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Microbiology > Chapter 5: Viruses and their multiplication > Flashcards

Chapter 5: Viruses and their multiplication Flashcards

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Biology 101 flashcards
1
Q

Virus description

A

genetic element that can multiply only in a living (host) cell
 Not living, not found on tree of life
 Obligate intracellular parasite: Needs host cell for energy, metabolic intermediates, protein synthesis
 Has its own nucleic acid genome

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2
Q

Virion

A

Virion- extracellular form of a virus
 Exists outside host and facilitates transmission from one host cell to another
 Replication/reproduction occurs only upon infection (entry into host cell)

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3
Q

Viral surface proteins

A

Capsid: the protein shell that surrounds the
genome of a virus
 Naked viruses (e.g., most bacterial and plant
viruses) have no other layers
 Enveloped viruses (e.g., many animal viruses) have
an outer layer consisting of a phospholipid bilayer
(from host cell membrane) and viral proteins
 Nucleocapsid: nucleic acid + protein in enveloped
viruses

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4
Q

V surface proteins significance

A

 Virion surface proteins important for host cell attachment and may include enzymes involved in infection/replication

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5
Q

virulent (lytic) infection

A

replicates and destroys host
 Host cell metabolism redirected to support multiplication and virion assembly

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6
Q

lysogenic infection

A

host cell genetically altered because viral genome becomes part of host genome

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7
Q

Viral genome

A

 either DNA or RNA genomes
 single-stranded or double-stranded
 usually smaller in size and gene content than cells

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8
Q

Group I:

A

double-stranded DNA viruses

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9
Q

Group II

A

single-stranded DNA viruses

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10
Q

Group III

A

double-stranded RNA viruses

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11
Q

Group IV

A

positive sense single-stranded RNA viruses

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12
Q

Group V

A

negative sense single-stranded RNA viruses

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13
Q

Group V

A

single-stranded RNA viruses with a DNA intermediate in their life cycle

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14
Q

Group VII

A

double-stranded DNA viruses with an RNA intermediate in their life cycle

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15
Q

beneficial virus examples

A

 e.g., Arabidopsis infected with plum pox virus
increases drought tolerance
 e.g., insect densovirus infection of rosy apple aphid
results in decreased size and offspring, but wings
form
 e.g., hepatitis G coinfection of HIV patients
decreases HIV replication and infectivity

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16
Q

virus size

A

Most viruses are smaller than prokaryotic cells; ranging from 0.02 to 0.3 μm
 Pandoravirus over 1 μm long
 Poliovirus (~28 nanometers) is size of ribosome

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17
Q

Virion Structure

A

 Some viruses only have one capsid protein
because small size of viral genomes restricts
number of proteins (e.g., tobacco mosaic virus)
 Capsids can be put together through selfassembly (spontaneous) or may require host
cell folding assistance

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18
Q

Capsomere

A

individual protein molecules
arranged in a precise and highly repetitive pattern
around the nucleic acid making up the capsid

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19
Q

 helical symmetry

A

: rod-shaped (e.g., tobacco mosaic virus or T M V)
 length determined by length of nucleic acid
 width determined by size and packaging of capsomeres

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20
Q

icosahedral symmetry

A

spherical
 20 triangular faces, 12 vertices; 5, 3, or 2 identical segments
 most efficient arrangement of subunits in a closed shell
 requires fewest capsomeres

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21
Q

Highly complex structures of virus

A

Virion contains several parts with their own shapes and symmetry
 Most complex are head-plus-tail bacteriophages (e.g., T4)
 Viruses of Acanthamoeba (e.g., Pandoravirus (ovoid with apical pore), Mimivirus (stargate)

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22
Q

 Enveloped Viruses characteristics

A

 have lipoprotein membrane surrounding nucleocapsid
 most (e.g., Ebola) use outer surface proteins to attach and infect
 relatively few enveloped plant or bacterial viruses because of cell walls surrounding cell membrane
 Entire virion enters animal cell during infection
 Enveloped viruses exit more easily

