Lecture 26: Virology I

Question 1

What is the definition of a virus?

Answer 1

An infectious, obligate intracellular parasite comprising genetic material surrounded by a protein coat (capsid) and, in some cases, a lipid membrane (envelope)

Question 2

Why are viruses considered as parasites?

Answer 2

Because they need to infect a cell in order to reproduce.

Question 3

What are glycoproteins?

Answer 3

They are protein spikes embedded in the lipid envelope that some viruses have.

Question 4

Are viruses alive?

Answer 4

No.

Question 5

Name 4 reasons why viruses are not considered to be living.

Answer 5

1. They do not reproduce on their own.
2. They only multiply from the assembly of pre-formed components made in the infected cell.
3. They cannot produce ATP on their own
4. They are not free-living, as they cannot reproduce outside of a cell.

Question 6

What are the 2 phases of viruses?

Answer 6

The virion, when it exists in the environment as an infectious particle, and the infected cell.

Question 7

What is the origin of viruses?

Answer 7

There is not definitive explanation for the origin of viruses, only hypotheses.

Question 8

Name 3 possible hypotheses for the origin of viruses.

Answer 8

1. From mobile genetic elements that gained the ability to move between cells
2. From free-living organisms that adapted a parasitic replication strategy
3. They may have evolved before cells and led to the evolution of cellular life.

Question 9

How do viruses replicate?

Answer 9

They replicate by assembly of pre-formed components into many particles. Once in the infected cells, they make the parts and assemble the final product.

Question 10

How does the growth curve of viruses differ from that of bacteria?

Answer 10

Viruses do not display an exponential growth phase, unlike bacteria.

Question 11

What are metagenomics?

Answer 11

It is a sequence analysis of genetic material from environmental samples.

Question 12

What are metagenomics used for?

Answer 12

They can be used to estimate the number of viruses on earth.

Question 13

Are viruses or bacteria more prevalent?

Answer 13

Viruses outnumber bacteria in every habitat.

Question 14

How many bacteriophages are present in the world’s waters?

Answer 14

More than 10^30

Question 15

How much of the species abundance and biomass on living beings do viruses make up?

Answer 15

Abundance: they comprise 94% of species
Biomass: they are small, so they only comprise 5% biomass

Question 16

Are the majority of viruses pathogenic?

Answer 16

No, most have no impact on our health. But some are, such as herpes.

Question 17

Explain how climate change impacts viral transmission.

Answer 17

Climate change is expected to bring over 15000 new causes of mammal transmitting viruses to other mammals, as species are moving to cooler locations due to rising temperatures and can interact with other species for the first time.

Question 18

Name 3 important roles that microorganisms play in marine ecosystems.

Answer 18

1. They are the main actors of the global carbon and ocean nutrient cycles
2. They promote the regeneration of CO2 and sequestration of CO2 in marine sediments
3. They kill 20% of marine biomass each day, particularly harmful algal blooms.

Question 19

How big are most viruses?

Answer 19

20-300 nm

Question 20

How does the size of viruses compare to other organisms?

Answer 20

They are smaller than bacteria but bigger than cell organelles.

Question 21

How can viral pathogens be isolated from bacteria?

Answer 21

Via filtration, as viruses can pass through 0.2 micron filters that bacteria cannot.

Question 22

What kinds of media can viruses reproduce in?

Answer 22

Only in the host, not in broth (unlike bacteria)

Question 23

When were the first human virus and yellow fever virus discovered?

Answer 23

1901

Question 24

When was the rabies virus discovered?

Answer 24

1903

Question 25

When was the variola virus discovered?

Answer 25

1906

Question 26

When were chicken leukemia virus and poliovirus discovered?

Answer 26

1908

Question 27

What was rous sarcoma virus discovered?

Answer 27

1911

Question 28

When were bacteriophages discovered?

Answer 28

1915

Question 29

When was influenza virus discovered?

Answer 29

1933

Question 30

What was the first giant virus discovered and when was it founded?

Answer 30

Mimivirus, discovered in 2003

Question 31

How big is mimivirus?

Answer 31

400 nm capsid with protein filaments extending 100 nm from the surface.

Question 32

What phylum do giant viruses belong to?

Answer 32

Nucleocytoviricota.

Question 33

Giant viruses are also known as […]. Where do they replicate?

Answer 33

Nucleocytoplasmic large DNA viruses (NLDV). They can replicate both in the cell’s cytoplasm and nucleus.

Question 34

How big are giant viruses compared to bacteria?

Answer 34

They can be just as large as bacteria.

Question 35

What type of cells can giant viruses infect? Give 3 examples.

Answer 35

Eukaryotic unicellular hosts, such as amoebas, algae, and protists.

Question 36

What is the hypothesis about the role of giant viruses in evolution?

Answer 36

It is likely that they were small viruses that picked up genes by lateral transfer from bacteria or viruses with an intra-amoebal lifestyle.

Question 37

What are virophages? What type of genome do they have?

Answer 37

They are small DNA viruses that replicate only in cells infected with a giant virus. They use the viral replication factory established by the giant virus.

Question 38

The first virophage discovered was called […]

Answer 38

Sputnik.

Question 39

What is the nature of the relationship between virophages and the co-infecting giant virus?

Answer 39

It is a parasitic relationship. Since viruses interfere with the giant viruses, the virophages can actually improve the survival of the host.