1. Lecture 9 Viruses, Viroids, Prions

2. Viruses Parasites that are not cells or organisms
Infect all forms of life:Bacteria, Archea, Eukarya
Virus particle called virion
Comprised of two parts:
1. Nucleic Acid
2. Protein coat (capsid)
Nucleocapsid- capsid with nucleic acid inside
Each capsid made of identical protein subunits called capsomeres

3. Virus Shapes

7. Two basic types of virions
Naked: consist only of nucleic acid and capsid
Enveloped virus: consists of nucleic acid, capsid, and envelope

9. Virus attachment to host cells
All viruses must be able to attach to specific receptors on host cells
Attachment spikes project from the capsid or envelope
In viruses with tails- tail fibers attach the virus to the host cell

10. Virus size
Approximately 100 to 1000 fold smaller than the cells they infect
Size generally from 10 nm to 500 nm
The smallest viruses contain very little nucleic acid, perhaps as little as 10 genes

12. Viral Genome Structure of viral genome is unusual
Contain only single type of nucleic acid- DNA or RNA
DNA may be linear or circular, either double-stranded or single-stranded
RNA is usually single-stranded

13. Overview- Replication Cycle of Viruses
Can only multiply within living cells that are actively metabolizing
Viruses lack cellular components necessary to harvest energy and synthesize proteins
Viruses must use structures and enzymes of cells they infect to support their own reproduction- considered parasites

14. Multiplication of Bacteriophages

15. Bacteriophage Virus that infects bacteria Excellent model
for other bacteria

16. Two possible outcomes following viral infection of a bacterial cell
Lytic Infection: viruses multiply inside the cells they invade
Lysogenic Infection: integrate viral DNA into host cell chromosome; the virus DNA replicates as the bacterial chromosome replicates

17. Replication of lytic phages
Phage nucleic acid enters the bacterium and capsid remains outside
Nucleic acid replicated along with phage proteins
Many virions are formed
Phages exit by bursting the cell
Phages that go through this life cycle are called virulent
Virulent: has the ability to overcome host defenses and cause disease

20. Lysogenic Infection The viral DNA is incorporated into the host genome
Each time the host chromosomes replicated and split into new cells, so is the viral DNA
Phage may excise itself from the chromosome and later and revert to lytic growth

22. Host range of phages
Host range: number of different bacteria that a particular phage can infect
Two factors determine the host range of a phage:
Phage must be able to attach to receptors on host cell surface
The restriction modification system of the host cell

23. Receptors on bacterial surface
Receptors vary in chemical structure and location
Receptors are usually on cell wall, although a few phages attach to pili or flagella
Receptor sites can be modified, thereby creating a resistant cell
Some temperate phages can alter the cell surface, an example of lysogenic conversion
As a result receptor no longer available
Thus, prophage protects it’s host and, in turn, is able to keep replicating inside of it

24. Restriction Modification System
In some bacteria to protect themselves from viral infection
Bacterial cell makes restriction enzyme and methylating enzyme
Methylating enzyme adds methyl group to bacterial DNA
Bacteria now knows this is it’s own DNA
Uses restriction enzyme to cut any DNA that is not methylated
Cuts viral DNA- inactivating it

26. Multiplication of Animal Viruses

27. Multiplication of Animal Viruses
Similar to bacteriophage replication
Animal viruses attach to host plasma membrane via spikes on the capsid or envelope
Animal viruses are usually taken into the cytoplasm as intact nucleocapsids
Uncoating is the separation of the capsid from the genome

28. Entry of Animal Viruses into their Host Cells

31. Interactions of Animal Viruses with their hosts
Acute Infections
Latent Viral Infections
Persistant Viral Infections

33. Latent Viral Infections
When acute infection followed by symptomless period and then reactivation of disease
Symptoms of the initial and reactivated diseases may differ
Provirus: latent form of virus in which viral DNA incorporated into host DNA

35. Examples of Latent Infections- HSV-1

36. Examples of Latent Infections- varicella zoster virus
Initial infection of children- chicken pox
Then can remain latent for years with no disease symptoms
Can be reactivated and cause shingles
Chicken pox and shingles- different diseases caused by same virus

37. Persistent Viral Infections
Occurs gradually over a long period
Example: Measles
Several years after contracting measles can get Subacute sclerosing panenchaphalitis

38. Retroviridae
Family of viruses that carry their genetic information as ssRNA
Have enzyme reverse transcriptase which forms a DNA copy that is then integrated into the host genome

40. Viral-Induced Tumors Tumor- results from abnormal growth of cells
Benign tumor- growth remains within defined region, not carried to other areas
Malignant tumor- when abnormal growth spreads to other parts of body
Tumor-causing viruses- Oncogenic Viruses

41. Viral-Induced Tumors Proto-oncogenes- normally occurring genes
They can be converted to oncogenes by:
radiation
chemical carcinogens
DNA damage
Viruses
Oncogenes then cause tumors

43. Viriods Much smaller than viruses Just consist of small ssRNA molecule
No protein coat
Infect plants

44. Prions Proteinaceous infectious agents
Contain only protein, no nucleic acid
Linked to number of fatal diseases in humans and animals
Obligate intracellular parasite
How does it replicate if no nucleic acid?
Prion protein converts host protein to prion protein

46. Prions
Cannot be killed by UV light or nucleases, can be killed by proteases and heat
Usually cannot be transmitted across species