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23
Q

Envelope + fibrils functions

A

 Specificity and penetration controlled in part
by envelope biochemistry
 Fibrils: hairlike polymer structures that attract
amoeba hosts
 Apical pore and stargate function as portals to
release genome

24
Q

Virus enzymes

A

lysozyme
neuraminidases
nucleic acid polymerases

25
Q

lysozyme

A

makes hole in cell wall to allow nucleic acid entry
 also lyses bacterial cell to release new virions

26
Q

neuraminidases (influenza)

A

 destroy glycoproteins and glycolipids
 allows liberation of viruses from cell

27
Q

nucleic acid polymerases

A

 RNA replicases: RNA-dependent RNA polymerases
 Reverse transcriptase: RNA-dependent DNA polymerase in retroviruses

28
Q

Virus culturing

A

 Bacterial viruses are easiest to grow (hosts in liquid medium or spread as “lawns” on agar and
inoculated with virus).
 Animal and plant viruses cultivated in tissue cultures (from animal organ in culture medium or hairy
root-based system in liquid medium).

29
Q

Titre

A

number of infectious virions per volume of fluid

30
Q

Plaque assay description

A

 Plaques are clear zones of cell lysis that develop on lawns of host cells where successful viral infection
occurs.
 Calculate titre from # of plaques
 Analogous to counting colonies
 Similar process for animal viruses
 Plants much harder: viruses must be purified and counted microscopically or through viral proteinspecific methods

31
Q

Plaque assay mechanism

A
32
Q

Why is the number of plaque-forming units (pfu) is always lower than direct counts by electron microscopy

A

 efficiency of infection usually much less than 100%
 defective virions or conditions inappropriate for infectivity
 Useful for estimating appropriate titre to yield plaques

33
Q

Five steps of viral replication in a permissive (supportive) host

A

 attachment (adsorption) of the virion
 penetration (entry, injection) of the virion nucleic acid
 synthesis of virus nucleic acid and protein by host cell as redirected by virus
 assembly of capsids and packaging of viral genomes into new virions
 release of new virions from host cell

34
Q

The Replication Cycle of a Lytic Bacterial Virus

A
35
Q

one-step growth curve phases

A

virion numbers increase
when cells burst
-Eclipse phase: genome replicated, and proteins translated
 Maturation: packaging of nucleic acids in capsids
 Latent period: eclipse + maturation
 Release: cell lysis, budding, or excretion
 burst size: number of virions released

36
Q

Most complex penetration mechanisms found intailed bacteriophages (e.g., T4)

A

 Virions attach to cells via tail fibers that interact
with polysaccharides on E. coli LPS layer
 Tail fibers retract, and tail pins contact cell wall
 T4 lysozyme forms small pore in
peptidoglycan
 Tail sheath contracts, and viral DNA enters
cytoplasm similar to syringe injection
 Capsid stays outside

37
Q

Prokaryotes possess mechanisms to diminish viral infections

A

Genome injection does not ensure infection
 toxin-antitoxin molecules
 antiviral CRISPR
 restriction endonucleases: enzymes that cleave foreign DNA at specific sites
 Some viruses have modified genome that is unaffected by restriction enzymes

38
Q

 Production of T4 Virions and Release

A

 Virion synthesis takes <30 minutes and ends in release of new virions from lysed cell
 Within 1 minute of entry, host-specific protein synthesis ends and phage-specific protein synthesis starts
 T4 genome encodes three major sets of proteins: early, middle, and late proteins.
 early proteins: enzymes needed for DNA replication and proteins that modify host enzymes to express
viral genes
 middle and late proteins: head and tail proteins and enzymes required to liberate mature phage particles

39
Q

Time Course of Events in Phage T4 Infection

A
40
Q

Production of T4 Virions and Release

A

 Genome is pumped into capsid under pressure using energy-linked packaging motor
 Host cell metabolism produces viral proteins and supplies ATP

