Viruses

1. Viruses Chapter 25

2. Structure • 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

3. Structure • 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

4. Structure

5. Structure • 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)

6. Structure • 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

7. Structure

8. Shape • Shape can be determined by capsid or nucleic acid • Icosahedron: geometric shape with 20 triangular faces • Herpes simplex, polio, chickenpox

9. Shape • Helix: coiled spring shape • Viral nucleic acid responsible for shape • Rabies, measles, tobacco mosaic virus, helical virus

10. Types • 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

11. Types • Retrovirus: contains RNA & reverse transcriptase • Reverse transcriptase is an enzyme that uses RNA as template to make DNA (reverse) Examples: HIV, leukemia

12. Viroids • 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

13. Prions • 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

14. Viral Replication • 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

15. Bacteriophage • 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

16. Bacteriophage

17. Lytic Cycle • Virus invades host cell, produces new viruses, destroys host cell, releases new virus • “Virulent” virus - undergoing lytic cycle, causes disease

18. Lytic Cycle 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

19. Lysogenic Cycle • 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

20. Lysogenic Cycle • 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

21. Lysogenic Cycle • 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

22. Viruses • 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.

23. Viruses 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")

24. Human Disease • Viral diseases among most widespread illnesses in humans • Mild fevers, forms of cancer, fatal diseases • Transmitted • Human Contact • Water • Insect Bite (parasite vectors)

25. Infectious Diseases • 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

26. Infectious Diseases • Chickenpox • Highly contagious (direct contact & air) • Multiplies in lungs & networks through blood to skin • Fever, skin rash • Mild, recovery followed by lifelong resistance

27. Infectious Diseases • 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

28. Prevention & Treatment • 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

29. Virus Vaccines • 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

30. Virus Vaccines • Booster: additional dose of vaccine, extend person’s protection

31. Virus Vaccines • 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

32. Smallpox • 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

33. Antiviral Approaches • 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

34. Viruses and Cancer • 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)

35. Emerging Viruses • Ebola • Causes hemorrhagic fever, death • 1st identified in Zaire, 1977 • Avian influenza (bird flu) H1N1 • Outbreak spread to humans, 2008