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Viruses and Infectious Disease Viral Structure and Reproduction What is a Virus? • Viruses are very tiny, nonliving, particles made of either DNA or RNA which is enclosed in a protein coat called a capsid. What makes them NONliving? • Viruses do not exhibit ALL of the characteristics of living things. Characteristic Viruses Living Cells Structure DNA or RNA core, protein capsid Cell membrane, cytoplasm; eukaryotes also contain nucleus and organelles Reproduction Only with the help of a host cell Independent cell division either asexually or sexually Genetic Code DNA or RNA DNA Growth and Development No Yes Obtain and use energy No Yes Change over time Yes Yes Naming Viruses • Most viruses are named after the disease they cause. • Some are named for the organ or tissue that they infect. Naming Viruses • Viruses that infect bacteria are called bacteriophages, or phage for short. Virus Structure • Viruses have two basic parts: – Inner core made of nucleic acid (DNA or RNA) – Outer protein coat called a “capsid”. Virus Structure • Some large viruses have an envelope surrounding their capsid. – Made of lipids, just like the plasma membrane of cells. – Makes viruses more infectious because they can more easily infect living cells. Virus Structure • Viral DNA or RNA contains only the instructions for making more copies of the virus. Virus Structure • The arrangement of proteins in the capsid determines the virus’ shape. • Tobacco Mosaic virus – long; helical • Polyhedral – resemble crystals • Shape helps determine what cell the virus infects and how. Viral Reproduction Viral Reproduction • The cell in which a virus replicates is called a host cell. •Before a virus can replicate, it must attach to the host cell and inject its nucleic acid (DNA or RNA) into the cell. Viral Reproduction • Once initial infection has taken place, a virus will enter one of two reproductive cycles. – Lytic - Lysogenic Lysogenic Infection • Viral DNA integrates itself into host DNA. As the host cell’s DNA replicates itself, it replicates the viral DNA as well. • Imbedded DNA is called a prophage. • Viral DNA remains inactive for a period of time, slowly incorporating itself into more and more cells as the host cell divides. • The host organism may not feel sick or show any signs of infection in a lysogenic infection. Lysogenic Infection Lytic Infection • The viral DNA enters the host cell and begins to break down the host’s DNA. • It then uses the host cells nucleotides to produce more viral DNA copies. • Soon, the cell is so full of new copies of the virus that the cell bursts and thousands of viruses leave the cell to infect other cells. Lytic Infection Retroviruses • Contain RNA as their genetic material. • Viral RNA enters the host cell and produces a DNA copy of itself which becomes a prophage. – Ex. HIV (AIDS) • Retroviruses may remain dormant (lysogenic) for a very Infectious Disease What are pathogens? • Bacteria and viruses are found everywhere in nature and most are completely harmless. • A FEW cause disease. • Disease-causing bacteria and viruses are called pathogens. Bacterial Disease • There are 2 ways that bacteria produce disease. – Some damage cells and tissues by breaking down the cells for food • Ex. Tuberculosis is inhaled where it destroys lung tissue. Mycobacterium tuberculosis Bacterial Disease – Some release toxins (poisons) that travel through the body, and interferes with the normal activity of the host. • Streptococcus – – Causes Strep Throat – Can release toxins into the bloodstream, causing Scarlet Fever. • Tetanus – – Toxins cause severe muscle spasms, paralysis and death. Preventing Bacterial Disease • Vaccine – a preparation of weakened or killed pathogens. – Prompts the body to develop immunity to the disease. – Immunity is the body’s ability to destroy new pathogens. Preventing Bacterial Disease • Antibiotics – Compounds that block the growth and reproduction of bacteria. • Ex. Penicillin and tetracyclins Controlling Bacterial Growth • Sterilization by heat – Most bacteria cannot survive high temperatures for very long. Controlling Bacterial Growth • Disinfectants – Chemical solutions that kill bacteria – Overuse of antibacterial compounds increases the likelihood that bacteria will evolve to become resistant to them which makes them more difficult to kill. Controlling Bacterial Growth • Food Storage and Processing – Refrigeration • Food stored at low temperatures lasts longer because it takes bacteria longer to multiply. – Canning • Preserves food almost indefinitely – Food is sterilized by heat and then immediately placed in sterile jars or cans. Viral Disease • Viruses produce disease by disrupting the body’s normal equilibrium. – Viruses attack and kill living tissue. • Viral Diseases cannot be treated with antibiotics. • The best treatment is prevention, usually through the use of vaccines. Common Infectious Diseases Bacterial Viral Lyme Disease Tetanus Diptheria Bacterial Meningitis Strep throat Tooth decay Common cold Influenza Smallpox Warts AIDS (HIV) Chickenpox Measles Polio Hepatitis A, B, and C West Nile