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• Nonliving particle that acts as an “obligate intracellular parasite” • A piece of nucleic acid (either DNA or RNA) • Surrounded by a protein coat called a “capsid” • Cause many diseases to living organisms • Can change how cells function • Originally thought viruses were tiny primitive cells (bacteria ancestors) • Ideas of something smaller than bacteria causing disease in late 1800’s • Virology: study of viruses • 1935 Wendell Stanley: nature of viruses – Crystallized tobacco mosaic virus – Found it was made of chemicals, rather than tiny cells Viruses Cells 1. Contain either DNA or RNA 2. Structure: capsid, no cell membrane or wall 3. Does not grow (in size) 4. Requires a host cell for reproduction 5. Does NOT carry out metabolic activity 1. Always have both DNA and RNA 2. Structure: cell wall/membrane, cytoplasm, organelles...etc... 3. Grows 4. Able to reproduce by mitosis/meiosis 5. Carries out metabolic activity • Viruses are generally classified according to their: – – – – SIZE (nm) capsid SHAPE TYPE of nucleic acid (DNA, RNA) they carry. Presence of an envelope • Diameter: 20 nm (poliovirus) to 250 nm (smallpox) • Viral nucleic acid may be DNA or RNA – Shape of nucleic acid may be helical, closed loop, or long strand; ds or ss • Capsid: protein coat surrounding nucleic acid • Envelope: membrane-like structure outside capsid - Taken from host cell membrane during replication - Allows new viruses to attach/infect host cells during 1st stage of viral replication • Shape can be determined by capsid or nucleic acid • Icosahedron: geometric shape with 20 triangular faces – Herpes simplex, polio, chickenpox • Helix: coiled spring shape • Viral nucleic acid responsible for shape • Rabies, measles, tobacco mosaic virus, helical virus • DNA & RNA viruses differ in the way they use host cell’s machinery to produce new viruses • DNA virus – Directly produce RNA that makes viral proteins – Join with host’s DNA to direct synthesis of new viruses • RNA virus – Releases RNA into host’s cytoplasm – Uses host’s ribosomes to produce new viral proteins • Retrovirus: contains RNA & reverse transcriptase • Reverse transcriptase is an enzyme that uses RNA as template to make DNA (reverse) Examples: HIV, leukemia • • • • Smallest known particles able to replicate Disease-causing agents Short, single strand of RNA, no capsid Disrupt plant cell metabolism & damage entire crops – Potatoes, cucumbers, avocados, oranges • Abnormal forms of proteins that clump together inside a cell • Clumps block cell traffic; kills cell • Found on surface of mammalian cells & brains of host – Scrapie, Mad Cow Disease (BSE), Creutzfeld-Jacob syndrome • Can replicate only by invading host cell & using enzymes & organelles of host to make more viruses • “Obligate intracellular parasites”; depend on host cells for replication • Outside host, virus is lifeless particle, no control of movements • Spread by direct contact (body fluids), wind, water, food • Viruses that infect bacteria • Commonly studied: T phages – infect bacterium in human digestive tract, E. coli • Icosahedral capsid contains nucleic acid • Contractile tail; collar & sheath – Inject nucleic acid into host • Base plate; tail fibers attach to host • Virus invades host cell, produces new viruses, destroys host cell, releases new virus • “Virulent” virus - undergoing lytic cycle, causes disease 1) Attachment- phage attaches to cell surface 2) Entry- phage injects DNA into host cell 3) Replication- phage DNA "tells" host to make more phage DNA and protein coats 4) Assembly- new viruses are assembled (host cell becomes a "virus factory" 5) Release- cell lyses (breaks open), releasing viruses • Viruses can infect cell without causing immediate destruction • Stay in host cell for days, months, or years • Called “Temperate” Viruses • Examples: • • • • Herpes simplex -> Herpes, Cold Sores Varicella Zoster Virus -> Shingles Human Papilloma Virus -> Warts Human Immunideficiency Virus -> HIV / AIDS • After attachment and entry, Integration: virus integrates its instructions (genes) within the host cell’s DNA • Prophage: (provirus) viral DNA segment that integrates itself into specific site of host cell’s genome • Viral Multiplication: each time the host cell DNA replicates so does viral DNA, each bacterial offspring infected with prophage • Prophage does not initially harm host cell • When triggered, a prophage becomes virulent, enters lytic cycle, replicating and destroying cell • Viruses are usually contracted through direct contact. • • Antibiotics work against bacteria but NOT viruses. • Some mammalian cells are capable of making small amounts of a special protein called INTERFERON, which can block (“interfere” with) viral activity when cells are attacked. Other defenses: • Phagocytes: large white blood cells that can engulf and destroy "invaders" • Lymphocytes: white blood cells capable of making ANTIBODIES which surround and disable invaders (recognize foreign "antigens") • Viral diseases among most widespread illnesses in humans • Mild fevers, forms of cancer, fatal diseases • Transmitted – Human Contact – Water – Insect Bite (parasite vectors) • Common human viral diseases • Chickenpox, Measles, Mumps, Polio, Rabies, Hepatitis, Common Cold • Viral infections can affect brain, liver, heart, lungs, & skin • Rabies – Animal Bite (saliva) – Virus travels, wound to central nervous system – Fever, headache, throat spasms, paralysis, coma – Lethal if untreated • Chickenpox – Highly contagious (direct contact & air) – Multiplies in lungs & networks through blood to skin – Fever, skin rash – Mild, recovery followed by lifelong resistance • Shingles – Chickenpox not destroyed, persists in nerve cells as provirus & cause shingles later during adulthood – Higher fever, Immune system weakens, pneumonia can occur – Rash in particular area (one side of chest) – Shed chickenpox viruses • Control of viral diseases accomplished by vaccination and antiviral drugs • Antiviral drugs: interfere with viral nucleic acid synthesis (few compared to bacteria) • Vaccination is most successful • Vaccine: preparation of pathogens or other materials that stimulates body’s immune system to provide protection against that pathogen • Inactivated – Do not replicate in a host system • Attenuated – Genetically altered so incapable of causing disease under normal conditions – Vaccines from attenuated preferred – Protection is greater & lasts longer • Booster: additional dose of vaccine, extend person’s protection • • • • 1960’s Measles, Mumps, Rubella 1980’s Hepatitis B 1990’s Hepatitis A & Chicken pox Continuous work done for AIDS vaccine – Genetic diversity & mutability – Best tool for now is educating people about HIV virus • Once killed 40% of people infected • Other 60% scarred & blind • DNA virus transmitted by nasal droplets emitted during sneezing & coughing • Fever, Headache, backache, skin rash • Eradication program 1967-1980 • Last case 1977 in Somalia • Control of animals that spread viral disease (“vector control”) – Ex: yellow fever eradicated by mosquito control • Rabies vaccinations kept pets free of infection; wildlife (meat with oral rabies vaccine) • Antiviral agents (like acyclovir) interfere with synthesis of viral nucleic acids & synthesis of viral capsids during replication • Some viruses contain oncogenes, which disrupt the cell cycle of infected host cells • Human Papilloma Virus (HPV) can cause cervical cancer • Hepatitis B Virus can cause liver cancer • Epstein-Barr Virus linked to Burkitt’s lymphoma (malignant tumors) • Ebola • Causes hemorrhagic fever, death • 1st identified in Zaire, 1977 • Avian influenza (bird flu) H1N1 • Outbreak spread to humans, 2008