VIROLOGY
Virology
Lecture 1 – Virus features and structure
Important:
• Try to get the big picture about virology, this is applicable to the whole field of virology.
• Don’t memorize all different individual viruses.
Virus
• A virus is an infectious, obligate in intracellular parasite.
• From Latin meaning ‘toxin or poison’
• They infect all living things.
• There are some many viruses around, so we constantly eat, breath viruses.
• Virion = infectious particle of the virus.
• Probably, viruses are beneficial for humans, because there are so many, and we still exist.
Viruses are NOT living cells, because …
Viruses are simple, because of 2 simple facts:
1. All viral genomes are obligate molecular parasites that can function only after they replicate
in a cell.
2. All viruses must make mRNA that can be translated by host ribosomes: they are all parasites
of the host protein synthesis machinery.
Keep in mind that …
• Viruses depend on their hosts to survive.
o If viruses are too successful and very quickly kill their hosts, they eliminate
themselves.
o SARS-Cov2 is right in the middle; not very successful, but also not successful. It only
makes the host sick after a few days but did not replicate.
o If viruses are too passive and the host can stop their growth, they eliminate
themselves.
1
, VIROLOGY
History of viruses
• Viruses are old, at least 300 million years.
• The origin of viruses is unclear:
o 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)
o In the late 1800s, researchers hypothesized
that a particle smaller than bacteria caused
the disease.
o Tobacco mosaic disease was discovered; the
leaves were colored when sick.
o They already knew that filters could be used
to filter micro-organisms, however, they
never heard of viruses before.
o He found that the cause of the disease could
not been seen under microscope, not be filtered.
o The extracted fluid of a sick plant could still make a healthy plant sick, but it was no
microorganism.
o They called it a contagious living fluid.
o In 1931 was the first electron microscopy developed. In 1935, they could then see
the viruses, while DNA and RNA were still unknown!
• In 1915 the first bacterial viruses were discovered and called bacteriophages. Also these
viruses were discovered by accident.
• In 1939 was a one-step growth experiment. They used a
lot of bacteria and took samples. If there was virus in the
supernatant (green) or pellet (blue). After 10 minutes
was an extremely burst of viruses secreted, and after 30
minutes all the viruses were made.
o 1. Eclipse phase: no virions found in the cell yet
o 2. Maturation phase: the virions mature inside
the viruses but are not released yet.
o 3. Latent phase: 1 + 2 phases together.
Most viruses are between 30 – 300 nm.
2
, VIROLOGY
Some facts and figures about viruses
• Greatest biodiversity on earth.
• Viruses are small, but there are many.
• They infect all types of organisms, from animals and plants to bacteria.
• The most abundant (overvloed) type of biological entity.
Virus structure
• The genetic material of a virus is either DNA or RNA. It can both be ds or ss.
• Genetic material is in a protein capsid: helical or icosahedral.
o Capsid protects the nucleic acid from chemical, physical and enzymatic damage.
• The combination of nucleic acid and capsid is called the nucleocapsid.
• Some viruses are enveloped: enveloped viruses.
• Viruses without envelope: naked viruses.
3
, VIROLOGY
Virus design
• Viruses use only a limited number of designs.
• Why?
o Repeating protein subunits requires less genetic material.
o Capsid subunits (capsomeres) are arranged symmetrically around the viral genome
for stability.
• It is in the best interest of the virus to be as small as possible.
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.
o The proteins are quite small, because a virus itself is also small.
• Self-assembly: the structure is self-ordered and therefore corresponds to a free energy
minimum.
o We need the minimum amount of energy to make the structure.
• Fidelity: the smaller the protein, the less chance of an error occurring.
o A smaller protein has a smaller chance of an error occurring.
• Economy: if more small proteins form a larger subunit, incorrectly folded proteins can be
easily discarded (minimal waste).
Virus design
• Viruses use only a limited number of designs
• Why?
o Capsomeres are identical subunits.
o Capsomeres are arranged symmetrically around the viral genome for stability.
o Repeating protein subunits requires less genetic material.
