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What is a virus?
A virus is a genetic element that cannot replicate independently of a living (host) cell.
Can viruses conserve energy or metabolise organic molecules?
No, viruses are incapable of conserving energy or metabolising/synthesising organic molecules themselves. However, large viruses are beginning to challenge this idea
What is meant by an 'obligate intracellular parasite'?
An obligate intracellular parasite refers to a virus, which must infect a host cell to replicate and survive.
What is meant by an 'obligate intracellular parasite'?
An obligate intracellular parasite refers to a virus, which must infect a host cell to replicate and survive.
How does a viral genome differ from a host genome?
A viral genome is independent of the host genome and cannot function without infecting a host cell.
What types of cells can viruses infect?
Viruses can infect both prokaryotic and eukaryotic cells, and even other viruses.
What is a virion?
A virion is the extracellular form of a virus that exists outside the host and facilitates transmission from one host cell to another.
When does virus replication/reproduction occur?
Replication/reproduction of a virus only occurs upon infection, when the virus enters a host cell.
What is a capsid?
A capsid is the protein shell that surrounds the viral genome.
What is a nucleocapsid?
A nucleocapsid consists of the nucleic acid (genome) and the first protein shell (capsid)
What are naked viruses?
Naked viruses are viruses that have no additional layers beyond the capsid. Examples include most bacterial viruses and norovirus.
What are enveloped viruses?
Enveloped viruses have an outer layer called the envelope, which includes at least a lipoprotein membrane. Examples include many animal viruses
How are viral genomes packaged in virions?
Viral genomes are packaged in virions, which vary greatly in size and morphology.
What are giant viruses?
Giant viruses are very large in size and have a high gene number
How do dsDNA virions compare to ssDNA and RNA virions in size?
dsDNA virions tend to be larger than ssDNA and RNA virions.
What is the most striking morphology of certain viruses?
The most striking morphology is that of the ‘head-plus-tail’ bacteriophages.
What is a theory about giant viruses?
According to Wong (2017), giant viruses may just be small viruses that stole genes from their host.
What is a capsomere?
A capsomere is an individual protein molecule arranged in a precise and highly repetitive pattern around the nucleic acid, making up the capsid
How can nucleocapsids be assembled?
Nucleocapsids can be assembled either through spontaneous self-assembly or by requiring host cell folding assistance.
What symmetry is found in spherical viruses?
Spherical viruses, like human papillomavirus, exhibit icosahedral symmetry, which is the most efficient arrangement of subunits in a closed shell and requires the fewest capsomeres.
What symmetry is found in rod-shaped viruses?
Rod-shaped viruses, such as tobacco mosaic virus (TMV), exhibit helical symmetry, where the length of the virus is determined by the length of the nucleic acid, and the width is determined by the size and packaging of the capsomeres.
Which bacteriophages are structurally the most complex?
Head-plus-tail bacteriophages, such as the T4 virion, are structurally the most complex.
What symmetries are found in the T4 virion?
The T4 virion has helical symmetry in its tail and icosahedral symmetry in its head.
What is the function of the collar in the T4 virion?
The collar in the T4 virion connects the head and tail
What are the other structural elements of the T4 virion?
Other structural elements of the T4 virion include the tail, which connects to the base plate, tail pins, and tail fibers
What is the envelope surrounding the nucleocapsid of an enveloped virus made of?
The envelope consists of a lipoprotein membrane, with lipids derived from the host's cytoplasmic membrane
What role do viral surface proteins play in enveloped viruses?
Viral surface proteins are embedded in the lipid membrane and are critical for host specificity, allowing the virus to attach to specific host-cell receptors at the start of infection. They are also important for the release of the virion after replication
Do all enveloped viruses have a matrix protein shell?
No, not all enveloped viruses have a matrix protein shell, but some do, which covers the nucleocapsid.
Why are there relatively few enveloped plant or bacterial viruses?
There are relatively few enveloped plant or bacterial viruses because host cell walls surround the host cell membrane, making it more difficult for the virus to acquire an envelope.
How do enveloped viruses exit animal host cells compared to other viruses?
Enveloped viruses exit animal host cells more easily than non-enveloped viruses.
Why are enveloped viruses more easily destroyed?
Enveloped viruses are more easily destroyed by hand sanitizers due to their lipid membrane, which is disrupted by alcohol-based products.
What is the envelope of an enveloped virus made of?
The envelope surrounding the nucleocapsid consists of a lipoprotein membrane, with lipids derived from the host's cytoplasmic membrane.
What is the role of viral surface proteins in enveloped viruses?
Viral surface proteins are embedded in the lipid membrane and are critical for host specificity by attaching to specific host-cell receptors at the start of infection. They are also important for the release of the virion after replication.
