1. Chapter 19- Viruses

2. 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

3. 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

4. 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

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

6. 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

9. 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
4 min how flu attacks
6 min ebola virus explained

10. 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

11. Viral Reproduction Lytic Cycle- reproduction occurs, cells burst
Lysogenic Cycle- reproduction does not immediately occur (dormancy)
Virulent- virus that undergoes both cycles
11 min Viral Replication

12. 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.

13. 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.

14. 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 , p.344

15. 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

16. 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)

17. 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

18. 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)

19. Prions (proteinaceous infections particle)
Small proteins
Linked to human diseases
Creutzfeldt-Jakob disease (CJD)
Animal diseases
Bovine spongiform encephalopathy (mad cow disease)
1 min- prion
1 min- Mad Cow Disease

20. 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
3 min animation
Why vaccines work 8 min