L5 - Viruses and their global impact

Question 1

How were viruses first discovered?

Answer 1

• Through Tobacco Mosaic Disease
• Novel infectious agent that could pass through bacteria-retaining porcelain filters
• Coined VIRUS by Martin Beijerinck (1898)

Question 2

Define a virus

Answer 2

• Mostly occur as particles called virions
• Set of genomic nucleic (DNA or RNA) molecule(s)
• Normally encased in protective coat/coats of protein or lipoprotein
• Able to mediate its own replication only within suitable host cells

Question 3

Describe virus replication in infected cells in three points

Answer 3

• Dependent on host cell’s protein synthesising machinery
• Assembled from pools of raw materials in cell
• located at sites not separated from cell contents by lipoprotein bilayer membrane e.g. replication in cytoplasm

Question 4

Describe the shape of proteins

Answer 4

• Generally the coat proteins arranged in geometrically definable forms
• e.g. helical or spheres

Question 5

What’s the range in genome size of viruses compared to cells?

Answer 5

• Mostly about 1 million times smaller than host genome
• Generally orders of magnitude smaller than bacteria
• ‘Giant’ virus genomes e.g. Mimivirus can overlap w/ small bacterial genomes

Question 6

Can viruses be defined phylogenetically?

Answer 6

• Hard to define phylogenically
• Have arisen multiple times independently
• Better defined functionally

Question 7

Name some classes of viruses based on what they infect

Answer 7

• Phage (infect bacteria, most abundant)
• Cyanophage (infect cyanobacteria)
• Mycovirus (infects true fungi or oomycetes)

Question 8

Describe how viruses can be classified based on their genomes

Answer 8

Baltimore classification:
- broad groupings based on genome type and replication strategy
- dsDNA, dsRNA, ssDNA
- (+)ssRNA = converted directly into viral proteins in replication
- (-)ssRNA = must convert to (+)ssRNA first
- reverse transcription (i.e. from DNA to RNA to DNA or vice versa) in retroviruses

• most phage = dsDNA
• most plants = (+)ssRNA
• most fungi = dsRNA

Question 9

Do viruses always form parasitic symbioses?

Give some examples

Answer 9

• No, can also form mutualistic interactions
• E.g. root colonising fungus infected by virus allows growth of ‘Panic grass’ at high temperatures
• Similar response when transferred to tomato - fungi + virus needed
• Endogenous retrovirus genetic elements co-opted by placental vertebrates
• Crucial in forming of placenta

Question 10

How abundant are viruses in the oceans?

What groups are most of them?

Answer 10

• Most abundant and variable biological entities in ocean (and whole earth)
• Seen through next generation meta-genomics
• Conservative = 10^30 virions
• Mostly phage-like, as prokaryotes are most abundant cellular microorganisms

Question 11

How are viruses essential to the maintenance of major biochemical cycles? Outline in 5 steps

Sketch the relevant diagram

Answer 11

The Viral Shunt:
- mass lysis of cellular biomass by viral infection
- stimulated prokaryotic productivity
- allows microbial life at great depths
- recycles C to surface and atmosphere
- ensures most CO2 absorbed by ocean is returned = continuation of photosynthesis

• See diagram on pg 32