Module 5

Question 1

what was the first virus discovered?

Answer 1

tobacco mosaic virus

Question 2

who discovered viruses

Answer 2

Dimitri Ivanovsky

Question 3

who discovered bacteriophages

Answer 3

Felix d’Herelle

Question 4

who discovered yellow fever virus

Answer 4

Walter Reed in the US Army

Question 5

why was the discovery of the yellow fever virus unethical

Answer 5

Reed researched on military participants
put mosquitoes on healthy individuals to test if they caused it

Question 6

compare the size of viruses to bacteria

Answer 6

~1/100th the length of a bacterial cell
the genome is much smaller

Question 7

how does the DNA of viruses differ

Answer 7

some are encoded by RNA, others DNA
can be single-stranded or double-stranded
closed and circular or linear

Question 8

definition: capsid

Answer 8

protein shell that encircles the virus’ genome

Question 9

definition: nucleocapsid

Answer 9

capsid + genome

Question 10

definition: envelope

Answer 10

additional layer of the outside of the nucleocapsid (not always present)

Question 11

where does the material for viral envelopes come from

Answer 11

any membrane, e.g. plasma membrane, nuclear membrane, ER

Question 12

what are the capsid structures

Answer 12

helical (exact length required to enclose the genome)
icosahedral (20 triangles, each triangle is 3 linear polypeptides)
complex (e.g. syringe-like shape)

Question 13

what is the shape of the smallpox virus

Answer 13

shaped like dumbbells
enclosed within a circular membrane

Question 14

what are examples of enveloped viruses

Answer 14

viruses for:
smallpox
influenza
HIV
Ebola

Question 15

what are examples of non-enveloped viruses

Answer 15

poliovirus
tobacco mosaic virus

Question 16

what are the steps for virus replication

Answer 16

attach to host cell
enter the cell
uncoat
replicate genome
expression
assembly
exit

Question 17

what is needed for a virus to attach to the host cell

Answer 17

must recognize specific:
cell-surface proteins
proteins modified w/ specific sugars
polysaccharides

Question 18

what are ways viruses get into animal cells

Answer 18

enveloped virus: endocytosis or membrane fusion
non-enveloped virus: endocytosis

Question 19

describe endocytosis of viruses

Answer 19

1. virus attaches to specific cell receptor
2. endocytosis is initiated
3. virus is enclosed in an endosome

if virus is non-enveloped:
virus is released and uncoats to release the genome

if virus is enveloped:
low pH of the endosome initiates fusion of the viral envelope w/ the endosome membrane

Question 20

why is it harder to infect plant cells

Answer 20

viruses can’t get through cell wall
can only access the cell if the wall is damaged or broken externally

Question 21

how do viruses infect bacterial cells

Answer 21

1. tail fibers attach to receptors
2. conformational change in the tail fibers bring base of the tail in contact with the host cell surface
3. rearrangement of tail proteins allows inner core tube proteins to extend down into cell wall
4. contact with plasma membrane initiates transfer of DNA through a pore formed in the lipid bilayer

Question 22

definition: lytic/virulent phages

Answer 22

only undergo the lytic cycle

Question 23

definition: temperate phages

Answer 23

undergo both lytic and lysogenic cycles

Question 24

why is the lysogenic cycle advantageous

Answer 24

it allows the virus to be stable for many generation
doesn’t immediately lyse the cell

Question 25

definition: prophage

Answer 25

stably maintained viral genome within a cell

Question 26

what causes the cell to return back to the lytic cycles

Answer 26

- some form of stress, e.g. exposure to UV light, DNA damage - influx of nutrients, cells begin dividing

Question 27

what are the theories for the origin of viruses

Answer 27

coevolution hypothesis: viruses have been around for a long time, may have been encoded before cellular life regressive hypothesis: were originally cellular organisms, lost genes they no longer needed progressive hypothesis: the right combinations of genetic material were put together (plausible b/c plasmids exist)

Question 28

how do you cultivate bacteriophages

Answer 28

1. small of volume of susceptible bacterial host cells are added to the phage sample 2. mixture is added to the molten nutrient agar and quickly mixed 3. agar mixture is poured onto a nutrient agar base and allowed to solidify 4. plaques appear after sufficient incubation

Question 29

what are ways to cultivate animal viruses

Answer 29

- prepare the virus in fertilized eggs of chicken or ducks - grown in tissue culture - many animal virus cultivation methods are dependent on the first human cell line, HeLa cells

Question 30

what are ways to purify viruses

Answer 30

filtration differential centrifugation gradient centrifugation

Question 31

what are ways to quantify viruses

Answer 31

direct count hemagglutination assay plaque assay endpoint assay: TCID50 and LD50

Question 32

where are virus particles in a gradient centrifugation

Answer 32

beneath cell debris

Question 33

how is a gradient centrifugation made

Answer 33

successively filling layers of decreasing concentration of sucrose

Question 34

which ways of quantifying viruses differentiates between infectious and noninfectious ones

Answer 34

plaque assay endpoint assay

Question 35

how are viruses historically named

Answer 35

simple letter/number combinations organisms they infect place they were discovered appearance disease caused

Question 36

what are standardized ways of naming

Answer 36

International Committee on Taxonomy of Viruses (ICTV) Baltimore Classification System

Question 37

what are some characteristics involved with ICTV naming

Answer 37

virion morphology genome structure biological features

Question 38

what is the Baltimore Classification System based on

Answer 38

based on the viruses genome there are 7 classes that encompass all possible genome characteristics

Question 39

what are ways to identify viruses

Answer 39

observe under an electron microscope nucleic acid analysis

Question 40

what are some virus-like particles

Answer 40

viroids - RNA with some spectacular secondary structure prions - proteinaceous infectious particles

Question 41

how is virology used in medicine

Answer 41

cancer-causing oncoviruses cancer-destroying oncolytic viruses gene therapy

Question 42

where are viroids found

Answer 42

only in plants

Question 43

what causes mad cow disease & Creutzfeldt-Jacob disease

Answer 43

prions

Question 44

what is CRISPR

Answer 44

a mechanism that bacteria use to prevent infection by viruses