CHAPTER 19 BACTERIA AND VIRUSES
19.2 Viruses I. What is a Virus? A. Viruses are particles of nucleic acid, proteins, and in some cases lipids. 1. Viruses can reproduce only by infecting living cells. 2. All viruses have one thing in common: they enter living cells and once inside, use the machinery of the infected cell to produce more viruses.
19.2 Viruses B. A typical virus is composed of a core of DNA or RNA surrounded by a protein coat. 1. The virus binds to a host and tricks the cell into allowing it inside.
19.2 Viruses 2. Once inside, the viral genes are expressed and the cell is used to make more viruses. • Viruses are highly specific to the cells they infect. C. capsid - a virus’s protein coat.
Viral Structure • Variety of shapes *Helical-spiral *Rod- little sticks *Spherical- round *Polyhedral- multisided Helical Rod
Viral Structure spherical polyhedral
RNA DNA Head RNA Capsid Capsid proteins Tail sheath Tail fiber Surface proteins Membrane envelope Figure 19-9 Virus Structures Influenza Virus Tobacco Mosaic Virus T4 Bacteriophage
19.2 Viruses II. Viral Infection (two types) A. Lyticinfection: a virus enters a cell, makes copies of itself, and causes the cell to burst or lyse.
19.2 Viruses B. Lysogenic Infection: a virus integrates its DNA into the DNA of the host cell, and the viral genetic information replicates along with the host cell’s DNA.
19.2 Viruses 1. Lysogenic viruses do not lyse (kill) the cell right away. 2. Instead they may remain inactive for a period of time.
III. Retroviruses - contains RNA as their genetic information.
19.2 Viruses A. Get their name because their genetic information is copied backwards. RNA DNA B. Responsible for some types of cancers in humans and also AIDS.
19.2 Viruses IV. Viruses and Living Cells A. Viruses are nonlivingthings. 1. Viruses do not grow, metabolize, or carry out homeostasis.
19.2 Viruses 2. Yet viruses, after infecting living cells, can reproduce, regulate gene expression, and evolve. B. Because viruses are completely dependent upon living things, it is more likely that viruses evolved after living cells.
19.3 Diseases Caused by Viruses I. Bacteria and viruses are everywhere in nature but only a few cause diseases. A. Pathogen =disease-causing agent. B. All viruses reproduce by infecting cells and diseases result when infection causes harm to the host.
19.3 Diseases Caused by Viruses . IV. Viral Disease in Humans A. Viruses produce disease by disrupting the body’s normal equilibrium.
Common Diseases Caused by Viruses Type of Virus Nucleic Acid Disease Cancer Cancer, AIDS Respiratory infections Chickenpox Smallpox Oncogenic viruses Retrovirus Adenoviruses Herpes viruses Poxviruses DNA RNA DNA DNA DNA
19.3 Diseases Caused by Viruses B. Viruses attack and destroy certain cells in the body causing symptoms of the disease. C. Viral diseases cannot be treated with antibiotics. D. The best way to protect against most viral diseases is by prevention, often by the use of vaccines before infection.