1. Today’s lesson, we cover:
1. viruses 2. viroids 3. Prions All are infectious agents

2. VIRUSES Viruses are non-living.
Virus (Latin): meaning poisonous slime of plant or animal origin
Viruses are non-living.
1. Acellular: lack cell membrane and organelles
2. No metabolic activity
3. Lack energy generating system
4. They are obligate parasites - only capable of reproduction in a host cell.
5. Genetic material: DNA or RNA, housed within a protein coat called a capsid
Virus (Latin): meaning poisonous slime of plant or animal origin
1nm = 1000 um
A complete virus particle is called a virion.
Virus size: nm

3. Virus Characteristics (cont.)
Viruses that infect animal cells can:
Disguise from the host’s immune system by “wearing” an envelope made of the host’s cell membrane
Protein spikes protrude outward to help with attachment of the virus to the host cell.
Blow up next page for student
Influenza viruses (100nm each), with capsid and membrane visible

4. Viruses are classified based on
1. Capsid’s size and shape (Cylindrical, Polyhedron, Enveloped, Binal (complex) shape)
2. Types of genetic materials (i.e. DNA or RNA)
3. The number of strands in Nucleic Acid - either single stranded or double stranded form
4. Nature of host (plant, animal or bacteria)
5. Presence or absence of envelope
VIRUS CLASSIFICATION
Polyhedron
The viral genome, often with associated basic proteins, is packaged inside a symmetric protein capsid. The nucleic acid-associated protein, called nucleoprotein, together with the genome, forms the nucleocapsid. In enveloped viruses, the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins.
Icosahedron is a polyhedron with 20 triangular faces, 30 edges and 12 vertices.
Tobacco Mosaic Virus. This tobacco mosaic virus was the first virus to be discovered. It was first seen with an electron microscope in 1935.
Cylindrical/helical
Binal (irregular) shape
Spherical

5. PRACTICE IDENTIFY VIRAL SHAPES
Influenza spherical; 2. ebola viruses:u shaped 3. tobacco mosaic viruses: helical/cylindrical
News on ebola: causes haemorrhage
Like a ‘Transformer’, this protein of the Ebola virus adopts different shapes for different functions, it revises a central dogma of molecular biology—that a protein molecule has one shape that predestines one biological function.
EBOLA VIRUS

6. BACTERIOPHAGE STRUCTURE
Bacteriophage: is a virus that infects bacteria
capsid (protein coat)
nucleic acid
tail
tail fibres
cell membrane
viral RNA/DNA being injected into host cell

7. Virus Reproduction Incubation period: time taken before symptoms show
2 possible cycles: lytic and lysogenic
1. The LYTIC Cycle (shorter incubation period)
Share one important trait with living thing: they reproduce
E.g. Bacteriophage T4 infects E. coli

8. B. Insertion A. Attachment E. Lysis & Release D. Assembly
bacterial DNA
virus
- Host’s DNA/RNA get digested
- Virus injects its nucleic acid into host cell
Viral DNA
Bacterial host cell
E. Lysis & Release
Cell lysed due to viral enzyme; new phage go on infect other hosts
C. Replication
D. Assembly
The host’s metabolism replicates the viral RNA/DNA
New viral particles are assembled
See blown up version of this page

9. Virus Reproduction 2. The LYSOGENIC cycle (longer incubation period)
provirus
A- Attachment & entry:
The virus invades a host cell and injects its DNA (or RNA) into the host.
B- Integration
The viral DNA is merged with the host DNA.
C- Multiplication of Genome
Viral DNA duplicates as host cell divides
The viral DNA remains dormant for a long period of time (provirus)
D- Cell division
As host cell divides, viral particles also get made
After this point,
the virus may
enter lytic cycle by
an environmental
trigger

