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 capsid

Virus size: 10-300 nm

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Virus Characteristics (cont.) There may be a further layer referred

to as the envelope, which surrounds the capsid - this is found in animal viruses and is derived from the membrane of the host animal cell.

Influenza viruses (100nm each), with capsid and

membrane visible

•Protein spikes may protrude from the viral particle - these are involved in the attachment of the particle to the host cell.

3http://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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VIRAL SHAPES EXAMPLES

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BACTERIOPHAGE STRUCTURE

capsid (protein coat)

nucleic acid

tail

tail fibres

cell membraneviral RNA/DNA injected into host cell

Bacteriophage: is a virus that infects bacteria

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Largest Virus Found in 2011!

•Pandoravirus (~1 m) •Irregular shape,•Look like a bacterium•DNA as genetic material•Infects amoebas•Not a threat to human•Found in sediments in Chile, Australia

Virus Microviewer Lab

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Virus Reproduction

• Viruses undergo replication within a host cell

• incubation period: time between when the virus

infects a cell and when the virus ruptures the cell

• 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

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Bacteria are NOT defenceless against phage infections

Bacteria use restriction enzymes that act like scissors to cut up phage’s DNA, making it useless

- Bacteria with effective restriction enzymes survive and reproduce- Any phages that develop resistance to the restriction enzymes also have a reproductive advantageBacteria and virus arms race!

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Summary of lytic cycle (HW- self note)

1. Attachment & 2. Insertion:

3. Replication:

4. Assembly:

5. Lysis & Release:

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A. Attachment & Entry

bacterial DNA/RNA

virus

Cells undergoing this cycle can be triggered into reproducing in the lytic cycle

Virus Reproduction

2. The LYSOGENIC cycle (longer incubation period)

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B. Integration

provirus

Viral nucleic acid becomes a part of the bacterial nucleic acid

Virus Reproduction

2. The LYSOGENIC cycle (longer incubation period)

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C. Multiplication of Genome

Virus Reproduction

2. The LYSOGENIC cycle (longer incubation period)

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May now

undergo

the lytic

cycle

D. Cell Division

Virus Reproduction

2. The LYSOGENIC cycle (longer incubation period)

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• Attachment: The virus invades a host cell and injects its DNA (or RNA) into the host.

• The viral DNA is merged with the host DNA.

• The viral DNA remains dormant for a period of time (provirus)

• The viral DNA is extracted from the HOST by a "trigger".

• The host cell replicates the parts of the virus and other materials in viral DNA LYTIC CYCLE

• New viral particles are assembled in the host cell.

The LYSOGENIC Cycle (longer incubation period)

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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 db stranded DNA 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 (newly added note) A different kind of sore: canker sore

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Retro-virusesVIRUSES and DISEASES

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• Normal genetic information transfer is from DNA to RNA• Retro-viruses use the reverse process (i.e. RNA to DNA):- After injecting RNA into host cell, retro-viruses have an

enzyme called Reverse transcriptase that can then convert their RNA into DNA

Retro-viruses

-The viral DNA merges with host’s DNA and can replicate for many years showing no harm to the host.

- Can switch to lytic cycle any time. What triggers this switch is not yet known.

VIRUSES and DISEASES

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

What’s the inference for showing HIV video under retro-viruses topic?

To sum it up…

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

VIRUSES and BIOTECHNOLOGY

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VIRUSES and BIOTECHNOLOGY•Viruses well-known for inserting their DNA/RNA into host’s DNA

•can be used in ‘gene therapy’ to repair bad genes

•The virus must be disarmedTo correct for a genetic disorder permanently, a stem cell is used.

-The virus carrying the desired gene is thus let infect the patient’s stem cells.

- The result is the desired gene is permanently expressed once the stem cells are injected into the bone marrow of the patient

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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 triggered 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

28(not in your note)

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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