • Antibiotics are ineffective against viral diseases. • Viral drugs, such as acyclovir, block specific steps in viral replication.

  31. Section 2 Viral Diseases Chapter 24 Virods and Prions • Viroids are short, circular, single strands of RNA lacking a capsid that infect plant cells. • Prions are infectious particles containing protein but no nucleic acids. • Prions cause mad cow disease and similar degenerative brain diseases.

  32. Section 2 Viral Diseases Chapter 24 Important Viral Diseases