10. Bring 2 cycles together

11. Compare lytic and lysogenic cycles
Lytic cycle
Lysogenic cycle
Fate of virus DNA/RNA after infecting the host
Take over host DNA
Becomes part of host DNA
Incubation time in host cell
Short; new viruses released within host’s lifetime or in the next generation
Long; new viruses released after several generations of host
Onset of viral diseases symptoms
Immediate
delayed

12. VIRUSES and DISEASES
Caused by herpes simplex virus (a DNA-containing virus).
The sores appear during lytic cycle
The sores disappear (when the viruses is in its provirus stage) on the skin throughout the person’s lifetime.
Lifetime latency is established when virus migrates to trigeminal nerve ganglion
Use Polysporin to make the sore disappear quicker; no cure still
Cold sores are caused by a herpes simplex virus (HSV-1) closely related to the one that causes genital herpes (HSV-2). Both of these herpes simplex viruses can affect your mouth or your genitals, and can be spread via oral sex.
Canker sore: not caused by herpes virus
COLD SORE (Kissing Disease)
Virus is periodically reactivated in times of physical or emotional stress
A different kind of sore: canker sore

13. VIRUSES and DISEASES AIDS (acquired immune deficiency syndrome):
Caused by HIV- a retro virus
The HIV destroys the body’s T-lymphocytes, which help fight off infections from other diseases
When leaving the host cell, new HIV take a piece of the host cell membrane to disguise themselves from the host body’s immune system.
epidemic  vs Pandemic
Epidemic: seasonal outbreak affecting smaller # of living things than a pandemic
Pandemic: global scale outbreak
HIV (AIDS)
HIV video:

14. What is a retro-virus?
A virus that use an enzyme called reverse transcriptase to converts its RNA into DNA once inside a host cell

15. can be used to make lots of copies of desirable genes
VIRUSES and BIOTECHNOLOGY
can be used to make lots of copies of desirable genes

16. Viruses can precisely insert their DNA/RNA into host’s DNA
can be used in ‘gene therapy’ to repair faulty genes
The virus must be disarmed before use
Virus carrying the desired gene is allowed to infect the patient’s stem cells.
VIRUSES in GENE THERAPY
Virus is another famous vector beside bacteria.
Used instead of plasmid when the DNA strand to be manipulated is quite long
Genetically engineered viruses can be used to deliver healthy working alleles of gene to the a human chromosome.
Shown here as a spherical virus but actually a T4 phage
Why injected the stem cell with the recombinant DNA into bone marrow? Because you need the healthy genes to be able to multiply over the course of the person’s life so stem cells in bone marrow is the best location. Stem cells give rise to other cells that become specialized later on that now can contain the healthy version of the gene

17. How vaccines work? 1 2 3 How vaccine works- from the Gates Foundation

18. Vaccines
antigens
Vaccines are mixtures of dead or weakened forms/parts of dangerous viruses
When inside a host, the antigens on the surface of the weakened viruses trigger white blood cells to produce antibodies, without causing an infection
Antibodies are chemicals produce by your body to recognize foreign antigens
attenuated  = modified or weakened viruses or parts
These antibodies has long lasting memories that allows the immune system to react quickly if it contacts the real virus
NPR Flu Attack video:

19. Vaccines antibody Foreign Invader macrophage antigen White Blood Cell19

20. Mis-shaped potatoes caused by viroids
Smaller than virus, with no capsid
Single stranded, circular RNA that do not code for proteins (unlike viruses; viral RNA do code for proteins)
Size of viroids: ~359 nucleotides; 10 molecules enough to infect plants
Mis-shaped potatoes caused by viroids
Viroids under microscope

21. Prions Are single proteins Found in brain of infected animals
Once eaten, prions go to the animal’s brain
Cause diseases by interacting with normally shaped proteins causing them to become abnormal and infectious
Responsible for brain diseases such as mad cow diseases in cow, scrapie in sheep and Creutzfeldt-Jakob disease in human
BSE = bovine spongiform encaphelopathy or mad cow disease

22. News Update
Univ. of Cadiff- School of Med