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Viruses

Sunday, 20 July 2014

9:13 AM

Discuss whether viruses are living or non-living organisms and explain why viruses are obligate parasites
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They do not contain cytoplasm or cellular organelles
• They are strict obligates intracellular parasites
...
This includes the ability to direct their host cell's machinery to
synthesis viral components
• Viruses contain genes and show inheritance
• They are able to undergo spontaneous mutation, allowing for evolution to occur
Obligate intracellular parasites:
• When viruses are not in a host cell, they do not appear to carry out many of the functions ascribed to living
organisms, and only perform the maintenance of arrangement of the capsid coat independently of the host cell
• When they infect a host cell, they acquire these capabilities as they take over the host cell's cellular machinery
• Because viruses do not seem to be alive until it infects a cell, they are considered obligate intracellular parasites

Nature:
• Microscopic, intracellular parasites
• Non-cellular, lacks cytoplasm and cellular
organelles
• Can only grow and reproduce inside host
cells by invading and controlling the host
cell's cellular machinery for synthesis of new
viral components and assembly

Describe the structural components of viruses
...
bacteriophages that reproduce via a lytic cycle, e
...
T4 phage;
ii
...
g
...
an enveloped virus e
...
influenza;
iv
...
g
...

Life cycle

Lytic cycle/T4 phage

Attachment • Attachment of tail fibres to
/Absorption specific receptors on surface
of bacterium

Lysogenic cycle/Lambda (λ) phage

• Attachment of tail fibres to specific
receptors on surface of bacterium

Enveloped virus/Influenza

Penetration • Contraction of the contractile • Contraction of the contractile sheath
• Endocytosis ; the host cell surface
/Entry
sheath which drive a hollow
which drive a hollow tube facilitating
invaginates and pinches off to form an
tube facilitating the entry of
the entry of viral DNA into the host's
endosome containing the virus
viral DNA into the host's
cytoplasm
• Membrane of the vesicle fuses with the
cytoplasm
• Empty protein capsid coat is left outside viral envelope; nucleocapsid is released
• Empty protein capsid coat is
the cell
• Uncoating of capsid; viral RNA and
left outside the cell
enzymes are released
• 8 strands of viral RNA, accessory
proteins, RNA-dependent RNA
polymerase
Replication

• Expression of viral genes and
replication of viral genome
occur in the cytoplasm using
host cell's machinery
• Viral enzymes degrade

• Viral DNA is inserted into a specific site
on the bacterial chromosome
• Inserted DNA --> Prophage
○ Codes for a repressor
protein that inhibits

Bio Notes Page 1

Retrovirus/HIV

• Attachment of glycoprotein
• Attachment of glycoprotein (gp120)
(hemagglutinin) of viral envelope to
of viral envelope to specific receptors
specific receptors (containing salic acid) on (CD4) on the membrane surface of
the membrane surface of host cells
host cells (T helper cells)
(epithelial cells)
• Conformation change of receptors
• Conformation change of receptors
facilitate entry of virus
facilitate entry of virus
• Fusion of viral envelope with the
plasma membrane of host cell;
nucleocapsid is released
• Uncoating of capsid; viral RNA and
enzymes are released
• 2 copies of viral RNA and
reverse transcriptase

• Viral RNA and accessory proteins from a
• Reverse transcriptase synthesises
complex and enter the nucleus; serves as a double-stranded DNA from singletemplate for replication of more RNA
stranded RNA
• Viral RNA exits nucleus and serves as
• Viral DNA enters nucleus and inserts
mRNA for synthesis of viral proteins
into host's DNA, catalysed by

• Viral enzymes degrade
bacterial genome and shut
down the synthesis of protein,
DNA and RNA of host

protein that inhibits
transcription of prophage
genes
• Host cell: Lysogen
• Prophage induced to exit host genome
under certain specific conditions

• Expression of viral genes and replication
of viral genome occur in the cytoplasm
using host cell's machinery
• Viral enzymes degrade bacterial
genome and shut down the synthesis of
protein, DNA and RNA of host
Maturation • Assembly of viral DNA, capsid • Assembly of viral DNA, capsid coat, tail
coat, tail apparatus, etc
...
into mature viruses
mature viruses
• In cytoplasm
• In cytoplasm
Release

• Lysis of host bacterium by
phage-coded lysosome
releases new viruses

mRNA for synthesis of viral proteins
• Neuraminidase and Hemagglutinin
• Viral proteins (Neuraminidase and
Hemagglutinin) are transported through
Golgi apparatus to host cell's surface
membrane, serving as exit points for the
new viruses

• Assembly of nucleocapsid in nucleus

into host's DNA, catalysed by
integrase
• Inserted DNA: Provirus
• When host cell is activated, viral DNA
is transcribed into mRNA
• Serves as template for
translation into viral proteins,
or is incorporated as genetic
material for new viruses
• Viral proteins (gp120) are
transported through Golgi apparatus
to host cell's surface membrane,
serving as exit points for the new
viruses
• Assembly of viral RNA, reverse
transcriptase into nucleocapsid in
cytoplasm

• Lysis of host bacterium by phage-coded • New viruses bud off from plasma
• New viruses bud off from plasma
lysosome releases new viruses
membrane, with the help of neuraminidase membrane

Explain how viral infections cause disease in animals, e
...
mammals, through the disruption of host tissue and
functions (e
...
HIV and T helper cells [details of the immune system are not required], influenza and epithelial cells of
the respiratory tract)
...

○ In serious cases, pneumonia
• High lethality of certain influenza subtypes may be due to the triggering of a massive immune response
○ May cause severe illness and death in high risk groups (children < 2years, elderly >65years, people with
chronic respiratory illnesses and impaired immune systems)
• High rate of mutation due to antigenic drift and antigenic shift
○ ↑ occurrence of different subtypes → difficulty in developing effec ve vaccines and an bodies

Bio Notes Page 2



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