Module 4 - Virology Flashcards
What is a virus?
A structure that has evolved to transfer nucleic
acid (genetic element) from one cell to another.
Genetic element that cannot replicate independently of a living (host) cell.
Outside a living cell a virus is an inert macromolecule
Viruses are very small:
*20 to 350nm (mimivirus ~ 700 nm)
*Only poxviruses and mimiviruses are visible
(just!) by light microscopy
*Bacteria >1um
*Pandora virus (~1um)
What are the characteristics of a virus?
Intracellular parasite
– Cannot replicate outside living cell
– Large inert macromolecules
Possess only one kind of nucleic acid
– DNA or RNA - not both
Limited amount of genetic material
– 3 genes in simple viruses
– 200 genes in complex virus
– (bacteria >500 genes)
Viruses - not enough genetic information to code for energy production of high potential;
do not possess ribosomes;
do not grow from the integrated sum of their constituents – they replicate;
Viral nucleic acid and viral protein synthesis occur separately - come together during maturation.
RNA viruses are only organisms to use RNA as genetic material;
Lipid and carbohydrate are acquired from the host cell (not coded for by viral genome)
What is the composition of a virus?
All viruses contain a nucleic acid genome (RNA or DNA)
and protein
Protein usually coded for by viral genome
– (some protein acquired from host during replication)
Structural proteins – part of the virus particle (virion).
– cell attachment and penetration
– Virus assembly and release
– Protection of nucleic acid and (replication enzymes in some viruses)
Non-structural proteins – most not found in virion
– replication (polymerases, helicases)
– virus assembly and release (proteases)
What is the nucleocapsid?
The viral genome is surrounded by a protein shell (Capsid) composed of many subunits of one or more structural proteins (capsomers)
The capsid together with the genome form the nucleocapsid.
Capsomers are arranged either in icosahedral or helical symmetry.
What are icosahedral viruses/nucleocapsid?
– the capsomers are arranged to form equilateral
triangular faces
– may appear spherical
What are helical nucleocapsids?
Helical symmetry - capsomers are arranged in helical pattern around a central core
Helical nucleocapsids tend to be rod-shaped rather then
spherical.
Helical Nucleocapsids – flexible virion structure
– can be spherical, pleomorphic (eg influenza virus), filamentous
What are naked and enveloped viruses?
Viruses composed only of nucleic acid and
protein are called naked viruses
- Naked Viruses often very stable in the environment
- nucleic acid “encased” in protein eg. FMDV
Viruses that acquire an outer layer of membrane
from the host during replication are called
enveloped viruses
– Enveloped virus generally more fragile
* more susceptible to inactivation
– because of lipid content eg. HIV
Explain the structure of an enveloped virus.
Enveloped viruses may have icosahedral or helical
nucleocapsid
– all helical viruses of animals are enveloped
– naked helical viruses only found amongst plant and
bacterial viruses
List some examples of enveloped viruses with helical nucleocaspids.
Orthomyxoviruses (eg. influenza)
Paramyxoviruses (eg. measles, mumps, Hendra)
Filoviruses (eg. Marburg, Ebola)
List some examples of enveloped viruses with icosahedral nucleocaspids.
-Togaviruses
-Ross River
-Rubella
-Flaviviruses
-dengue
-yellow fever
-Herpesviruses
-Herpes simplex
-chicken pox
What type of shapes can enveloped viruses take on.
Spherical/Icosahedral, bullet, Bacilform/rod-shaped, Pleomorphic.
What are complex viruses?
Neither helical nor icosahedral
Large viruses with complex structure
– Poxviruses
What are the characteristics of the complex virus, T- even Bacteriophage?
- Icosahedral head
- Contractile tail/sheath
- Base plate
- Tail fibres/legs!
What is a mimivirus?
A giant DNA virus (i.e. “girus”) infecting species of the genus Acanthamoeba, was first identified in 2003. With a particle size of 0.7microm in diameter, and a genome size
of 1.2Mb encoding more than 900 proteins, it is the most
complex virus described to date.
What is the structure of viruses determined by?
Size and coding capacity of the genome:
- Genome has to fit into nucleocapsid
- Number of structural proteins available
- Most economic nucleocapsid symmetry
Functional requirements also important:
- Protection of genome from environment
- Mode of cell attachment and entry
- Mode of replication
- Mode of virion assembly and release
What is a capsid?
Protein shell that encloses the nucleic acid
What are capsomers?
Individual units that make up capsid
What is an envelope and how is acquired?
Lipid membrane aquired by nucleocapsid as it
buds from cells. The envelope is acquired by the virus as it matures and buds through the cell membrane
What is a virion?
The VIRION is the complete infective virus particle.
What are the major differences between bacteria and viruses?
Visibility: Viruses are not usually visible by light microscopy, whereas bacteria are visible.
Growth/Replication: Viruses replicate within host cells, while bacteria grow independently.
