Today’s lesson, we cover:

1. viruses

2. viroids

3. Prions

All are infectious agents1

VIRUSESVirus (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 capsidVirus size: 10-

300 nm

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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

Influenza viruses (100nm each), with capsid and

membrane visible

•Protein spikes protrude outward to help with attachment of the virus to the host cell.

4http://www.youtube.com/watch?v=7iVm1uEIyP0

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 form4. Nature of host (plant, animal or bacteria)5. Presence or absence of envelope

VIRUS CLASSIFICATION

Polyhedron

Binal (irregular) shape

Cylindrical/helical

Spherical

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EBOLA VIRUS

PRACTICE IDENTIFY VIRAL SHAPES

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BACTERIOPHAGE STRUCTURE

capsid (protein coat)

nucleic acid

tail

tail fibres

cell membrane

viral RNA/DNA being injected into host cell

Bacteriophage: is a virus that infects bacteria

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Virus Reproduction

• Incubation period: time taken before symptoms

show

• 2 possible cycles: lytic and lysogenic

1. The LYTIC Cycle

(shorter incubation

period)

E.g. Bacteriophage T4 infects E. coli

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A. Attachment B. Insertion

C. Replication

D. Assembly

E. Lysis & Release

bacterial DNA - Host’s

DNA/RNA get digested

- Virus injects its nucleic acid into host cell

Viral DNA

Bacterial host cell

The host’s metabolism replicates the viral RNA/DNA

New viral particles are assembled

virus

Cell lysed due to viral enzyme; new phage go on infect other hosts

See blown up version of this page

Virus Reproduction2. The LYSOGENIC cycle (longer incubation period)

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 divisionAs host cell divides,

viral particles also get made

After this point,the virus mayenter lytic cycle byan environmentaltrigger

provirus

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Bring 2 cycles together

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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

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VIRUSES and DISEASES

COLD SORE (Kissing Disease)

•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.

• Virus is periodically reactivated in times of physical or emotional stress A different kind of sore: canker sore

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•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.

HIV (AIDS)

VIRUSES and DISEASES

•HIV video: http://www.youtube.com/watch?v=v_AUuMbtO-E

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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

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•can be used to make lots of copies of desirable genes

VIRUSES and BIOTECHNOLOGY

VIRUSES in GENE THERAPY •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.

How vaccines work?

1

2

3How vaccine works- from the Gates Foundationhttp://youtu.be/JZvpF6gaGH4

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Vaccines 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

antigens

These antibodies has long lasting memories that allows the immune system to react quickly if it contacts the real virus

NPR Flu Attack video: http://www.youtube.com/watch?v=Rpj0emEGShQ

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White Blood CellForeign

Invader

antibody

antigen

macrophage

Vaccines

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• Smaller than virus, with no capsid• Single stranded, circular RNA that do not code for

proteins (unlike viruses; viral RNA do code for proteins)

Viroids

Mis-shaped potatoes caused by viroidsViroids under microscope

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• 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

Prions

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News Update

Univ. of Cadiff- School of Med

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http://youtu.be/jFpwHe6KjDA