Chapter 6 Flashcards
What is a noncellular particle that must infect a host cell, where it reproduces?
Virus
What does a virion consist of?
- a single nucleid acid
- contained in a capsid
Are viruses specific or general in what kind of host cell they can affect?
- Specific; the hosts are limited to a particular host range of closely related strains or species
What is a virus that infects bacteria called?
- bacteriophage
What is the measles virus?
- a human virus
What is the TMV?
- tobacco mosaic virus
- plant virus
What happens after a RNA virus infects the cell?
- Virions are assembled within “virus factories”, virus-induced cell compartments called a replication complex
- Complexes move around within the cell
Do viruses only replicate in the host?
- No some integrate their genomes into that of the host
What is a prophage?
- a virus the integrates its genome into the DNA of a bacterial genome
What is a provirus?
- an integrated viral genome within a human cell
What is an endogenous virus?
- a permanently integrated provirus transmitted via the germ line
What are the three different forms a virus may interconvert to?
- Virion
- Intracellular replication complex
- Viral genome integrated within host DNA
What is the Virion form?
- an inert particle that does not carry out any metabolism or energy conversion
What is the Intracellular replication complex form?
- Within a host cell, the virla gene products direct the cell’s enzymes to asseble progeny virions at “virus factories” called replication complexes
What is the Viral genome integrated within host DNA form?
- This may be a permanent condition
What type of virus is related to the lysogenic cycle?
provirus
Do viruses play a role in the ecosystem?
yes
How do acute viruses affect the ecosystem?
- act as predators or parasites to limit host population density - recycle nutrients from their host bodies
How does Virus-associated mortality affect the ecosystem?
- they may increase the
genetic diversity of
host species
How do persistent viruses affect the ecosystem?
- they may evolve traits
that confer positive
benefits in a virus-host
mutualism
What role do viruses play in marine ecosystems?
- significant role in the cycling of food molecules - also important in carbon balance
What is a host range?
- a particular group of
host species that a
virus can infect
Are chronic or acute virus disease more common?
Chronic
What keeps the viral genome intact and enables infection of the appropriate host cell?
- the structure of the
virion
What delivers the viral genome to the host cell?
- the capsid
What are Icosahedral viruses?
- polyhedral with 20 identical triangular faces - have a structure that exhibits rotational symmetry
What encloses some icosahedral viruses?
envelope
What does this envelope contain?
- glycoprotein spikes (encoded by the virus) - Tegument proteins (in between the envelope and capsid)
What are Filamentous viruses?
- capsid consists of a long tube of protein with genome coiled inside - vary in length depending on genome size - include bacteriophages as well as animal viruses - show helical symmetry
What is an example of a tailed virus?
- T4 bacteriophages
- Has an icosahedral
“head” and helical
“neck”
What are RNA viruses that lack capsid symmetry?
- Influenza viruses
- RNA segments are
coated with
nucleocapsid proteins
What is special about Poxviruses?
- Their genome is surrounded by several layers. - large number of accessory proteins
What contains so many enzymes that they appear to have evolved from degenerate cells?
- Large asymmetrical
viruses
What are RNA molecules that infect plants?
Viroids
What are some characteristics of viroids?
- no protein capsid
- replicated by host
RNA polymerase - catalytic ability
What are proteins that infect animals?
- Prions
What are some characteristics of prions?
- no nucleic acid component - abnormal structure that alters the conformation of other normal proteins
Viral genomes can be:
- DNA or RNA
- Single or double-
stranded
-Linear or circular
What has key consequences for the mode of infection, and for the course of a viral disease?
- the form of the
genome
What is the criteria that viruses are classified by?
- Genome Composition
- Capsid Symmetry
- Envelope
- Size of the virion
- Host range
David Baltimore proposed that the main distinctions among classes of viruses be:
- genome composition
- the route used to
express messenger
RNA
Group I
Double-stranded DNA viruses
Group II
Single-stranded DNA viruses
Group III
Double-stranded RNA viruses
Group IV
(+) single stranded RNA viruses
Group V
(-) single stranded RNA Viruses
Group VI
RNA retroviruses
Group VII
DNA paraetroviruses
How can the relatedness of different herpes viruses that evolved from a common ancestor be measured?
- by comparing their
genome sequences
What is this comparison based on?
- Orthologs
- Genes of common
ancestry in two
genomes that share
the same function
What is useful for viruses because of their small genomes encode few proteins?
- Proteomic
classification
What are gut bacteriophages called?
- Coliphages
- modulates human
digestion, the
immune system, and
mental health
What is mediated by cell-surface receptors?
