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Viruses I. What are viruses? • A. non-living segments of nucleic acids (DNA or RNA) contained in a protein coat. • They reproduce by invading other living cells. •Comes from the Latin word for “poison.” •obligate intracellular parasites. Viruses: • Do not grow. • Have no homeostasis. • Do not metabolize. • They do cause disease in many organisms… a “pathogen.” B. Structure: • 1. Inner core- made of the nucleic acid (DNA/RNA). • 2. Capsid- a protein coat that surrounds the inner core. • The arrangement of these proteins determines the shape of the virus. • They do not have a nucleus, cytoplasm, cell membrane or organelles. C. Types of Viruses: • 1. Virulent- one that will cause disease in a living cell (host cell). • 2. Temperate- one that does not immediately cause disease. II. Virology • The study of viruses. • Scientists use bacteriophages to study viruses because they reproduce so quickly. Bacteriophages •Are viruses that infect bacteria as a host cell. III. Reproduction •A. Lytic cycle –reproductive process for virulent viruses. B. Steps in the Lytic Cycle: •1. Attachment –bacteriophage attaches its tail to a bacterial host cell. 2. Entry •Bacteriophage punches a hole in the host cell and injects its nucleic acid into the host cell. 3. Replication • The host cell’s DNA is destroyed. • The bacteriophage’s nucleic acid takes over and controls the production of proteins in the host cell. 4. Assembly • New viruses are made. • The host cell fills with about 200 new viruses in about 90 minutes. 5. Lysis and Release •The host cell will burst open (lysis) releasing the newly made viruses free to attach and invade other cells. C. Lysogenic Cycle •Reproductive process for temperate viruses. •Examples: –HIV and –Herpes Simplex I (cold-sore) • 1. After viral DNA is injected into the host cell, it combines with the host cell’s chromosomes and becomes a provirus. • It can be undetected for many years. • 2. The host cell divides by mitosis and the provirus is now in the daughter cells (in a dormant state). •3. Proviruses can “pop out” of a cell at any time, enter a lytic cycle and cause disease. IV. AIDS • Acquired • Immunodeficiency • Syndrome • caused by HIV. Statistics • The World Health Organization estimated that in 1998 5.8 million people were newly infected with HIV. • Currently, it is estimated that 30 million people are infected worldwide. • Africa has the highest rate of infection. • Global Daily Death Toll – 8,000 • HIV is spread when body fluids of an infected person is passed on to an uninfected person by direct contact or contaminated objects. • 1. Sexual contact. (semen and vaginal fluid) • 2. Blood to blood contact. • 3. Intravenous needles. • 4. Breast milk. • The average incubation period may last 11 years before any symptoms begin to appear. • In some cases it has been documented to last as long as 20 years. V. Important Viral Diseases: • Chicken pox • influenza • rabies • smallpox • Hepatitis A and B • common cold • mumps and rubella