Virology Flashcards
What are viruses?
Acellular, obligate intracellular parasites known to infect every type of cell. They are considered nonliving and cannot live outside a host cell.
General Characteristics of Viruses?
- Obligate intracellular parasites of living cells
- Lack a metabolic system & must obtain energy from host cells (No ADP or ATP, free amino acids, or sugars. Lack enzymes/machinery for synthesizing proteins, must reproduce inside a host cell.)
- Multiply by taking control of the host cell’s processes & regulating the synthesis and assembly of new viral particles
- Most viruses only infect a limited number of cell types
- Viruses infect every type of cell
- Protected from much of the hosts immune response
- Contain either DNA or RNA.
- Genome encodes info needed for viral replication & synthesis of progeny virions.
What are the size of viruses?
Animal- 22nm-450nm
Require an Electron Microscope to view
How are viruses different from other pathogens?
Acellular, reproduce via assembly process, haploid (except retroviruses), contain EITHER DNA or RNA, lack organelles/cytoplasm/nucleus/nucleoid.
The genome of the virus is surrounded by?
An outer protein coat (Capsid). And sometimes an envelope.
What is an envelope?
A lipid membrane derived from the host cell (either the cell membrane or internal organelle membrane), which contains glycoproteins of viral and host cell origins.
True or False? 13 of the 19 families of animal viruses are enveloped.
True
Most Enveloped Viruses cannot survive in the GI tract
……
The capsid is made up of identical protein subunits called…..
Capsomers.
What are the two symmetrical nucleocapsid structures found among medically important human and animal viruses?
Helical and Icosahedral
How are the Nucleocapsids of helical viruses arranged?
They are tightly wound and rigid in naked viruses, nucleocapsids of enveloped viruses are looser.
Icosahedral capsids have ____Triangular faces and ____corners.
20 Triangular faces and 12 corners.
What is the function of the capsid and envelope?
To protect the virus core, which contains the genome.
The viral envelope plays the following roles….
Host Cell recognition and binding to host cell upon infection. Capsid proteins fulfill these roles in the absence of an envelope.
Capsid+Genome=
Nucleocapsid
True or False. A virus contains DNA and RNA?
False. DNA or RNA.
DNA Viral Genomes can exist as…..
Single Stranded Linear (+ or -), Double Stranded Linear, or Double Stranded Circular.
RNA Viral Genomes can exist as…..
Single Stranded Linear (+ or -), Single Stranded Negative Sense Segmented, Single Stranded Circular Negative Sense, or Double Stranded Linear Segmented.
Why are virus genomes small?
Physically not enough space.
Describe “Strain”, “Type”, and “Variant”.
Strain- Same virus but isolated form different patients or geographical locations.
Type- Same virus but responding differently to antibody detection (serotypes)
Variant- Virus whose phenotype differs from wild-type (genetic mutation is not known)
Spontaneous mutations occur often in viral genomes. What are their rates of incidence? And why do RNA viruses have a higher rate?
DNA Virus- 1/10^-9
RNA Virus- 1/10^-3
RNA dependent RNA polymerases are typically more error prone than their DNA counterparts.
All RNA viruses encode an RNA-dependent RNA polymerase EXCEPT…..
Retroviruses (RNA Dependent DNA Polymerases)
Reverse transcriptase have a rate of error of……
One per cycle
What are the two methods of recombination?
Intramolecular recombination by strand breakage and religation or strand switching (occurs in all viruses that use a DNA intermediate)
Intramolecular recombination by copy-choice, a viral polymerase switches template strands during replication. (Occurs only in RNA)
Reassortment is…..
Exchange of genetic material between two segmented genomes. (Commonly seen in Influenza viruses each year)
How is viral genetic diversity relevant to human disease?
Drug resistance, escape anitbody recognition, or recognize a new host.
The life cycle of an animal virus includes the following steps…..
Recognition of host cell, Attachment, Penetration, Uncoating, Synthesis of nucleic acids and proteins, Assembly, Release from host cell.
People who will get an A on this exam…
Karin Hwang
Terry Parsons!
