Virology Course 10BM
Lesson 1: HPV: virus that can lead to cervical cancer
OPEN QUESTION ON TEST: viral life cycle!
A virus is an infectious, obligate intracellular parasite. Virion = infectious particle
Viruses are not living things, they infect living organisms. Viruses are complicated assemblies of
molecules, including proteins, nucleic acids, lipids, and carbohydrates, but on their own they can do
nothing until they enter a living cell.
Without cells, viruses would not be able to multiply.
When a virus encounters a cell, a series of chemical reactions occur that lead to the production of
new viruses. These steps are completely passive, that is, they are predefined by the nature of the
molecules that comprise the virus particle.
Viruses don’t actually ‘do’ anything. Everything they do is passive. Often scientists and non-scientists
alike ascribe actions to viruses such as employing, displaying, destroying, evading, exploiting, and so
on. These terms are incorrect because viruses are passive, completely at the mercy of their
environment.
- All viral genomes are obligate molecular parasites that can function only after they replicate
in a cell
- All viruses must make mRNA (or bring their own mRNA) that can be translated by host
ribosomes (almost all viruses use the host ribosomes): they are all parasites of the host
protein synthesis machinery, they hijack the ribosomes and use that
- viruses depend on their hosts to survive: if viruses are too successful and very quickly kill
their hosts or if viruses are too passive and the host is able to stop their growth, they
eliminate themselves
- there is always this balance: for example: ebola is very lethal but therefore doesn’t spread as
quickly due to not having a living host. The common cold spreads very easy but isn’t lethal
The origin of viruses is unclear, some possibilities: some may have evolved from plasmids or
transposons, evolved from bacteria
However it is unlikely that all current viruses evolved from a single progenitor...so viruses might have
arisen multiple times
First virus discovered: Tobacco Mosaic Virus (TMV): stunts growth of tobacco plants
Extracts from diseased tobacco plants could transmit disease to healthy tobacco plants. They filtered
out the bacteria so they knew it wasn’t from the bacteria but from a virus.
You can only see viruses by a electron microscope, so not by a normal microscope
Bacteriophages: 1.Eclipse phase (no virus inside or outside bacteria, no virions present yet)
(bacterium eaters) 2.Maturation phase (virions already formed inside cells but not released yet)
3.Latent phase = 1+2 (both phases together)
lysed bacteria don’t grow on agar: clear spots
Some facts about viruses: they infect ALL types of organisms, from animals and plants to bacteria,
present all over the world, the most abundant type of biological entity, we eat and breathe billions of
virion, we carry viral genomes as part of our own genetic material, viruses are small, but many!
Virus structure: The genetic material of a virus is
either DNA or RNA
Genetic material is in a protein capsid: helical or
icosahedral
Capsid protects the nucleic acid from chemical,
physical and enzymatic damage, especially when
,compared to the envelope protein which only consists of phospholipids. This can easily be destroyed
by detergents as soap.
Viruses are around 30-300 nm
The combination of nucleic acid and capsid is called the nucleocapsid
Some viruses are enveloped: enveloped viruses and viruses without envelope: naked viruses
The envelope of the virus consist of phospholipids.
A naked virus is composed of nucleic acid and the capsid.
Icosahedral: consist of triangles (20) and looks like a sphere
The complex viruses consist of a mix of helical and icosahedral (bacteriophages), head is icosahedral
and the ‘body’ is helical
The nucleic acids of viruses consist of double stranded DNA/RNA or single stranded DNA/RNA, both
can be linear of circular
RNA viruses are more likely to mutate than the DNA, no proofreading etc. The smaller the better
Most plant viruses are not enveloped and helical, as well as animal viruses. We need to know based
on pictures whether viruses are naked/enveloped, helical/icosahedral
Virus designs: Viruses use only a limited number of designs= Why? Repeating protein subunits
requires less genetic material. It is in the best interest of the virus to be as small as possible. So the
symmetry helps the virus to stay small
Capsid subunits (capsomeres) are arranged symmetrically around the viral genome for stability
Capsids are made of subunits (capsomeres)
•Necessity: A nucleic acid codes for 15% of its weight as a protein. A subunit with multiple small
proteins, require a smaller gene than one large protein. So you can better have 3 small ones than 1
large one
•Self-Assembly: the structure is self-ordered and therefore corresponds to a free energy minimum.
•Fidelity: the smaller the protein the less there should be transcribed/translated so the less chance
of an error occurring.
•Economy: if more small proteins form a larger subunit, incorrectly folded proteins can be easily
discarded (minimal waste)
Helical symmetry Icosahedral symmetry (the closest shape to the sphere, 20 triangles)
The most elementary viral globular capsid= icosahedron (second structure). Closest shape to a
sphere: most internal volume for the least amount of surface area, made of 20 triangles,
rearrangement of identical asymmetrical subunits
Based on 2 fold axis, 3 fold axis and 5 fold axis it still stays the same figure
Every triangle can be made of 3 proteins 20 triangles so 20 x 3 = 60 proteins are used
The icosahedral capsid is made of 1 protein.
T=1, the smallest particle all subunits have the same arrangement (5 neighbours)
T=1: 60 capsid proteins
3 different proteins is 1 triangle so T=3, when there are 20 triangles this means 60 proteins, and in
, The larger the number of subunits the more stable the virus becomes. The larger the virus particle
and the bigger and more complex its genome can be.
Exam question:
- The capsid is composed 60 asymmetric units made of 3 protein (T=3), for a total of 180
capsid proteins (3 * 60)
- The capsid is composed 60 asymmetric units made of 4 proteins (T=4), for a total of 240
capsid proteins (4 * 60)
- The capsid is composed 60 asymmetric units made of 7 proteins (T=7), for a total of 420
capsid proteins (7 * 60)
T was the number of facets in a triangular shape: 1 triangular shape is made of 3 proteins
T=1 gives 1 x (3 (proteins) x 20 (faces)) = 60
T=3 gives 3 x (3 (proteins) x 20 (faces)) = 180
T=4 gives 4 x (3 (proteins) x 20 (faces)) = 240
T=16 gives 16 x (3 (proteins) x 20 (faces)) = 960
T=25 gives 25 x (3 (proteins) x 20 (faces)) = 1500
Virus structures are not flat, they’re 3D structures
virions are metastable structures:
metastability: the ability of a non-equilibrium state to hold for a long time : e.g. diamonds
•must protect the genome (requires stability), must come apart upon infection, virions self-
assemble: This requires minimal energy and is reversible
head-tail morphology (tailed viruses)
function of virion proteins
•protection of the genome;
assembly of a stable protective protein shell
specific recognition and packaging of the nucleic acid
interaction with host cell membranes to form the
envelope (if applicable)
•delivery of the genome •additional functions
recognition and binding to host cell receptors ensure infectious cycle
uncoating of the genome ensure assembly
fusion with cell membranes interact with the host immune system
Naked viruses are composed only of the nucleocapsid. Enveloped viruses have a
layer of plasma membrane taken from the previous host cell that surrounds the
nucleocapsid. This layer of membrane is called the viral envelope. On the surface of
either the capsid of naked viruses or the viral envelope are special proteins and
glycoproteins called spikes. The spikes aid in attachment of the virus to a host cell
membrane.
Viral envelopes:
Some viruses have membranous envelopes that help them infect hosts. These viral envelopes
surround the capsids of influenza viruses and many other viruses found
in animals. Viral envelopes, which are derived from the host cell’s
membrane. Viral genome does not encode lipid synthesis
Enveloped virus particles: about 4nm thick lipid bilayer. Detergents or
organic solvents disrupt membranes and thus infectivity, because they
