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63 Cards in this Set
- Front
- Back
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What is a virus? |
It is a genetic element that cannot replicate by itself. Requires a host cell |
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What is a virion? |
A extracellular form of a virus |
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What are the two forms of viruses that can exist? |
1) Naked virus 2) Enveloped virus |
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What is a nucleocapsid? |
Contains both the capsid and the nucleic acid |
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What is the capsid made of? |
Composed of capsomeres |
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How are spike cells made and what are their roles? |
Spike cells are made by the host cell and they allow the virus to attach to their host |
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What are the genomes of viruses? |
It can be either DNA or RNA genomes (Some are circular, but most linear) |
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What is special about positive ssRNA genomes? |
Ability to interact with the host ribosomes |
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What is special about negative ssRNA genomes? |
Do not have ability to interact with the host ribosomes (contain enzymes the convert the negative form into positive ssRNA) |
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How can viruses be classified? |
Basis of the host they infect |
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Typically, are viruses smaller than prokaryotic cells? |
Yes |
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What is the capsid? |
The protein shell that surrounds the genome of a virus particle |
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What is a capsomere? |
The subunit of the capsid |
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What is a enveloped virus? |
A virus that contains additional layers around the nucleocapsid |
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What are the two types of orientations that nucleocapsids are constructed? |
1) Helical symmetry (length determined by length of nucleic acid) 2) Icosahedral symmetry (spherical) |
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Is it true that the envelop of enveloped viruses makes initial contact with the host cell? |
True |
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What are the three enzymes necessary for viruses to create infection? |
1) Lysozyme (makes hole in cell wall) 2) Nucleic acid polymerase ( 3) Neuraminidases (cleave glycosidic bonds; allows viruses to leave host cell) |
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What are the 5 steps of the virus replication life cycle? |
1. Attachment 2. Entry (injection of viral DNA into host cell) 3. Synthesis (of virus nucleic acid and protein by cell metabolism) 4. Assembly (of capsids and packaging of viral genomes into new virions) 5. Release of mature virions from host cell |
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The virus life cycle has a latent period. What makes up the latent period? |
Eclipse and maturation |
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What makes up the burst size? |
The number of virions released |
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What is Titer? |
The number of infectious units per volume of fluid |
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What are plaques? |
Clear zones that develop on lawns of host cells |
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What does plaques represent? |
Infection by a single virus particle |
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Is the number of plaque forming units always lower than direct counts by electron microscopy? |
Yes |
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Why is the number of plaque forming units always lower than direct counts by electron microscopy? |
1. Inactive virions 2. Conditions not appropriate for infectivity |
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Is attachment of virion to host cell highly specific? |
Yes (requires complementary receptors on the surface of a host and its infecting virus) |
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What happens when a virus attaches to a host cell? |
Changes to both virus and cell surface that facilitates penetration |
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What is considered a permissive cell? |
Host cell that allows the complete replication cycle of a virus to occur |
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How do eukaryotes diminish viral infections? |
Possess mechanisms such as immune defense, and RNA interference |
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How do prokaryotes diminish viral infections? |
CRISPR (similar to RNA interference) and Restriction modification system |
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What is the restriction modification system? |
Its a DNA destruction system which is effective only against double stranded virus DNA |
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How do restriction enzymes work in the restriction modification system? |
Cleave DNA at specific sequences |
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What are the two virus mechanisms that allow them to evade bacterial restriction systems? |
1. Chemical modification of viral DNA (glycosylation or methylation) 2. Production of proteins that inhibit host cell restriction systems |
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T4 bacteriophages contain a special modified base. What is that base and what does it do? |
5-hydroxymethylcytosine which makes the viral DNA resistant to virtually all known restriction enzymes |
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What are the 3 main parts of the T4 genome? |
1. Early proteins 2. Middle proteins 3. Late Proteins |
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What are the early and middle proteins required for? |
Enzymes needed for DNA replication and transcription |
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What are late proteins required for? |
Head and tail proteins and enzymes necessary to release virions |
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How is double stranded DNA pumped into the head of a new mature virion? |
Under pressure using ATP |
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What are the 2 types of viral life cycles? |
1. Virulent mode 2. Temperate mode |
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What happens in the virulent mode of the viral life cycle? |
Viruses lyses host cell after infection (known as lytic cycle too) |
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What happens in the temperate mode of the viral life cycle? |
Viruses replicate their genomes in tandem with host genome and without killing the host |
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What is special about the temperate mode of the viral life cycle? |
It can be converted to the lytic cycle (virulent mode) via induction |
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Can temperate viruses have a stable relationship within the host? |
Yes |
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What is lysogeny? |
State where most virus genes are not expressed and the virus genome is replicated in synchrony with the host chromosome |
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What is a prophage? |
When the viral genome is integrated into the host genome |
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What is a lysogen? |
A bacterium containing a prophage |
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What are the 2 key elements that control which viral pathway will occur? |
Repressor proteins: 1. cl protein- causes repression of lytic events 2. Cro repressor- controls activation of lytic events |
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Is it true that bacteriophages are really diverse? |
Yes |
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What kind of genome do most bacteriophages have? |
Double stranded DNA |
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What is unique about animal viruses? |
The entire virion enters the host cell, unlike in prokaryotes |
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Are most animal viruses naked or enveloped? |
Most animal viruses are enveloped |
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How do animal viruses obtain and build their envelop? |
As the animal virus leaves the host cell, it can remove part of the host cell's lipid bilayer |
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What are the four main consequences of virus infections in animal cells? |
1. Persistent infections 2. Latent infections 3. Transformation 4. Cell fusion |
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What are persistent infections? |
Infected host cell remains alive and continues to produce virus particles |
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What are latent infections? |
Delay between the infection by the virus and lytic events |
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What is transformation? |
Conversion of normal cell into tumor cell |
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What is cell fusion? |
Two or more cells become one cell with many nuclei |
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What are retroviruses? |
RNA viruses that replicate through DNA intermediate |
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What are the important features of retroviruses? |
1. Enveloped virus 2. Contain a reverse transcriptase (copies information from its RNA genome into DNA) 3. Virion contains specific tRNA molecules |
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What are the specific genes of retroviruses? |
1. GAG- encode structural proteins 2. POL- encode reverse transcriptase and integrase 3. ENV- encode enveloped proteins |
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What is the process of replication of a retrovirus? |
1. Entrance 2. Removal of virion envelope at the membrane 3. Reverse transcription of one of the two RNA genomes 4. Integration of retroviral DNA into host genome 5. Transcription of retroviral DNA 6. Assembly and packaging of genomic RNA 7. Budding of enveloped virions; release from cell |
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Where does most of Earth's genetic diversity reside? |
Viruses |
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Viral Metagenome is.....? |
The sum total of all viral genes in a particular environment |