- Contract and
attachment
What are cell-surface receptors?
- Proteins that are specific to the host species and which bind to a specific viral component - normally used for important functions for the host cell
True or False. Most bacteriophages inject only their genome into a cell through the cell envelope.
True
What are the two different life cycles that bacteriophages can undergo?
- Lytic Cycle: Virulent Phage( kills host) ex) T4 - Lysogenic cycle: Temperate Phage (forms prophage)
Steps of the Lytic replication cycle:
1. host recognition and attachment 2. Genome Entry 3. Assembly of phages 4. Exit and transmission
What is a temperate phage?
- can infect and lyse cells like a virulent phage, but it also has an alternative pathway: to integrate its genome as a prophage
What is the state called lysogeny?
- Bacteriophage is
quiescent - Can reactivate to
become lytic
How does a slow-release replication cycle differ from lysis and lysogeny?
- phage particles reproduce without destroying the host cell.
What extrudes individual progeny through the cell envelope?
- Filamentous phages
What are three types of defense mechanisms that bacteria have?
- genetic resistance
- restriction endonucleases
- CRISPR
What is genetic resistance?
- altered receptor proteins
What is restriction endonucleases?
- Cleave viral DNA lacking methylation
What is CRISPR?
- Clustered regularly interspaced short palindromic repeats
- A bacterial immune system of sorts
What is the best understood phage community?
Gut Virome
Phage effects that are positive:
- Phages may limit the bacterial numbers to levels that the human immune system can tolerate.
- Phage particles may modulate immune system activity by suppressing T-cell activation and tumor formation.
- Phages may attack biofilms
What determines Tropism?
- the receptors of the cell`
How do most animal viruses enter the host as?
- Mostly as virions
- Internalized virions undergo uncoating, where genome is released from its capsid.
What is the primary factor determining the life cycle of an animal virus?
- the form of its genome
Replication Cycle: DNA Viruses
- can utilize the host replication machinery
Replication Cycle: RNA Viruses
- Use an RNA-dependent RNA-polymerase to transcribe their mRNA
Replication Cycle: Retroviruses
- Use a reverse transcriptase to copy their genomic sequence into DNA for insertion in the host chromosome
True or False. All animal viruses make proteins with host ribosomes.
True
-Translation occurs in the cytoplasm
Assembly of new virions
- Capsid and genome
- May occur in the cytoplasm or nucleus
- Envelope proteins are inserted into a membrane
What happens when the release of progeny viruses from host cell happens?
- Lysis of cell
- Budding
What happens during Budding?
- Virus passes through membrane
- Membrane lipids surround capsid to form envelope
- All enveloped viruses bud from a membrane
(plasma or organelle membrane)
What virus causes many human cancers?
- Oncogenic Viruses
What does an Oncogenic Virus do to the host cell?
- it transforms it to become cancerous
What are the mechanisms of oncogenesis?
- Insertion of an oncogene into the host genome
- Integration of the entire viral genome
- Expression of viral proteins that interfere with host cell cycle regulation
Entry of plant viruses into host cells usually requires…..
- mechanical transmission
What are the three routes that plant viruses enter?
- Contact with damaged tissues
- Transmission by an animal vector
- Transmission through seed
What prevents a lytic burst or budding out of virions in a plant?
- thick cell walls
Instead of this plant viruses are transmitted to uninfected cells by…
-plasmodesmata
What is plasmodesmata?
- Membrane channels that connect adjacent plant cells
- Inner channel connects the ER
Since viruses are ubiquitous what are three defense mechanisms that animal and plants have?
- Genetic Resistance
- Immune System
- RNA interference (RNAi)
What is genetic resistance?
- Host continually experiences mutations
What is the immune system?
- “inactive immunity”: interferons
- “Adaptive immunity”: antibodies
What is the RNA interference?
- Widespread among eukaryotes and archaea
What is the RNA interference?
- Widespread among eukaryotes and archaea
How does a “new” virus emerge to sicken humans?
- As a result of human consumption of wildlife
2. As variants of endemic milder pathogens
What requires growth in host cells?
- Culturing Viruses
Bacteriophages may be cultured in….
- batch culture(in liquid)
- isolated plaques on a bacterial lawn (on a plate)
What generates a step curve?
-Batch culture of viruses
What can be cultured within the whole organism by serial inoculation?
- Animal Viruses
What does serial inoculation ensure?
- that the virus strain maintains its original virulence, but the process is expensive and laborious
What is the alternative way to grown animal viruses?
- In human cell tissue culture
