BMS2052 Lecture Notes - Lecture 7: P53, Cell Cycle, Influenza A Virus
30 May 2018
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Week 3. Virology
VIROLOGY I
• About 2 billion people worldwide have been infected by HBV (hepatitis B virus)
• HBV genome contains only four genes
• Viruses infect all living organisms
• We eat and breathe billions of viruses regularly
• We carry viral genomes as part of our own genetic material (8.3% of human genome)
• Carbon content of viruses corresponds to 75x106 blue whales
• Viruses are not living things
• Have non-cellular organisation
• Cannot make energy or proteins independently of a host
• What is a virus:
o Non-living in extracellular state
o Infectious obligate intracellular parasites
o Virus particle: supramolecular complexes consisting of
Nucleic acid (DNA or RNA)
Protective shell (capsid)
Sometimes membranous lipid envelope and viral membrane proteins
o Viruses exist in 2 states/phases:
1. Extracellular state: virus particle (virion)
-no biosynthetic function
-structure by which virus genome is carried from cell in which it has been
produced to a new target cell
2. Intracellular state:
-reprogramming host cells, virus replication occurs along with production of new
viral components
• Virion = virus particle unit that infects the appropriate host cell
1. Entry step
2. Release of genetic material into cell
3. Goes into cell nucleus and starts replicating
4. Makes protein and assembles virus
• Virus is like a car manufacturer – needs to assemble body parts then generate a functional virus
• Virus particles are produced from the assembly of pre-formed components
• Virus particles themselves do not grow or undergo division
• Where do viruses come from?
o Likely from jumping genes (mobile genetic elements) provided the raw material needed
for the construction of viruses
o May be an unavoidable consequence of rapid gene evolution
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o Still evolving today
• Historical classification of viruses was disease related
i.e. respiratory viruses, enteric viruses, STV, hepatitis viruses etc
• Modern virus taxonomy:
o Viruses are classified by shared properties rather than the properties of cells/organisms
they infect i.e. DNA/RNA, ss or ds etc
o No consideration of the disease caused by a virus; related viruses can cause very
different diseases
o More size and molecular basis
• General features of the viral reproductive system:
Attachment (adsorption)
Entry
Uncoating (release of viral genome)
Replication and amplification (two steps)
Viral gene expression
Synthesis of mRNA (transcription)
Synthesis of viral proteins (translation) – reprogramming of host cell
Assembly of capsid subunits into particles
Packaging of nucleic acid
Release of virus particles
• Virion (virus particle):
o Complete physical entity that occurs extracellularly and can infect new host cells
o Composed of:
Virus genome (nucleic acid)
Protein capsid (coat) – helical, icosahedral or complex structure
May have an envelope
Range in size
o Function:
Protection of genome – assembly of capsid, recognition and packaging of nucleic
acid genome, interaction with host cell membrane to form envelope
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Delivery of genome – binding to external receptors, transmission of signals
Other function – interaction with cellular components
o Structure of virus particles:
Rod shaped coat of repeating units
Promoters associate with nucleic acid in a spiral of helical arrangement
Helical symmetry: identical units, proteins have self-assembly capability -> come
together -> helical -> build tube like structures
eg. paramyxovirus (measles, mumps, Hendra virus) – many animal RNA viruses
Icosahedral symmetry: solid shape consisting of 20 triangular faces arranged
around the surface of a sphere, rotational 2-3-5 symmetry, need 60 proteins to
build a small capsid, need 3 proteins per triangular face
-contain only small genomes eg. parvoviruses
Helical viruses have it easier, icosahedral is more compressed
Icosahedral or helical capsids are surrounded by an envelope
o Envelope:
Host derived lipid from cell nuclear or plasma membrane
Contains virus-encoded proteins or glycoproteins projected as spikes (allows
attachment) or peplomers
May be flexible, unusual shapes
o Complex structures:
Mix of shapes or no consistent symmetry
eg. poxiviruses
Adenovirus: icosahedral
Rotavirus: -causes severe diarrhoea, can be fatal to children , big problem in
developing, need digestive enzymes to open particles, very robust
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