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VIRUSES Chapter 9.8 and 9.9 Biology 391 • What are they – – • • Details & Examples Life Cycles – – – – • Structure & Classification Discovery Lytic Lysogenic Evolution Prions and Viroids Impacts & Applications Viruses – What are they? • Could they be the beginnings of life? – (Sec 17.5) – Are viruses even alive? • • Life = “self-sustained chemical system that is capable of undergoing Darwinian, or biological evolution” - NASA Biological Evolution includes 1. Self-reproduction 2. Mutation that can be inherited 3. Natural selection Defined & Classified • Non-living – what characteristics are they missing? • Genetic material surrounded by a protein coat • Require a host cell to reproduce • Categorized by genetic material – Single stranded or double stranded DNA – Single stranded or double stranded RNA • Categorized by shape of protein coat (CAPSID) – Enveloped, Helix, Isohedron (polyhedron) More classification • Retroviruses – Have RNA genomes & contain special enzymes that transcribe RNA to DNA within host – Reverse Transcriptase – Ex: HIV • Bacteriophages – Viruses that attack bacteria – Have DNA Discovery… Tobacco Mosaic Virus • 1883 – 1935 – The idea of a virus the crystallization of TMV • Experiment: – Something wrong with the plants – tested for bacteria by straining homogenate – Filtrate still infectious – http://www.cellsalive.com/howbig.htm Viruses are Host Specific • In multicellular organisms viruses specialize in attacking particular cell types • Cold viruses attack membranes of respiratory tract • Measles viruses infect the skin • Rabies viruses attack nerve cells • Some viruses linked to cancer (e.g. T-cell leukemia, liver cancer, cervical cancer) • Herpes virus attacks mucous membranes of mouth and lips (causing cold sores) – Other herpes virus type causes genital sores • HIV virus attacks specific white blood cell type, causing AIDS (a) Envelope (lipid bilayer) Spikes Core Proteins HIV (b) Herpes Viruses Viral RNA in protein coat Glycoproteins Chapter 19 Protein Coat 9 Reverse Transcriptase Swine Flu? http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/Influenza.html Viral Replication (2 methods) • LYTIC – Virulent, immediate, quickly kills • LYSOGENIC – Long-term and may lie dormant – Similar to lytic (eventually) but integrates into genome • Viruses are parasites. - Any good parasite does not kill it’s host. Why? - http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter17/animation_quiz_2.html The Lytic Infection 1. ATTACHMENT – virus connects with cell surface 2. INSERTION – virus injects its genome (DNA or RNA) into host cell 3. REPLICATION – the viral genome is replicated – The DNA of the host cell is inactivated, and the viral DNA takes over making viral proteins and viral nucleic acid. – The viruses are assembled (nucleic acid inside protein coat) 4. BURST - The cell then bursts open (lyses) and the newly formed virus particles are released, free to infect other cells. The Lysogenic Infection 1. ATTACHMENT – virus connects with cell surface 2. INSERTION – virus injects its genome (DNA or RNA) into host cell 3. INTEGRATION – the viral DNA inserts itself into host cell’s DNA 4. REPLICATION – the viral genome is replicated – The DNA of the host cell is still replicated and the life of the cell may continue as normal, replicating the virus too. – The virus can stay dormant for years 5. BURST – at some point, the virus becomes virulent. - The cell then bursts open (lyses) and the newly formed virus particles are released, free to infect other cells. Evolution: Reproduction & Survival • Viruses evolve very quickly – Faulty genome reproduction enzymes = lots of mistakes – Lots of copies made per cycle compounds mistakes – Multiple strains come about very quickly • Survival Mechanisms – Are not killed by antibiotics (designed for prokaryote membranes & metabolism) – Upon lysing, pick up surface membranes • Evade host immune destruction in future Viroids & Prions • VIROIDS – Short RNA segments capable of self replication inside a host – Cause many plant diseases (degraded by most animals) • PRIONS – Small protein particles that cause disease – Chronic Wasting disease, CJD (Creutzfeldt-Jakob Disease), Scrapie in sheep, Bovine spongiform encephalopathy (BSE or “Mad Cow Disease”) • These diseases create holes in brain tissue 9.9 Impact of Viruses • Viruses are a serious threat to cellular life – Antibiotics are useless against viruses – Modern technologies allow viruses to spread at a greater rate – Vaccines are best defense – Anti-viral drugs interfere with viral protein production and/or specificity proteins – Some diseases caused by viruses: chicken pox, small pox, common cold, flu, ebola, mumps, measles, rabies and AIDS • Usefulness of Viruses – Delivering DNA in Cloning Experiments – Delivering gene segments in Gene therapy – Attacking Cancer cells Gene Therapy (p410) • Treating the genetic defect itself rather than treating the symptoms of a disease • Genetically engineer viruses using recombinant DNA technology (insert desired genes into viral genome) • GERM-LINE THERAPY – Change genetic sequence of gametes • SOMATIC CELL THERAPY – Target the cells that express the gene – Ex: insulin gene in pancreas; growth hormone in pituitary • Trouble: directing to correct cell Controlling area of integration Other Applications • Because viruses are host specific & specialized to attack specific host cell, we can use them too • Bacteriophages can treat bacterial diseases – Rise in bacterial antibiotic resistance makes standard drugs less effective – Bacteriophages specifically target host bacteria – Bacteriophages are harmless to human body cells Chapter 19 22 Recap • How are viruses defined? • Why do viruses evolve so quickly? • Why might viruses be useful in fighting cancers? • Are viruses alive? Why or why not? • What is the difference between viruses, viroids and prions? • What is the difference between lytic and lysogenic infections? Extra Slides How Viruses Replicate: HIV 1a. Virus attaches to receptor 1b. Core disintegrates; viral RNA enters the cytoplasm cytoplasm nucleus vRNA 3b. DNA transcribed into mRNA & viral RNA, which move to cytoplasm DNA mRNA 2. Viral reverse transcriptase makes DNA using viral RNA Chapter 19 25 3a. DNA enters nucleus & chromosomes How Viruses Replicate: HIV 6. Viruses bud from plasma membrane 5. Viral proteins & RNA assembled Chapter 19 4. Viral proteins made 26 using mRNA How Viruses Replicate: Herpes 2a. Viral envelope merges with nuclear membrane 2b. Protein coat disintegrates; viral DNA copied & enters nucleus nucleus envelope coat DNA DNA mRNA (cytoplasm) 1. Virus enters cell by endocytosis Chapter 19 3. Viral DNA transcribed to mRNA, which 27 moves to cytoplasm How Viruses Replicate: Herpes envelope coat 5. New viruses assembled & bud from nucleus, get envelope from inner nuclear membrane nucleus DNA DNA mRNA (cytoplasm) 4. MRNA makes proteins, which enter nucleus Chapter 19 28 6. Newly formed viruses leave the cell by exocytosis Influenza A structure • ~100 nm in diameter • sheathed in a lipid bilayer (derived from the plasma membrane of its host) • Studded in the lipid bilayer are two integral membrane proteins – ~ 500 molecules of hemagglutinin ("H") and – ~100 molecules of neuraminidase ("N") • Within the lipid bilayer are – ~3000 molecules of matrix protein – 8 pieces of RNA • Each of the 8 RNA molecules is associated with many copies of a nucleoprotein • several molecules of the three subunits of its RNA polymerase • some "non-structural" protein molecules of uncertain function