Viruses are acellular parasites that require a host cell to replicate. They consist minimally of a protein coat that protects their nucleic acid core. Many viruses have an outer envelope derived from the host cell membrane. Viral shapes include polyhedral, icosahedral, helical, and complex structures. Viruses infect both animal and plant hosts, causing diseases. Bacteriophages infect bacteria and can undergo lytic or lysogenic cycles. Animal virus replication involves attachment, penetration, uncoating, biosynthesis of viral components, assembly, and release of new virus particles. Prions are pure protein particles that can convert normal proteins into more prions, causing fatal neurological diseases in animals and humans.

1. Chapter 13 viruses,viroids, prions Viruses: are acellular, obligate intracellular parasites require a host cell to multiply Not assigned to any kingdom Virion is another name for a complete virus particle Viral structure Small size = 20 – 1000 nm, need EM to see, go thru filters that retain bacteria

2. Viral structure pg 389 Core – contains nucleic acids – DNA or RNA (contains genetic info) but not both Capsid – outer protein coat that protects the core and maintains shape Composed of identical protein subunits = capsomeres Envelop – membrane around capsid = enveloped virus Not all viruses have an envelop = nonenveloped or naked virus

3. Virus Architecture Minimally, a virus is a proteinaceous carrier of nucleic acid. Many viruses are more complicated than that, such as having a lipid envelope surrounding the protein capsid.

4. Viral structure Envelop continued May have spikes to aid in attachment to host Partially derived from host cell plasma membrane during viral replication cell Shapes Polyhedral = many sided Icosahedral – has 20 triangular faces Enveloped – herpes simplex Nonenveloped – Adenovirus, poliovirus

5. Viral structure Shapes continued Helical Nonenveloped = rabies, Ebola Enveloped = influenza Complex Bacteriophages with polyhedral head and a helical tail Pox viruses with several coats around the nucleic acids but no clear capsid pix pg 391

6. Not-Complex Virions

7. Complex (Tailed) Phage Virion Note that this head actually is elongated top to bottom rather than isometric.

8. Host range = range of host cells in which a virus can multiply Animal viruses – contain either DNA or RNA Chickenpox, smallpox – humans Rabies - dogs Plant viruses – RNA Tobacco mosaic Usually polyhedral or helical Can produce both internal and external effects and have an economic impact on agriculture Some plant diseases are caused by viroids = short pieces of RNA

9. Bacteriophages = bacterial viruses Some have tail-like fibers thru wh/ they inject their nucleic acid into the bacterial cell Multiplication of bacteriophages All viruses carry only the genetic info needed for replication of their nucleic acids and synthesis of their protein coats They require energy production and ribosomes from the host cell

10. Multiplication or bacteriophages Phages can cause lytic (virulent) infections or lysogenic (temperate) infections in E. coli Lytic infections – caused by T-even phages cause lysis and death of the host cell Lysogenic infection – caused by phage lambda don’t cause death or lysis of host cell Phage can spontaneously become lytic

11. Lytic cycle of T-even phage Adsorption Tail fibers adheres to specific sites on bacterial cell wall Penetration Enzymes released from the phage tail dissolve a hole in cell wall Phage DNA is injected into cell thru hollow tail Capsid remains outside the bacterial cell – no uncoating needed

12. Lytic cycle of T-even phage Biosynthesis Viral DNA core directs synthesis of viral parts by the bacteria Maturation = phage DNA and capsids which were formed separately are assembled into virions (individual virus particles) Release of virus causes lysis of bacterial cell

13. Virulent Phage Life Cycle

14. Lysogenic cycle – phage lambda Phage lambda attaches to (adsorption) and penetrates into bacterial cell but are not replicated and released immediately Phage DNA forms a circle Can now go thru lytic or lysogenic cycle Prophage = phage DNA incorporated into bacterial DNA and remains latent or inactive

15. Lysogenic cycle – phage lambda Bacteria divides producing more cells with viral DNA Rare – prophage can leave the hosts DNA and initiate the lytic cycle Phage may take some of the hosts DNA w/ it and infect a new cell taking with it new genes The new host cell may exhibit some properties of the old host cell Ex. Toxin production – Corynebacterium diphtheria can only produce disease when it carries a lysogenic phage because the phage carries the gene coding for the toxin