Genetic Material: Viruses do not code for energy production; bacteria have genetic material that codes for energy production.
DNA/RNA Content: Viruses contain either DNA or RNA, while bacteria contain both.
Replication: Viruses cannot replicate outside a living cell; bacteria usually can.
Ribosomes: Viruses do not have ribosomes; bacteria contain ribosomes for protein synthesis.
What are the mechanisms of virus attachment to host cells?
Brownian Motion: Allows random collisions of viruses with cell receptors.
Virus Attachment Sites:
- Enveloped viruses use spikes/peplomers.
- Naked viruses attach anywhere on the virus surface (capsid).
Cell Receptors: Surface molecules on the host cell, e.g., CD4 and CCR5 for HIV.
Attachment Mechanism:
- Involves physical complementarity between the virus and the receptor.
- Specific interaction leads to host and tissue specificity, meaning most viruses can only infect certain cell types.
How do viruses penetrate host cells?
Viruses penetrate host cells through several mechanisms:
- Endocytosis: The virus is engulfed into a cytoplasmic vacuole.
- Membrane Fusion (for enveloped viruses): The viral envelope fuses with the host’s cytoplasmic membrane, allowing only the nucleocapsid to enter the cell.
- Direct Entry (for some naked viruses): The virus shell (capsid) undergoes molecular rearrangement, enabling either the whole virus or just the viral genome to enter the host cell.
What are the two primary mechanisms for the entry of naked viruses into host cells?
Naked viruses can enter host cells through:
Direct Entry: The nucleocapsid undergoes molecular rearrangement, allowing the viral genome to directly enter the cell.
Exosomes: Naked viruses are excreted from infected cells in exosomes and subsequently enter new cells within membrane-bound particles.
What occurs during the initiation of viral replication?
The initiation of viral replication begins with uncoating, which allows the viral genome to be released into the cytoplasm, often spontaneously or facilitated by host cell or viral enzymes. Some viruses may inhibit the host cell’s macromolecular synthesis early in replication to prevent competition, directing the cell to synthesize only viral proteins. Additionally, certain enveloped viruses rely on healthy host cells to synthesize cellular proteins and membranes necessary for their replication.
Where do DNA and RNA viruses replicate, and how do they utilize host cellular machinery?
DNA viruses typically replicate in the cell nucleus, where all necessary replicative enzymes are found (except for poxviruses). In contrast, RNA viruses generally replicate in the cytoplasm, although some, like retroviruses and influenza, have exceptions. Host RNA is not replicated during this process. RNA viruses code for their own replicative enzymes but rely on host cell ribosomes (specifically polyribosomes) to translate these enzymes from mRNA. Importantly, many enzymes required for viral replication are not coded by the virus itself, necessitating the use of host cell enzymes.
) Briefly describe the replication strategy of :
a) A positive strand RNA virus
b) A negative strand RNA viruses
Give two examples of each
a) Positive Strand RNA Virus Replication Strategy
Positive strand RNA viruses, such as Dengue virus and Hepatitis C virus, have their genomic RNA that can function directly as mRNA. Upon entering the host cell, the viral RNA is translated by host ribosomes to produce viral proteins. The viral RNA-dependent RNA polymerase synthesizes a complementary negative strand RNA, which then serves as a template to produce more positive strand RNA genomes for new virions. The new virions assemble and bud off from the host cell, taking a portion of the host membrane as their envelope.
b) Negative Strand RNA Virus Replication Strategy
Negative strand RNA viruses, such as Respiratory Syncytial Virus (RSV) and Influenza virus, require the viral RNA polymerase to transcribe their genomic RNA into positive strand mRNA upon entry into the host cell. The negative strand RNA serves as a template for synthesizing complementary positive strand RNA, which can then be translated into viral proteins. After translation and the replication of genomic RNA, the new negative strand RNA genomes are assembled into new virions, which then bud off from the host cell membrane.
How do viruses mature and release from host cells?
After viral nucleic acid and protein synthesis, assembly occurs.
Simple viruses often undergo self-assembly.
More complex viruses may require multiple assembly stages.
Naked viruses accumulate in the host cell:
- Burst open for sudden and complete release of progeny viruses.
- May also be secreted gradually in exosomes.
Enveloped viruses bud through the cell membrane for gradual release.
What are the key targets for antiviral agents and how do viruses utilize host cell machinery?
Viral Utilization of Host Machinery:
- Protein Synthesis: Viruses rely on the host’s translation system and enzymes for protein processing.
- DNA Replication: For DNA viruses, they utilize cellular enzymes.
- Energy Requirements: Viruses depend on cellular energy to drive synthesis and enzyme reactions.
Challenges in Antiviral Treatment:
- Interfering with viral replication can also harm host cells.
- Antiviral agents typically target viral-specific enzymes or nucleic acids to minimize toxicity to host cells.
