Chapter 19- Viruses

Properties of Viruses Non living because they lack metabolism and are not made of cells No membranes, ribosomes, or other cellular components (non-cellular infectious agent) Protein coat wrapped around a nucleic acid core) They can not grow They can only reproduce inside a host Smaller than a cell and only visible with electron microscope A virus can not be treated with antibiotics, it can only run its course until your immune system kicks it out. Can lay dormant within the cells

Viral Body Plans Complex virus (bacteriophage) Inner core: Genetic material is DNA or RNA, various enzymes Covering: Coat is protein , some have envelope Helical virus Polyhedral virus

Viral Envelopes Many viruses that infect animals have a membranous envelopes Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell Some viral envelopes are formed from the host cell’s membrane as the viral capsids exit; can form from nuclear envelope and are replaced by golgi membrane

Enveloped Virus (HIV) viral protein lipid envelope (derived from host) viral RNA reverse transcriptase viral coat (proteins)

Capsid and viral genome enter the cell Capsid Fig. 19-7 Capsid and viral genome enter the cell Capsid RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Copy of genome (RNA) Glyco- proteins Figure 19.7 The reproductive cycle of an enveloped RNA virus New virus

Parasitic Nature Obligate intracellular parasites (they can not exist independently) Specific to hosts- human, dog, some can cross species Specific to cells- common cold is a virus that attacks cells of the respiratory track; HIV attacks white blood cells https://www.youtube.com/watch?v=Rpj0emEGShQ 4 min how flu attacks https://www.youtube.com/watch?v=sRv19gkZ4E0 6 min ebola virus explained

Viral Multiplication - Basic Steps Attachment- Attach to host cell Penetration- Enter host (virus or just genetic material) Biosynthesis-Direct host to make viral genetic material and protein Maturation-Assemble viral nucleic acids and proteins Release-Release new viral particles

Viral Reproduction Lytic Cycle- reproduction occurs, cells burst Lysogenic Cycle- reproduction does not immediately occur (dormancy) Virulent- virus that undergoes both cycles https://www.youtube.com/watch?v=EqK1CYYQIug 11 min Viral Replication

Lysis of host cell is induced; infectious particles escape. Lytic Pathway Tail fibers and other parts are added to coats. Virus particles bind to wall of suitable host. Viral genetic material enters cell cytoplasm. Viral protein molecules are assembled into coats; DNA is packaged inside. Viral DNA directs host machinery to produce viral proteins and viral DNA.

Viral DNA usually becomes integrated into the bacterial chromosome. Lysogenic Pathway Viral DNA is excised from chromosome and cell enters lytic pathway. Prior to prokaryotic fission, the chromosome and integrated viral DNA are replicated. After binary fission, each daughter cell will have recombinant DNA.

e Lysis of host cell is induced; infectious particles escape. d The coats get tail fibers, other parts. Lytic Pathway a Virus particle injects genetic material into a suitable host cell after binding to its wall. c Viral proteins are assembled into coats around viral DNA. b Viral DNA directs host cell to make viral proteins and replicate viral DNA. a-1 Viral DNA is integrated into the host’s chromosome. a-4 Viral DNA is excised from the chromosome. Lysogenic Pathway a-2 Before prokaryotic fission, the bacterial chromosome with the integrated viral DNA is replicated. a-3 After cell division, each daughter cell will have recombinant DNA. Fig. 21-15, p.344

Retroviruses RNA virus that have a DNA stage HIV RNA inside a protein coat Reverse Transcriptase make DNA from the viral RNA DNA inserts into host DNA; proteins are assembled, and virus is released Hard to make vaccines because RNA mutates easily and each virus can be slightly different from the others http://highered.mcgraw-hill.com/sites/007352543x/student_view0/chapter20/how_the_hiv_infection_cycle_works.html

Nature of Disease Contagious disease pathogens must directly contact a new host Epidemic- incidence rate exceeds what is expected (SARS) Pandemic (AIDS)- occurs worldwide Sporadic- “singular cases” Endemic- infection maintained in a population (no malaria in UK)

Emerging Viruses Viral strains can jump species (avian flu) Emerging Pathogens Ebola virus Bird flu SARS virus Drug-resistant strains Could be mutations of known viruses, viruses exposed when new areas are developed, new hosts Host and pathogen are coevolving

Viroids Smaller than viruses Strands or circles of RNA Cause many plant diseases Horizontal transmission (enter through damaged cell walls) or Vertical Transmission (inherit from parent)

Prions (proteinaceous infections particle) Small proteins Linked to human diseases Creutzfeldt-Jakob disease (CJD) Animal diseases Bovine spongiform encephalopathy (mad cow disease) https://www.youtube.com/watch?v=tfv3xAw0XOE 1 min- prion https://www.youtube.com/watch?v=aP-ShyyHiIc 1 min- Mad Cow Disease

How do vaccines work? Once you have gotten a virus, your body develops the immunity to that virus Vaccines are made by growing a weakened or killed form of the virus (often grown in eggs) The form of the virus is injected into a persons body, which causes an immune response, and immunity to the virus https://www.youtube.com/watch?v=zcEn7p6CKfE 3 min animation https://www.youtube.com/watch?v=3aNhzLUL2ys Why vaccines work 8 min