Sarah Stamper
William Gunther
The viral attachment protein for binding to a cellular protein is usually a ____ if the virus is enveloped, or a ____ if the virus is naked.
Glycoprotein if enveloped, Surface peptide if naked.
Receptor molecules on host cells are usually…..
Glycoproteins (specific) or Glycolipids.
The interaction of a viral attachment protein with a cellular receptor protein establishes the _____ of a virus.
Tropism. Specificity of a certain cell type. Hep. A has a tropism for hepatocytes.
How do naked viruses attach to their host cells?
Direct interaction between viral capsid proteins and the host cell receptor. (This presents a problem for curing the common cold, because the ligand on rhinovirus is inaccessible to antibodies.)
How do enveloped viruses attach to their host cells?
Viral “spikes” extend from the virion surface. One of them is a ligand for the host cell receptor.
Penetration can be by 3 ways…..
Direct Penetration, Fusion, or Endocytosis.
Direct Penetration is seen only in…..
Naked Viruses. Capsid remains extracellular while the genetic material enters the host cell.
Fusion is seen only in…..
Enveloped Viruses. The viral envelope merges with the host cell, both capsid and genome enter the host cell.
Endocytosis is seen in both Naked and Enveloped viruses.
…….
Now I’m standing alone in a crowded room, and we’re not speakinnnnn.
And I’m dying to know is it killing you, like it’s killin me yeahhhhhhh.
Which type of viruses commonly keep the genetic material in the cytosol for replication and gene expression after uncoating?
RNA Viruses
Which type of viruses commonly transport the genetic material to the nucleus for replication and gene expression after uncoating?
DNA Viruses
Describe the production of mRNA from DNA Genomes.
+ssDNA—>Synthesis of - strand—->+-dsDNA intermediate OR +-dsDNA —–>Transcription of - strand—–> +mRNA
Describe the production of mRNA from RNA Genomes.
+-dsRNA—>Transcription of - strand —->+mRNA
+ssRNA—-> +mRNA
-ssRNA—-> Transcription of - strand —-> +mRNA
+ssRNA —-> Reverse Transcriptase makes DNA—-> -ssDNA —-> Replicase copies - strand —-> +-dsDNA Intermediate —-> Transcription of -strand —->+mRNA
Where is mRNA translated?
Cytoplasm
dsDNA viruses typically replicate in the nucleus, with the exception of…..
Poxviridae (cytosol)
ssDNA viruses replicate in….
Nucleus, via a dsDNA intermediate
T or F. DNA genomes are replicated by direct DNA–>DNA copying by either viral or cellular DNA polymerases.
True
Non retrovirus RNA genomes are replicated by direct RNA–>RNA copying and require a…..
Viral Replicase enzyme (RNA Dependent RNA Polymerase)
Retroviral RNA replication includes an RNA–>DNA step that requires a……
Reverse Transcriptase enzyme.
What are inclusion bodies?
Compact masses of viruses that may be present in the nucleus or cytoplasm of the host cell.
Before progeny virus particles assemble and leave the infected cell, each virion needs to contain:
Viral nucleic acid, accessory proteins, and viral enzymes required for infection.
How are icosahedral virions assembled?
Spontaneously into an empty capsid due to a packaging sequence in the genome
How are helical virions assembled?
Using a Pac site in which capsomer subunits bind to begin the process of assembly. It then proceeds spontaneously.
What occurs during the maturation phase that, if not done, will result in the virus being noninfectious?
Processing of protein precursers into their final products through protease activity.
Release of the virions from the host cell occurs via what 3 mechanisms?
Budding, Cell Lysis, or Exocytosis.
What is Budding?
Virions distend through a membrane, becoming enveloped. This takes place through the plasma membrane, nuclear membrane, ER, or vesicles. This takes place in enveloped viruses and incorporates both viral and host cell proteins
What is Cell Lysis?
When the cell becomes overwhelmed by viral load and bursts. Used by naked viruses.
What is Exocytosis?
