VIRUSES

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VIRUSES
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 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.

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

Influenza viruses (100nm each), with capsid and
membrane visible
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• Viruses are classified based on
• 1. Capsids 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
Cylindrical/helical
Binal (irregular) shape
Spherical
http//www.youtube.com/watch?v7iVm1uEIyP0
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VIRAL SHAPES EXAMPLES
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BACTERIOPHAGE STRUCTURE
Bacteriophage is a virus that infects bacteria
capsid (protein coat)
nucleic acid
tail
tail fibres
cell membrane
viral RNA/DNA injected into host cell
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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

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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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B. Insertion
A. Attachment
bacterial DNA
virus
- Hosts 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 hosts metabolism replicates the viral RNA/DNA
New viral particles are assembled
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Bacteria are NOT defenceless against phage
infections
Bacteria use restriction enzymes that act like
scissors to cut up phages 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
  advantage
• Bacteria and virus arms race!

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Summary of lytic cycle (HW- self note)

• Attachment 2. Insertion
• 3. Replication
• 4. Assembly
• 5. Lysis Release

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(No Transcript)
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Virus Reproduction
2. The LYSOGENIC cycle (longer incubation period)

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

A. Attachment Entry
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Virus Reproduction
2. The LYSOGENIC cycle (longer incubation period)
B. Integration
Viral nucleic acid becomes a part of the
bacterial nucleic acid
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Virus Reproduction
2. The LYSOGENIC cycle (longer incubation period)
C. Multiplication of Genome
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Virus Reproduction
2. The LYSOGENIC cycle (longer incubation period)
May now undergo the lytic cycle
D. Cell Division
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The LYSOGENIC Cycle (longer incubation period)

• 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.

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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 hosts 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

• 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
  persons lifetime.

COLD SORE (Kissing Disease)

• 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-viruses
VIRUSES and DISEASES
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Retro-viruses
VIRUSES and DISEASES

• 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

• The viral DNA merges with hosts 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.

• HIV video http//www.youtube.com/watch?vv_AUuMbt
  O-E

Whats the inference for showing HIV video under
retro-viruses topic?
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To sum it up
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VIRUSES and DISEASES

• AIDS (acquired immune deficiency syndrome)
• Caused by HIV- a retro virus
• The HIV destroys the bodys 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 bodys immune system.

HIV (AIDS)

• HIV video http//www.youtube.com/watch?vv_AUuMbt
  O-E

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VIRUSES and BIOTECHNOLOGY

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

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VIRUSES and BIOTECHNOLOGY

• Viruses well-known for inserting their DNA/RNA
  into hosts DNA
• can be used in gene therapy to repair bad genes
• The virus must be disarmed
• To correct for a genetic disorder permanently, a
  stem cell is used.
• The virus carrying the desired gene is thus let
  infect the patients 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

• 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
?vRpj0emEGShQ
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(not in your note)
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Vaccines
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(No Transcript)
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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)

Mis-shaped potatoes caused by viroids
Viroids under microscope
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Prions

• Are single proteins
• Found in brain of infected animals
• Once eaten, prions go to the animals 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
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News Update
Univ. of Cadiff- School of Med
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http//youtu.be/jFpwHe6KjDA