Do all enveloped viruses have a matrix protein shell?
No, not all enveloped viruses have a matrix protein shell, but some do, which covers the nucleocapsid
Why are there relatively few enveloped plant or bacterial viruses?
the host cell walls surrounding the host cell membrane prevent the virus from acquiring an envelope.
How do enveloped viruses exit animal host cells?
Enveloped viruses exit animal host cells more easily than non-enveloped viruses.
What is the significance of glycosylation of viral surface proteins?
Many viral surface proteins are glycosylated, meaning they have a glycan 'shield.' This glycosylation may have implications for immune recognition, potentially helping the virus evade the immune system.
What is the role of enzymes inside some virions?
Enzymes inside some virions are an absolute requirement for successful infection and replication.
What is the function of the lysozyme-like enzyme in bacterial viruses?
The lysozyme-like enzyme makes a hole in the peptidoglycan cell wall to allow nucleic acid entry and also lyses the bacterial cell to release new virions.
What does neuraminidase do in enveloped viruses?
Neuraminidase degrades the glycan structure on glycoproteins and glycolipids, allowing the liberation of viruses from the host cell. An example is the influenza virus.
What are RNA replicases in viral virions?
RNA replicases are RNA-dependent RNA polymerases that replicate RNA genomes within the virus.
What is reverse transcriptase and where is it found?
Reverse transcriptase is an RNA-dependent DNA polymerase found in retroviruses, such as HIV. It converts RNA into DNA for integration into the host genome.
How much do viral genome sizes vary?
Viral genome sizes vary about 1000-fold from the smallest to the largest.
What is the smallest viral genome?
The smallest viral genome is the 1.75-kb single-stranded circovirus
What is the largest viral genome?
The largest viral genome is the 2.5-Mb genome of the Pandoravirus, which infects some marine amoebae. It has a larger genome than some bacteria.
How does the size of RNA genomes compare to DNA genomes in viruses?
RNA genomes are typically smaller than DNA viral genomes.
What are viroids?
Viroids are naked infectious RNA molecules (without a capsid) that cause plant diseases. They have the smallest microbial genomes.
What is the diversity in viral genome composition?
Viral genomes can be either DNA or RNA, single-stranded or double-stranded, and either linear or circular.
How do viral genomes compare in size to cellular genomes?
Viral genomes are usually smaller than cellular genomes.
What are the two types of single-stranded RNA viral genomes?
Single-stranded RNA viral genomes can be plus sense (same as mRNA) or minus sense (complementary to mRNA)
What challenge do viral genomes create?
Viral genomes create challenges for the flow of genetic information due to their diverse structures and replication mechanisms
What is the Baltimore Scheme?
The Baltimore Scheme is a classification system for viruses based on their genome structure and replication mechanisms
How are bacterial and animal viruses classified in the Baltimore Scheme?
In the Baltimore Scheme, bacterial viruses are classified at the top (except for classes V and VI), while animal viruses are classified at the bottom.
Which Baltimore class primarily infects prokaryotes?
Prokaryotes are primarily infected by class I (dsDNA) viruses.
Which Baltimore class primarily infects eukaryotes?
Eukaryotes are primarily infected by class IV (ss(+)RNA) viruses
Which Baltimore classes infect fungi?
Fungi are primarily infected by classes III and IV viruses.
Where are most class I and class V viruses found?
Most class I and class V viruses have animal hosts rather than plant hosts.
Where are more class II viruses found?
More class II viruses have plant hosts instead of animal hosts.
Which Baltimore class infects only animals?
Class VI (Retroviruses) infect only animals.
Where are class VII viruses more common?
Class VII viruses are more common in plants than in animals
What is the major difference between prokaryotic and eukaryotic viruses in their life cycle?
The major difference is that prokaryotic viruses inject nucleic acid into the host, whereas eukaryotic viruses enter the host as the whole virion.
What are the phases of viral replication in a permissive (supportive) host?
Attachment (adsorption) of the virion
Penetration (entry, injection) of the virion nucleic acid
Synthesis of virus nucleic acid and protein by the host cell, as redirected by the virus
Assembly of capsids and packaging of viral genomes into new virions
Release of mature virions from the host cell
What is a virulent (lytic) infection?
A virulent (lytic) infection is one where the virus replicates and destroys the host cell.
What is a one-step growth curve in virus replication?
A one-step growth curve in virus replication is characterised by an increase in virions when host cells burst, typically following a specific sequence of events.
What happens during the eclipse phase of the viral life cycle?
During the eclipse phase, the viral genome is replicated and viral proteins are translated within the host cell
What occurs during the maturation phase of virus replication?
During the maturation phase, viral nucleic acids are packaged into capsids to form new virions.
What is the latent period in virus replication?
