VIRUSES Chapter 9.8 and 9.9 Biology 391

What are they – Structure & Classification – Discovery Details & Examples Life Cycles – 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 –

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

Chapter 199 (a) (b) Glycoproteins Glycoproteins Envelope (lipid bilayer) Envelope (lipid bilayer) Protein Coat Protein Coat Core Proteins Core Proteins Reverse Transcriptase Reverse Transcriptase Viral RNA in protein coat Viral RNA in protein coat Spikes Spikes Herpes Viruses HIVHIV

Swine Flu?

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? -

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

Chapter 1922 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

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

Chapter 1925 How Viruses Replicate: HIV cytoplasm nucleus DNA vRNA mRNA 1a.Virus attaches to receptor 2.Viral reverse transcriptase makes DNA using viral RNA 1b.Core disintegrates; viral RNA enters the cytoplasm 3a.DNA enters nucleus & chromosomes 3b.DNA transcribed into mRNA & viral RNA, which move to cytoplasm

Chapter 1926 How Viruses Replicate: HIV 4.Viral proteins made using mRNA 5.Viral proteins & RNA assembled 6.Viruses bud from plasma membrane

Chapter 1927 How Viruses Replicate: Herpes (cytoplasm) envelope coat DNA nucleus DNA mRNA 1.Virus enters cell by endocytosis 3.Viral DNA transcribed to mRNA, which moves to cytoplasm 2a.Viral envelope merges with nuclear membrane 2b.Protein coat disintegrates; viral DNA copied & enters nucleus

Chapter 1928 How Viruses Replicate: Herpes nucleus mRNA (cytoplasm) envelope coat DNA mRNA 4.MRNA makes proteins, which enter nucleus 5.New viruses assembled & bud from nucleus, get envelope from inner nuclear membrane 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