15. Virology I: Introduction to Viruses

Question 1

what are viruses?

Answer 1

• acellular organisms
• genomes consist of nucleic acid
• obligately replicate inside host cells
  – using host metabolic machinery and ribosomes
• form pool of components assembling particles (virions)
  – protect the genome and transfer it to other cells

Question 2

how do viruses replicate

Answer 2

-obligately replicate inside host cells

• use host etabolic machinery and ribosomes

Question 3

what do viruses form, and what are the functions?

Answer 3

• a pool of components
• assemble into virions (particles)
• protect the genome
  – and transfer it into other cells

Question 4

what are the simple definitions of viruses?

Answer 4

-Viruses are acellular organisms with nucleic acid genomes, which make particles to protect the genome and transfer it between cells.

• An infectious, obligate intracellular parasite comprising genetic material (DNA or RNA) surrounded by a protein coat and/or a membrane.

Question 5

how big are viruses?

Answer 5

• bacterial
  – 1 micron
• lambda phage
  – 0.1 micron
• animal cell
  – 10 micron

Question 6

what do viruses consist of?

Answer 6

• acellular
• genetic material core
  – DNA / RNA enclosed by coat of proteins
• enveloped viruses (animal viruses)
  – enclosed by lipid envelope

Question 7

how are viruses classified and reproduce?

Answer 7

• reproduce in living cells
• classified:
  – nucleic acid characteristics
  – capsid symmetry
  – host
  – diseases caused

Question 8

what are the structural differences between non-enveloped / enveloped viruses?

Answer 8

• capsomers / envelope
• nucleocapsid
  – nucleic acid / DNA, RNA
  – capsid composed of capsomers / capsid

Question 9

why are viruses considered as dead?

Answer 9

• don’t:
  – metbolise
  – have cells
  – reproduce independently

Question 10

how do viruses not replicate independently?

Answer 10

• inert packages of nucleic acid
  – can’t replicate without host cell

Question 11

what traits define viruses as living?

Answer 11

• made of same building blocks
• replicate and evolve
• inside cell, they engineer environment suitably

Question 12

how are viruses non-living / living?

Answer 12

• alternate between inactive state (out of cell) and living
• metabolically active sate (inside cell)
  – virocell

Question 13

why are viruses important?

Answer 13

• greates biodiversity on earth
• drive global cycles
• beneficial
• pathogens

Question 14

how do viruses drive global cycles?

Answer 14

• carbon, nitrogen, sulphur cycles
• facilitate genetic transfer between different cell types

Question 15

how are viruses beneficial?

Answer 15

• control of pests

Question 16

how are pathogens caused by viruses important?

Answer 16

• main cause of death and illness

Question 17

what are the abundances of organisms?

Answer 17

• biomass:
  – mostly prokaryotes
  – 5% viruses
  – least protists
• abundance:
  – 94% viruses
  – prokaryotes
  – least protists

Question 18

why are we not always infected?

Answer 18

• most viruses pass through us
  – ingest many non-animal viruses with food
• example:
  – leafy vegetables (cabbage) contaminated with baculoviruses
  – 100 cm2 of leaf material contain 10*E8 particals of virus pathogenic for cabbage looper

Question 19

how do we classify viruses?

Answer 19

• nature and sequence of nucleic acid encapsidated
• symmetry of capside (protein shell)
• presence / absence of envelope (lipid membrane)
• dimensions of virion and capsid

Question 20

what rule do viruses follow?

Answer 20

• viral genomes make mRNA ably read by host ribosomes

Question 21

what terminology is used with DNA and RNA?

Answer 21

• DNA:
  – single stranded
  –double stranded
• RNA:
  – single stranded
  – double stranded

Question 22

what are the differences between single and double stranded RNA / DNA?

Answer 22

• single stranded:
  – linear / circular DNA
  – linear +ve / linear -ve / linear segmented RNA
• double stranded:
  – linear / linear with single chain breaks / circular DNA
  – linear segmented RNA

Question 23

draw the diagram of retro/DNA/RNA viruses

Answer 23

-

Question 24

what are the main components of the human genome?

Answer 24

• introns (26%)
  – protein coding genes (1.5%)
  – dispersed by non-coding introns
• LINEs (20%)
• SINEs (13%)
• miscellaneous unqiue sequences (12%)
• LTR retrotansponsons (8%)
• miscellaneous heterochromatin (8%)
  – remnants of retroviruses (fossil relics of former viral infections)
• segmental duplications (5%)
• DNA transposons (3%)
• simple sequence repeats (3%)

Question 25

what are retroviruses?

Answer 25

• type of RNA virus
  – inserts copy of genome into DNA of host cell (changing genome of cell)
• achieved through reverse transcriptase
  – many errors made: non-infectious/attenuated virions or more infections virions)
• virions consist of enveloped particles (100 nm diameter)
  – obtained from host plasma membrane during budding process
• virions contain two identical single-stranded RNA molecules

Question 26

draw retrovirus infection and reverse transcription

Answer 26

-

Question 27

coronaviruses

Answer 27

-

Question 28

what are the discrete virus phases?

Answer 28

• virion
  – infectious particle
• infected cell

Question 29

what is the mimivirus?

Answer 29

• Acanthamoeba polyphaga mimivirus (APMV)
• isolated from amoebal co-culture present in a water sample from cooling tower
• characteristics:
  – DNA virus
  – 700 nm diameter
  – genome approxiamately 1.2 MB (million base-pairs)
• structure:
  – fibrils
  – capsid
  – inner membrane
  – core
• core:
  – dsDNA virus
  – 90% coding capacity
  – 10% junk DNA
  – 1.2 MB
  – 911 protein coding genes
  – additional genes (aminoactyl tRNA synthetases; sugar; lipid; amino acid metabolism)

Question 30

draw the replication graph of a virus

Answer 30

-

Question 31

what is the pandoravirus

Answer 31

• pandoravirus salinus
  – from mouth of Chile’s Tunquen River
• pandoravirus dulcis
  – freshwater pond near Melbourne
• parasitize amoebas
• sizes
  – 1 micrometer in length
  – dwarfs: 50 - 100 nm
• supersize DNA
  – 2500 genes (compared to 10 in many viruses)

Question 32

what is the pithovirus?

Answer 32

• Pithovirus sibericum
  – zombie virus
• 1.5 micrometer length, 0.5 micrometer diameter
• 500 genes
• over 30 000 years in ancient permafrost
  – when exposed to host (amoebas), still active and quickly infected host
• giant viruses:
  – very tough,
  – special environments (deep ocean sediments, permafrost - preserve microbes, viruses)
• climate change exposes potential viruses in ancient ice