The latent period includes both the eclipse and maturation phases, where the virus is not yet visible outside the host cell.
What is the release phase in the virus life cycle?
The release phase involves the virus exiting the host cell, typically through cell lysis, budding, or excretion
What is meant by the burst size in viral replication?
The burst size refers to the number of virions released from the host cell during the release phase
What are the two pathways of temperate viruses?
Temperate viruses display two pathways: lytic infection and lysogenic infection.
How is the host cell genetically altered during a lysogenic infection?
During a lysogenic infection, the viral genome integrates into the host genome, genetically altering the host cell.
What is a prophage in the lysogenic cycle?
A prophage is the integrated viral DNA that is replicated along with the host genome during the lysogenic infection.
What role do the repressors cI and Cro play in the phage λ life cycle?
Repressors cI and Cro control the genetic switch between the lytic and lysogenic cycles in the phage λ life cycle.
Is the lytic pathway present in both animal and bacterial viruses?
Yes
What is the major difference in the entry of viruses into prokaryotic and eukaryotic cells?
In eukaryotes, the whole virion enters the host cell, whereas in prokaryotes, only the viral nucleic acid enters the host cell.
Where does viral replication typically occur in eukaryotic cells?
The nucleus
How does eukaryotic virion entry depend on the host cell?
Eukaryotic virion entry is dependent on a host receptor on the host cell surface.
Why do animal viruses infect only certain tissues?
Animal viruses infect only certain tissues because different tissues or organs express different host receptors.
What is the outcome of a virulent infection in animal cells?
A virulent infection leads to the lysis (destruction) of the host cell, also known as a lytic infection.
What is a latent infection in animal viruses?
A latent infection is similar to a lysogenic infection in temperate phages, where the viral genome remains dormant within the host cell without immediately causing cell death.
What is a persistent infection in animal viruses?
A persistent infection occurs when some enveloped animal viruses slowly bud virions from the host cell without killing the host cell.
What is an oncogenic infection or transformation in animal viruses?
An oncogenic infection or transformation happens when the viral genome converts a normal host cell into a tumour cell. Examples include HPV, hepatitis, polyoma, and herpes viruses.
What are the possible outcomes for temperate prokaryotic viruses?
Temperate prokaryotic viruses have two possible outcomes: lytic infection or lysogenic infection.
Are there more possible outcomes for animal viruses compared to prokaryotic viruses?
Yes, there are more possible outcomes for animal viruses compared to prokaryotic viruses.
Which type of viruses are easiest to grow and best studied?
Bacterial viruses (bacteriophages) are the easiest to grow and therefore best studied.
How are bacterial viruses cultured on a Petri dish?
A Petri dish with a nutrient agar layer is used. Permissive bacterial cells and virus sample are mixed in liquid medium or molten top agar, then spread as “lawns” on the agar.
What is a plaque or pock in virus culturing?
A plaque or pock is a halo that appears where the virus has lysed bacterial cells on the agar lawn.
What happens when the virus load is high in a culture?
When the virus load is high, a large part of the lawn can be lysed, and halos (plaques) appear.
How can viruses be harvested from halos?
Viruses can be easily harvested from the halos by washing the plate.
How can viral particles be directly counted?
Viral particles can be directly counted using Electron Microscopy, though it is laborious.
What is the plaque assay used for?
The plaque assay is used to detect and count viruses by plating specific dilutions of viral samples and counting the halos (plaques) that form.
What is the term for counting viruses using the plaque assay?
The term for counting viruses using the plaque assay is counting Plaque-Forming Units (PFU), similar to counting Colony-Forming Units (CFU) for viable bacterial cells.
What is a titer in the context of viruses?
A titer is the number of infectious units (PFU) per volume of the original fluid sample.
How are animal and some plant viruses cultivated?
Animal and some plant viruses are cultivated in tissue culture, with halos forming upon successful virus infection.
How does the number of PFUs compare to direct counts by electron microscopy?
PFU counts are always lower than direct counts by electron microscopy.
Why is the efficiency of infection usually less than 100%?
The efficiency of infection is usually less than 100% because of inactive virions or conditions that are inappropriate for infectivity.
What is the range of plating efficiency used in quantitative virology?
Plating efficiency is used to ensure workable titers for lab studies, typically ranging from 0.1% to 1% for animal viruses
What are some molecular diagnostic methods for viral infections?
Some molecular diagnostic methods include:
q(RT-)PCR
LAMP
Lateral flow tests
Serology (antibody testing)
Molecular probes
What is the significance of using different molecular diagnostic methods?
These diagnostic methods vary in specificity and sensitivity, enabling rapid detection of viral infections.
What are subviral agents?
Subviral agents are infectious agents that resemble viruses but are not classified as viruses.