16. Temperate Phage Life Cycle(s)

17. Replication of animal viruses Attachment or adsorption Virus attaches to host cell plasma membrane Specific attachment – viral spikes complementary to host cell attachment sites Penetration Virus enters host cell by: Pinocytosis Fusion – viral envelop fuses with plasma membrane and releases capsid into the host cell’s cytoplasm

18. Replication of animal viruses Uncoating Viral capsid dissolved by host enzymes releasing viral nucleic acids Manufacture of viral parts – Under the direction of the viral genome but carried out using the host’s machinery, proteins, energy

19. Replication of animal viruses Biosynthesis of DNA virus Viral DNA is synthesized in host’s cell nucleus Capsid proteins are synthesized in cell’s cytoplasm Capsid protein enter the nucleus and combine with DNA New viruses new viruses bud thru cell membrane and the cell membrane becomes part of the virus envelop

20. Replication of animal viruses Biosynthesis of RNA virus – multiplication takes place in the cytoplasm Assembly (maturation) virus parts are assemble to produce mature virions Release – mature virus leaves host cell Lysis of host cell if nonenveloped (naked) virus Budding out if enveloped virus pg 410

21. Virus culture methods Animal viruses are cultured by 3 methods living animals, embryonated eggs, cell cultures Living animals Some viruses can only be cultured in living animals –mice, rabbits, guinea pigs Studies of immune system response to viral infections – animals are observed for signs of disease or killed so the tissue can be examined for the virus Human AIDS virus can’t be grown in animals so vaccines are tested on humans but it takes years to determine the effectiveness

22. Virus culture methods In embryonated eggs Convenient and inexpensive, once was the most widely used method Viral growth = death of embryo, embryo cell damage, lesions to egg membranes Harvest egg contents for viable viruses If allergic to eggs need to know if vaccine was prepared in eggs

23. Virus culture methods In cell cultures – preferred growth for virus When viruses are grown in labs, use continuous diploid cell lines Cells are trypsinized (to break apart), suspended in a growth media, growth is in a monolayer of cells on the flask after incubation Virus cause cell deterioration = cytopathic effect (CPE) Problem with cell cultures is microbial contamination

24. Virus culture methods Bacteriophages – can be grown in bacterial suspensions of liquid media or agar Plaque method uses solid media for detection and counting of viruses Melted agar containing bacteriophage and host bacterial cell is poured into a Petri dish that contains a hardened layer of agar growth media Each virus infects a bacterium, multiplies, and releases hundreds of new viruses wh/ destroy the surrounding bacteria and produce clearings or plaques Each plaque represents one virus, reported as PFU

25. Viruses and cancer 10% of cancer is caused by viruses Viruses that cause tumors in animals = oncogenic viruses or oncoviruses An oncogene is a cancer causing gene Tumor cells undergo transformation – now have properties different from non tumor forming cells After being transformed by viruses, most tm cells contain a virus specific antigen on their cell surface = tumor specific transplantation antigen (TSTA) or an antigen in their nucleus = T antigen

26. Viruses and cancer Tm cells undergo transformation cont. Transformed cells – less round than normal cells, have chromosomal abnormalities = unusual # of chromosomes or fragmented chromosomes Oncogenic viruses DNA viruses Human papillomavirus – cervical cancer Hepatitis B causes liver cancer

27. Viruses and cancer Oncogenic viruses cont. RNA viruses: only viruses from the family Retroviridae cause cancer T-cell leukemia and lymphoma Prions – pure protein molecules that can catalyze the conversion of normal proteins into more prions No nucleic acids Infection develops slowly and is usually fatal Ex. Mad cow disease

28. Prions Prions cont. 9 animal diseases that are neurological diseases called spongiform encephalopathies – large vacuoles form in the brain ex. Mad cow disease Human disease are – kuru, Creutzfeldt - Jacob disease (CJD) Both diseases progressively destroy ms coordination and brain fx CJD has been transmitted w/ transplanted nerve tissue and contaminated surgical instruments

29. Viral infections Persistent or chronic viral infection = occurs gradually over a long period of time. Usually fatal Latent viral infections = virus stays in the host for long periods of time without causing disease Stress or other causes can trigger re-appearance of disease Ex. Herpes simplex –cold sores Chickenpox virus - shingles