3.31

Question 1

Viruses (4)

Answer 1

• simple organization
• DNA or RNA genome
• unable to reproduce outside of
living cells
• obligate intracellular parasites

Question 2

Cellular Organisms (4)

Answer 2

• complex organization
• DNA genome and RNAs
• carry out cell division
• some are obligate intracellular
parasites

Question 3

FIGURE 1 • Theories of viral
origin. Three theories have been
proposed to account for the origin of
viruses.

Answer 3

A. According to the theory
of reductive origin, viruses evolved
by degenerative evolution from
intracellular parasitic cells. 
B. According
to the theory of intracellular origin,
viruses evolved from functional parts
of cells that acquired an ability to
reproduce themselves uncontrolled by
the cell. 
C. The theory of independent
origin holds that viroid nucleic acids
could have evolved outside of cells
during the "RNA world" and acquired
the ability to infect cells.

Question 4

— virus is bigger by far than ant other known virus, and rivals bacteria

Answer 4

pandora

~1000 nm

Question 5

Multicomponent viruses genomes are segmented and the

segments are distributed into

Answer 5

separate viral particles

Question 6

Cultivation of viruses requires inoculation of

living host cell (4)

Answer 6

• suitable animals
• embryonated eggs
• tissue (cell) cultures
• cytopathic effects (CPEs)

Question 7

tissue (cell) cultures (3)

Answer 7

– monolayers of animal cells
– plaques
• localized area of cellular destruction and lysis

Question 8

cytopathic effects (CPEs) (1)

Answer 8

– microscopic or macroscopic degenerative changes or

abnormalities in host cells and tissues

Question 9

HERPES: — CAPSID SYMMETRY

Answer 9

ICOSAHEDRAL

Question 10

bacteriophage — capsule

Answer 10

T4

Question 11

Viruses are classified using a

combination of properties: (4)

Answer 11

• Type and structure of nucleic acid
used for viral genome
• Presence or absence of envelope
• Type of capsid symmetry
• Replication strategy

Question 12

Virus Replication (5)

Answer 12

–adsorption
–penetration and uncoating
–synthesis of viral proteins and nucleic acids
–assembly of virions
–release

Question 13

Adsorption of Virions

Answer 13

viral surface proteins and/or enzymes
mediate attachment to specific host
receptors

Question 14

Penetration and Uncoating

• three mechanisms

Answer 14

– injection of nucleic acid
– fusion of envelope with host membrane
– endocytosis

Question 15

Assembly of Virus Capsids (4)

Answer 15

• capsid proteins
• assembly of naked viruses
• assembly of enveloped viruses
• site of morphogenesis varies

Question 16

Assembly of Virus Capsids

• capsid proteins

Answer 16

– encoded by late genes

Question 17

Assembly of Virus Capsids

• assembly of naked viruses

Answer 17

– empty procapsids formed then nucleic acid

inserted

Question 18

Assembly of Virus Capsids

• assembly of enveloped viruses

Answer 18

– in most cases, similar to assembly of naked

viruses

Question 19

Virion Release

• naked viruses

Answer 19

– usually by lysis of host cell

Question 20

Virion Release

• enveloped viruses

Answer 20

– formation of envelope and release usually
occur concurrently
• virus-encoded proteins incorporated into host
membrane
• nucleocapsid buds outward and is surrounded
by modified host membrane

Question 21

FIGURE 6.23 • Bacteriophage reproduction: lysis and lysogeny.

Answer 21

B. Lysis
occurs when the phage genome reproduces progeny phage particles, as many as possible,
and then lyses the cell to release them. In phage lambda, lysogeny can occur when the
phage genome integrates itself into that of the host. The phage genome is replicated along
Nith that of the host cell. The phage DNA, however, can direct its own excision by
expressing a site-specific DNA recombinase. This excised phage chromosome then initiates
a lytic cycle.

Question 22

prophage (host infected): phage C1
(Clostridium botulinum)
bacterial product from prophage gene:

Answer 22

Botulnum toxin

c1

Question 23

progphage (host infected): Beta phage (Corynebacterium
diphtheriae)
bacterial product from prophage gene:

Answer 23

Diphtheria toxin

tox

Question 24

progphage (host infected):
Epsilon 34
(Salmonella enterica)
bacterial product from prophage gene:

Answer 24

LPS synthesis

rfb

Question 25

HERV-W

Answer 25

syncytin-1 (retroviral env protein, placental fusion)

Question 26

HERV-E

Answer 26

apolipoprotein c1 (liver fx)

Question 27

FIGURE 6.24 - Bacteriophage

replication cycle:

Answer 27

slow release. In the
slow-release replication cycle, a
filamentous phage produces phage
particles without lying the cell. The hos
continues to reproduce itself, but more
slowly than uninfected cells do because
many of its resources are being used to
make phages.

Question 28

Bacterial Defenses (against bacteriophages) (3)

Answer 28

Genetic resistance - mutations in bacterial genes (e.g. receptor)
Restriction endonucleases - enzymes that cut invading DNA
CRISPR - a bacterial immune system

Question 29

FIGURE 6.25

CRISPR defense of bacterial cell.

Answer 29

A piece of phage DNA
gets copied as a “spacer” into the host genome. If the bacterium survives
infection, later reinfection by the same kind of phage causes
transcription of the spacers into CRISP RNA. A processed spacer (crRNA)
joins the Cas complex to recognize and cleave the phage DNA.

Question 30

FIGURE 6.26 - The gut bacteriophage community.

Answer 30

Bacteriophages enter the intestine, where they infect
intestinal bacteria. Most intestinal bacteria carry prophages. Phages also modulate the immune system
Phages may limit the bacterial numbers to levels that the human immune system can
tolerate. Lysogenized bacteria may use quorum sensing to detect host cell populations and
“decide” whether to start a lytic cycle.
Phage particles may modulate immune system activity by suppressing T-cell activation and
tumor formation
Phages may attack biofilms. Biofilms of pathogens such as Pseudomonas aeruginosa may be
eroded by phage infection.

Question 31

Phage therapy being explored as a potential

Answer 31

antibiotic treatment