Acellular microbes

Question 1

how many viruses are predicted to be on earth?

Answer 1

10^31

Question 2

what are viruses?

Answer 2

composed of protein and nucleic acid, lack metabolism and replicate only in host cells

Question 3

what are satellites?

Answer 3

subviral infectious agents composed of nucleic acid and protein that require a helper virus to replicate

Question 4

what are viroids?

Answer 4

non-translated ssRNAs, lack protein (Infect plants)

Question 5

what are prions?

Answer 5

infectious proteins, cause spongiform encephalopathies (BSE, scrapie, CJD)

Question 6

what are obligated intracellular pathogens?

Answer 6

incapable of replication outside host cell

Question 7

how do viruses multiply?

Answer 7

Viruses multiply by independent synthesis of their constituent parts which are then assembled to reconstitute new viruses particles, rather than by growth and division

Question 8

what is the size of a virion?

Answer 8

~10-400nm

Question 9

what is the structure of all virions?

Answer 9

contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid)

Question 10

what is the function of a capsid?

Answer 10

Protect viral genetic material and aids in its transfer between host cells

Question 11

what are capsids made from?

Answer 11

Made of protein subunits called protomers

Question 12

what are the 3 types of capsids?

Answer 12

Capsids are helical, icosahedral, or complex

Question 13

what are helical capsids?

Answer 13

• Shaped like hollow tubes with protein walls
• Protomers self assemble
• Size of capsid is a function of nucleic acid

Question 14

what is an icosahedral capsid?

Answer 14

An icosahedron is a regular polyhedron with 20 equilateral faces and 12 vertices

Question 15

what are capsids of complex symmetry?

Answer 15

Some viruses do not fit into the category of having helical or icosahedral capsids

Question 16

what are viral envelopes?

Answer 16

• Many viruses are bound by an outer, flexible, membranous layer called the envelope
• Animal virus envelopes (lipids and carbohydrates) usually arise from host cell plasma or nuclear membranes

Question 17

what are viral envelopes made from? function?

Answer 17

• Envelope proteins, which are viral encoded, may project from the envelope surface as spikes or peplomers
• Many involved in viral attachment to host cell or in entry process

Question 18

what is viral genome made up of?

Answer 18

• Virus genome is composed of nucleic acid
• either DNA or RNA (NOT both)
• single or double stranded; segmented or circular
• Encode viral proteins, may also include regulatory elements

Question 19

what is Baltimore classification of viruses based on?

Answer 19

viral genome structure AND replication strategy

Question 20

what are the 7 classifications in Baltimores classification system?

Answer 20

• dsDNA viruses
• ssDNA viruses
• dsRNA viruses
• Positive sense ssRNA viruses
• Negative sense ssRNA viruses
• RNA reverse transcribing viruses
• DNA reverse transcribing viruses

Question 21

what are the 6 steps to the viral replication cycle?

Answer 21

1. Attachment (via binding specific host receptor)
2. Penetration / uncoating of nucleocapsid
3. Transcription & translation of viral proteins
4. Replication of viral genome
5. Assembly of viral progeny
6. Release

Question 22

what is the virus entry mechanism for enveloped viruses?

Answer 22

Membrane fusion at cell surface
OR
Receptor mediated endocytosis followed by membrane fusion in endosome

Question 23

what is the virus entry mechanism for nonenveloped viruses?

Answer 23

Receptor mediated endocytosis and uncoating or direct injection of nucleic acid

Question 24

how is the viral genome replicated in dsDNA viruses?

Answer 24

usually in host cell nucleus (except poxviruses, bacteriophages), using host DNA polymerase, typical flow of information dsDNA ➢mRNA ➢ protein

Question 25

how is the viral genome replicated in ssDNA viruses?

Answer 25

must convert ssDNA to dsDNA first

Question 26

how is the viral genome replicated in RNA viruses?

Answer 26

must use virally encoded enzymes to replicate genome

Question 27

what is the polarity of mRNA?

Answer 27

mRNA is a ‘Positive sense’ RNA molecule - can be translated by the ribosome into protein

Question 28

which polarity of RNA cannot be translated by ribosomes into proteins?

Answer 28

Anti-sense or ‘negative sense’ RNA

Question 29

what is the function of replication complexes?

Answer 29

Helps to avoid immune response and / or concentrate viral components to help assembly

Question 30

where does baceriophage assembly occur?

Answer 30

in cytoplasm

Question 31

where are the 2 places that animal viruses can be assembled?

Answer 31

• nucleus
• cytoplasm

Question 32

how are non-enveloped virions released from the host cell?

Answer 32

typically lyse the host cell

Question 33

how are enveloped virions released from the host cell?

Answer 33

usually released by budding

Question 34

what is the other way virion are released from the host cell?

Answer 34

Some viruses disseminate via direct cell to cell movement

Question 35

what are the 6 steps in the SARS-CoV-2 replication cycle?

Answer 35

1. Attachment
2. Uncoating
3. Production of viral proteins
4. Replication of viral genome
5. Assembly
6. Release

Question 36

what are virophages?

Answer 36

viruses that appear to infect other viruses

Question 37

what are satellites?

Answer 37

Sub-viral agents: defined as “entities that are smaller than viruses but with virus-like properties” and “which lack genes that could encode functions needed for replication”

Question 38

what does a satellite require for replication?

Answer 38

a helper virus

Question 39

what are viroids?

Answer 39

Infectious agents composed of closed, circular ssRNAs

Question 40

what does viroid replication require?

Answer 40

Replication requires host cell DNA-dependent RNA polymerase

Question 41

what are prions?

Answer 41

Infectious proteins – abnormally folded version of normal brain prion protein