LS10 Virology review
Lesson 1 Viruses
A virus is an infectious, obligate intracellular parasite
★ The Latin meaning of toxin/poison
★ Virion = the infectious particle
★ Viruses are not living things, but are complicated assemblies of molecules e.g. protein,
nucleic acids, lipids, and carbohydrates, and can do nothing until they enter a living cell
(host)
★ When a virus encounters a cell, a series of chemical reactions occur that lead to the
production of new viruses → these steps are passive, but are predefined by nature of the
molecules that comprise the virus particle
Viruses are simple
★ All viral genomes are obligate parasites that can function only after they replicate in a
cell
★ All viruses must make mRNA that can be translated by ribosomes: they are all parasites
of the host protein synthesis machinery once the host takes up the virus particle
Viruses depend on their host to survive
★ If viruses are too successful and quickly kill their hosts, they immediately eliminate
themselves too e.g. omicron
★ If viruses are too passive and the host can stop their growth, then they also eliminate
themselves
★ So end phase: viruses depend on their host
Viruses are at least 300 million years old
★ Origin is unclear
★ They May have evolved from plasmids or bacteria, but unlikely that all current viruses
evolved from a single progenitor so it is thought that viruses arise multiple times
The first virus discovered: Tobacco mosaic virus (TMV)
★ TM disease stunts the growth of
tobacco plants and gives their leaves a
mosaic coloration
★ In the late 1800s, researchers
hypothesized that a particle smaller than
bacteria caused the disease → extracts
from diseased tobacco plants
transmitted the disease to healthy ones
,There are several phases within a virus
★ Eclipse phase, here there are no viral particles yet
★ Maturation phase, here only cell-associated viruses are present
★ Latent phase, the release of the virus
Virus structure
★ Their genetic material either contains DNA or RNA, the genetic material is surrounded
by a protein capsid (helical or icosahedral)
★ The capsid protects the nucleic acid from chemical, physical and enzymatic damage
★ Nucleocapsid = nucleic acid + capsid
★ Some viruses are enveloped and some are not = naked virus
Viruses use only a limited number of designs
★ Because repeating protein subunits require less genetic material
★ Capsid subunits (capsomers) are arranged symmetrically around the viral genome for
stability
★ Isohedrical has 60 subunits, and there are 20 equal triangles made of 7 proteins (T=7),
so you have a total of 420 capsid proteins (7*60=420) *EXAM Q*
Capsids are made of subunits (capsomers)
★ Necessity: nucleic acid codes for 15% of its weight as a protein, a subunit with multiple
small proteins requires a smaller gene instead of a large protein
★ Self-assembly: the structure is self-ordered and therefore corresponds to a free energy
minimum
★ Fidelity: the smaller the protein, 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)
, Virions are metastable structures
★ Metastability = the ability of a non-equilibrium state to hold for a long time e.g.
diamonds
★ As they must protect the genome and come apart upon infection
★ They self-assemble and deliver the genome to interact with the host
Viral envelopes
★ Naked viruses are composed of only the nucleocapsid
★ Enveloped viruses have a layer of plasma membrane taken from the previous host cell
that surrounds the nucleocapsid = viral envelope, so they are derived from the host cell’s
membrane
★ On the surface of either the capsid of naked viruses or viral envelope are special proteins
and glycoproteins = spikes, they add to the attachment of the virus to a host cell
membrane
★ The viral genome does not encode lipid synthesis
Lesson 1 Viruses
A virus is an infectious, obligate intracellular parasite
★ The Latin meaning of toxin/poison
★ Virion = the infectious particle
★ Viruses are not living things, but are complicated assemblies of molecules e.g. protein,
nucleic acids, lipids, and carbohydrates, and can do nothing until they enter a living cell
(host)
★ When a virus encounters a cell, a series of chemical reactions occur that lead to the
production of new viruses → these steps are passive, but are predefined by nature of the
molecules that comprise the virus particle
Viruses are simple
★ All viral genomes are obligate parasites that can function only after they replicate in a
cell
★ All viruses must make mRNA that can be translated by ribosomes: they are all parasites
of the host protein synthesis machinery once the host takes up the virus particle
Viruses depend on their host to survive
★ If viruses are too successful and quickly kill their hosts, they immediately eliminate
themselves too e.g. omicron
★ If viruses are too passive and the host can stop their growth, then they also eliminate
themselves
★ So end phase: viruses depend on their host
Viruses are at least 300 million years old
★ Origin is unclear
★ They May have evolved from plasmids or bacteria, but unlikely that all current viruses
evolved from a single progenitor so it is thought that viruses arise multiple times
The first virus discovered: Tobacco mosaic virus (TMV)
★ TM disease stunts the growth of
tobacco plants and gives their leaves a
mosaic coloration
★ In the late 1800s, researchers
hypothesized that a particle smaller than
bacteria caused the disease → extracts
from diseased tobacco plants
transmitted the disease to healthy ones
,There are several phases within a virus
★ Eclipse phase, here there are no viral particles yet
★ Maturation phase, here only cell-associated viruses are present
★ Latent phase, the release of the virus
Virus structure
★ Their genetic material either contains DNA or RNA, the genetic material is surrounded
by a protein capsid (helical or icosahedral)
★ The capsid protects the nucleic acid from chemical, physical and enzymatic damage
★ Nucleocapsid = nucleic acid + capsid
★ Some viruses are enveloped and some are not = naked virus
Viruses use only a limited number of designs
★ Because repeating protein subunits require less genetic material
★ Capsid subunits (capsomers) are arranged symmetrically around the viral genome for
stability
★ Isohedrical has 60 subunits, and there are 20 equal triangles made of 7 proteins (T=7),
so you have a total of 420 capsid proteins (7*60=420) *EXAM Q*
Capsids are made of subunits (capsomers)
★ Necessity: nucleic acid codes for 15% of its weight as a protein, a subunit with multiple
small proteins requires a smaller gene instead of a large protein
★ Self-assembly: the structure is self-ordered and therefore corresponds to a free energy
minimum
★ Fidelity: the smaller the protein, 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)
, Virions are metastable structures
★ Metastability = the ability of a non-equilibrium state to hold for a long time e.g.
diamonds
★ As they must protect the genome and come apart upon infection
★ They self-assemble and deliver the genome to interact with the host
Viral envelopes
★ Naked viruses are composed of only the nucleocapsid
★ Enveloped viruses have a layer of plasma membrane taken from the previous host cell
that surrounds the nucleocapsid = viral envelope, so they are derived from the host cell’s
membrane
★ On the surface of either the capsid of naked viruses or viral envelope are special proteins
and glycoproteins = spikes, they add to the attachment of the virus to a host cell
membrane
★ The viral genome does not encode lipid synthesis
