Ch. 13 Viruses, Viroids, and Prions

Question 1

characteristics of viruses

Answer 1

• nonliving
• obligate intracellulare parasites
• can infect organisms of each domain
• commonly referred to by host range

Question 2

structures of viruses

Answer 2

• nucleic acid (DNA OR RNA)

- protein coat (capsid)

Question 3

capsid subunits

Answer 3

capsomeres

Question 4

virion

Answer 4

complete virus

Question 5

enveloped virus

Answer 5

steals some plasma or cell membrane from a host cell

• adds viral spikes
• necessary for this type of virus; they cannot survive without their envelope

Question 6

polyhedral virus

Answer 6

shape geometric (polyhedral)

Question 7

helical virus

Answer 7

long capsomeres, curve and loop;

e.g. ebola

Question 8

complex virus

Answer 8

odd shapes

• phage head, machinery to inject
• usually have DNA (no RNA)
• T-even bacteriophage
• when empty = ghost

Question 9

taxonomy of viruses

Answer 9

virions are classified by structures.

Question 10

viral multiplication growth curve

Answer 10

• one step growth curve
• eclipse period (virion number decrease to zero – all inside host cells)
• virions skyrocket in acute infection
• virions drop as immune system kicks in or death occurs

Question 11

two cycles of bacteriophage multiplication

Answer 11

• lytic cycle

- lysogenic cycle

Question 12

LYTIC CYCLE

Answer 12

(1) attachment: phage attach to host cell on receptor (receptor is for a different purpose)
(2) penetration: sheath contracts; phage penetrates host cell and injects DNA (becoming a ghost)
(3) biosynthesis: viral DNA replication; host DNA is fragmented; synthesis of viral components by host cell
(4) maturation: viral components assemble into virions (specific order)
(5) release: host cell lyses; new virions released. burst size = number of bacteriophages released; usually 100

Question 13

LYSOGENIC CYCLE

Answer 13

follow lytic cycle until step 2.

(3a) phage DNA integrates with bacterial chromosome by recombination, thus becoming a prophage.

bacteria is quiescent; may benefit cell to have new characteristics

(4b) lysogenic bacterial reproduce normally, with many cell divisions.
(5b) occasionally, prophage may excise from bacteriachromosomes (usually by radiation), initiating the lytic cycle

Question 14

temperate phage

Answer 14

lysogenic phage (doesn't kill host).
can do either lysogenic or lytic cycles, depending on the amount of energy available. if less energy is available, it will become a prophage.

Question 15

Specialized transduction (e.g. gal gene)

Answer 15

(1) prophage exists in galactose-using host (has gal gene)
(2) phage genome excises into lytic phase with adjacent gal gene
(3) phage matures and cell lyses, releasing phage with gal gene
(4) phage infects a cell without gal gene
(5) gal and prophage integrate into new host DNA
(6) lysogenic cell can now metabolize gal!

only transfers genes on either side of prophage.

Question 16

DNA Animal Virus Replication

Answer 16

(1) attachment to cell – enveloped virus fuses to plasma membrane OR receptor-mediated endocytosis
(2) entry and uncoating – whole virion taken up; cell takes apart capsids, viral DNA enters nucleus
(3) biosynthesis – viral DNA transcribed into early proteins, DNA replicates, capsid proteins are made last
(4) maturation
(5) release (host cell dies eventually)

Question 17

RNA Animal Viruses (Retroviruses) are made up off..

Answer 17

RNA and 3 viral enzymes: reverse transcriptase, integrase, protease

Question 18

RNA Animal Virus Replication

Answer 18

(1) attachment and entry
(2) uncoating
(3) reverse transcriptase makes double stranded molecule of DNA from viral RNA
(4) integrase integrates viral DNA into a host cell chromosome, becoming a provirus. can remain quiescent/latent for a long time.
(5) transcription of provirus: makes RNA etc for next virus. It is polysystronic and all translated in one go.
(6) protease processes/dissembles long viral proteins
(7) assembly and release of virus; typically occurs through budding (takes piece of host cell plasma membrane)

Question 19

viral infection types are calssified by…

Answer 19

length of time.

Question 20

acute infection

Answer 20

few days

e.g. common cold, influenza

Question 21

latent infection

Answer 21

initial acute virus.
detectable virions drop to zero for months or years
another set of symptoms emerges years later

e.g. shingles, cold sores, leukemia

Question 22

persistent infection

Answer 22

initial acute virus.
virions gradually increase over months or years. eventually fatal

e.g. HIV, measles, cervical cancer

Question 23

plant viruses vs viriods

Answer 23

viriods: “naked” RNA; cause severe plant diseases
virusoid: RNA enclosed in protein coat; only cause plant disease when cell is infected by another virus.

Question 24

prions

Answer 24

• protein alone, no genetic material.
• neurological diseases
• cannot be detected without autopsy; can mimic dementia, alzheimers, etc
• incubation can be decades
• originates genetically, from cannibalism, or eating infected beef

Question 25

PrPc

Answer 25

prion protein cellular: normal version of abnormal prion

Question 26

how are prions infectious if they are just protein?

Answer 26

(1) PrPc produced by cells secreted to cell surface as membrane-bound glycoproteins.
(2) PrPsc may be acquired OR produced by altered PrPc gene.
(3) PrPsc binds to normal PrPc on surface
(4) PrPc is converted to PrPsc by PrPsc (prion acting as an enzyme)
(5) continued conversion (cascade)
(6) PrPsc is taken up by the cell, accumulating in endosomes.

(7) endosomes eventually returned to surface; cell death occurs
- neurodegenerative
- brain cells die (e.g. spongiform)