41
Q

T4 packaging stages

A

 Empty proheads (bacteriophage head precursors) assembled
 Packaging motor assembled at prohead opening and genome pumped into prohead using ATP
 Motor discarded and capsid head sealed

42
Q

T4 after packaging mechanism

A

 After head is filled, T4 tail, tail fibers, and other components are self-assembled
 Late enzymes break membrane and peptidoglycan
 Lysis occurs, 100+ virions released

43
Q

Virulent

A

Viruses always lyse and kill host after infection

44
Q

Temperate

A

Viruses establish long-term, stable relationship but are capable of virulence

45
Q

Lysogeny

A

Temperate viruses can enter a stage where few viral proteins produced, viral genome is replicated with host chromosome and passed to daughter cells

46
Q

Lysogen

A

host cell that harbors temperate virus
- can result in lysogenic conversion with new genetic properties (e.g., virulence in pathogens)

47
Q

Life cycle of a temperate phage

A

 examples: lambda and P1
 in lysogeny, genome is integrated into bacterial
chromosome forming prophage (viral DNA)
 lysogeny maintained by phage-encoded repressor
protein
 Inactivation of repressor induces lytic stage
(induction)
 Viral DNA excised; phage early, middle, and late
proteins produced; virions produced and host lyses
 Cell stress (e.g., DNA damage) induces lytic pathway

48
Q

3 key differences between Eu viruses

A

 Entire virion enters the animal cell
 Eukaryotic cells contain a nucleus, the site of replication for many animal viruses
 Viroplasms (membrane-bound viral factories) form in some eukaryotic cells to increase virion assembly rate
and protect from host defense

49
Q

Viral Infection of Animal Cells features (specificity)

A

 Studied in cell culture
 Bind specific host cell receptors, typically used for cell-cell contact or immune function (e.g., poliovirus
and HIV receptors)
 Viruses often infect only certain tissues because different surface proteins expressed by different
tissues/organs

50
Q

Viral infection of animal cell mechanisms (entry)

A

Host cell entry occurs by fusion with
cytoplasmic membrane or endocytosis
 Uncoating occurs at the cytoplasmic
membrane or in the cytoplasm
 Viral DNA genomes enter nucleus, most viral
RNA genomes are replicated or converted
to DNA within nucleocapsid

51
Q

animal Virion envelope assembly

A

 After genome packaging, many animal viruses must be enveloped
 Occurs during exit through lysis or budding when virus picks up part of cell’s cytoplasmic membrane and uses it as
part of envelope

52
Q

Animal virion infection outcomes

A

 Virulent infection: lysis of host cell, most common
 Latent infection: Viral DNA exists in host genome as provirus (similar to lysogeny) and virions are not produced;
host cell is unharmed unless/until virulent pathway is triggered.
 Persistent infections: slow release of virions from host cell by budding does not result in cell lysis.
 Infected cell remains alive and continues to produce virus
 Transformation: conversion of normal cell into tumor cell

53
Q

Possible Effects of Animal Virus Infection of Host Cells diagram

A
54
Q

Viral Infection of Plant Cells similarity to animal cells

A

Plant viruses share many animal viral traits (e.g., mostly RNA genomes, complete virion enters cell, viral
factories form)

55
Q

Major differences for plant viruses

A

Three major differences:
 Wider host range
 Mostly not enveloped
 Transmission different

56
Q

Plant viruse overview

A

Plant cell wall prevents entry by endocytosis and fusion
 Viruses enter through wounds or penetration by insects, nematodes, fungi
 Vectors: pests that transfer viruses to other host cell types
 After entry, capsid removed, genome replicated, (in nucleus) new virions assembled
 Movement proteins help viruses travel through plasmodesmata (channels) connecting cells; can infect entire
plant
V

57
Q

Viral infection of plants mechanism

A

Microbiology (6 decks)

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