4
Virology
Lecture 1 – Virus features and structure
Important:
• Try to get the big picture about virology, this is applicable to the whole field of virology.
• Don’t memorize all different individual viruses.
Virus
• A virus is an infectious, obligate in intracellular parasite.
• From Latin meaning ‘toxin or poison’
• They infect all living things.
• There are some many viruses around, so we constantly eat, breath viruses.
• Virion = infectious particle of the virus.
• Probably, viruses are beneficial for humans, because there are so many, and we still exist.
Viruses are NOT living cells, because …
Viruses are simple, because of 2 simple facts:
1. All viral genomes are obligate molecular parasites that can function only after they replicate
in a cell.
2. All viruses must make mRNA that can be translated by host ribosomes: they are all parasites
of the host protein synthesis machinery.
Keep in mind that …
• Viruses depend on their hosts to survive.
o If viruses are too successful and very quickly kill their hosts, they eliminate
themselves.
o SARS-Cov2 is right in the middle; not very successful, but also not successful. It only
makes the host sick after a few days but did not replicate.
o If viruses are too passive and the host can stop their growth, they eliminate
themselves.
1
, VIROLOGY
History of viruses
• Viruses are old, at least 300 million years.
• The origin of viruses is unclear:
o 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)
o In the late 1800s, researchers hypothesized
that a particle smaller than bacteria caused
the disease.
o Tobacco mosaic disease was discovered; the
leaves were colored when sick.
o They already knew that filters could be used
to filter micro-organisms, however, they
never heard of viruses before.
o He found that the cause of the disease could
not been seen under microscope, not be filtered.
o The extracted fluid of a sick plant could still make a healthy plant sick, but it was no
microorganism.
o They called it a contagious living fluid.
o In 1931 was the first electron microscopy developed. In 1935, they could then see
the viruses, while DNA and RNA were still unknown!
• In 1915 the first bacterial viruses were discovered and called bacteriophages. Also these
viruses were discovered by accident.
• In 1939 was a one-step growth experiment. They used a
lot of bacteria and took samples. If there was virus in the
supernatant (green) or pellet (blue). After 10 minutes
was an extremely burst of viruses secreted, and after 30
minutes all the viruses were made.
o 1. Eclipse phase: no virions found in the cell yet
o 2. Maturation phase: the virions mature inside
the viruses but are not released yet.
o 3. Latent phase: 1 + 2 phases together.
Most viruses are between 30 – 300 nm.
2
, VIROLOGY
Some facts and figures about viruses
• Greatest biodiversity on earth.
• Viruses are small, but there are many.
• They infect all types of organisms, from animals and plants to bacteria.
• The most abundant (overvloed) type of biological entity.
Virus structure
• The genetic material of a virus is either DNA or RNA. It can both be ds or ss.
• Genetic material is in a protein capsid: helical or icosahedral.
o Capsid protects the nucleic acid from chemical, physical and enzymatic damage.
• The combination of nucleic acid and capsid is called the nucleocapsid.
• Some viruses are enveloped: enveloped viruses.
• Viruses without envelope: naked viruses.
3
, VIROLOGY
Virus design
• Viruses use only a limited number of designs.
• Why?
o Repeating protein subunits requires less genetic material.
o Capsid subunits (capsomeres) are arranged symmetrically around the viral genome
for stability.
• It is in the best interest of the virus to be as small as possible.
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.
o The proteins are quite small, because a virus itself is also small.
• Self-assembly: the structure is self-ordered and therefore corresponds to a free energy
minimum.
o We need the minimum amount of energy to make the structure.
• Fidelity: the smaller the protein, the less chance of an error occurring.
o A smaller protein has a smaller chance of an error occurring.
• Economy: if more small proteins form a larger subunit, incorrectly folded proteins can be
easily discarded (minimal waste).
Virus design
• Viruses use only a limited number of designs
• Why?
o Capsomeres are identical subunits.
o Capsomeres are arranged symmetrically around the viral genome for stability.
o Repeating protein subunits requires less genetic material.
4