Occurs in both naked and enveloped viruses. Similar to budding, but the virions do NOT gain a part of the cell membrane. Enveloped viruses that obtain their envelope from an internal membrane exit via this method.
What happens if a virus encounter a non permissive cell?
No infection. Lack of appropriate receptors.
What are the three broad types of viral infections?
Abortive, Acute/Lytic, Persistent.
Describe an abortive infection.
A virion enters a cell, but does not proceed through all the steps of replication. Mature virions are not produced.
Describe an Acute/Lytic infection.
Rapid onset, period of disease, clearance of virus. Usually result in the death of the host cell.
Describe a persistent infection.
Lingers, not readily cleared by the immune system.
What are the 3 types of persistent infections? Describe them.
Chronic Infection: Can be lifelong, continuous shedding of virions.
Latent Infection: Intermittent periods of viral replication and shedding with long periods of dormancy.
Transforming: Virus causes the cell to lose growth control, usually by overexpression of growth factors. May lead to tumor formation.
Describe the Cytopathic response in response to the following viruses- Smallpox, Herpes Simplex, Adenovirus, and Polio.
Smallpox- Cells round up, inclusion bodies in cytoplasm.
Herpes Simplex- Multinucleated Syncytia, inclusion bodies in nucleus.
Adenovirus- Clumping of cells, nuclear inclusions.
Polio- Cell lysis
Describe the Cytopathic response in response to the following viruses- Reovirus, Influenza, Rabies, Measles.
Reovirus- Cell enlargement, vacuoles and inclusions in cytoplasm
Influenza- Cells round up
Rabies- Cytoplasmic inclusions
Measles- Multinucleated Syncytia. (Syncytia= Fusion of infected cell with neighboring cell)
What are the basic steps of a viral infection?
Entry into host, Immune evasion, Entry into cells/primary replication, Spread within host, Cell injury/clinical illness, Shedding.
Most viruses enter the host through what? (Hint- Covers 90% of our bodies)
A mucosal surface.
Localized infections occur where?
Site where the virus entered the host.
Generalized infections occur where?
They spread beyond the point of entry to infect additional tissues or may become systemic.
What is the most common point of entry for virions?
The respiratory tract.
The alimentary tract (GI) is a common portal of entry for viruses. Viruses that enter via this route are typically what kind?
Naked, since bile disrupts the lipid bilayer of enveloped viruses.
What are a few ways viruses can enter through the skin?
Minor abrasions (Herpes simplex, poxviruses), Insect bites (West Nile), Needles (Hep B/C, retroviruses, Cytomegalovirus, Epstein-Barr, Ebola)
The genital tract is a route of entry for some viruses. Which cause localized infections and which caused generalized?
Localized- Herpes Simplex, Papillomaviruses
Generalized- HIV, HTLV, Hep B/C
The Conjunctiva is less resistant to viral infection than skin, but how does it prevent infection?
Lysozyme, IgA in tears, blinking of the eyelids.
How does a virus avoid detection via inhibiting antigen presentation?
Downregulating expression of MHC class I so the host cell cannot present viral antigens to T cells.
What is antigenic variation?
The ability of a virus to change or remove certain surface proteins to avoid antibody binding from previous infections. The host no longer recognizes the virus.
Describe molecular mimicry.
Viruses can make surface proteins very close to the host proteins, this can trick the immune system into believing a virus is a “self” molecule.
Places like the eye, brain, ovary, and testes are also known as what?
Privileged sites. Locations with little immune protection.
Why do papillomaviruses have a long incubation period?
They infect the basal layer of the epidermis, becoming keratinized as they mature.
Presence of virus in the bloodstream is called?
Viremia
When a primary viremia transfers to an organ, this liberates more viruses that spread via…..
Secondary viremia
How can HIV be transferred to a fetus?
In-Utero, during birth, or through breast feeding.
Parvovirus and Variola can both cross the placenta. What happens to the fetus?
Fetal death/abortion
Cytomegalovirus and Rubella both cross the placenta. What happens to the fetus?
Congenital defects. Deafness, blindness, heart/brain defects.
The TORCH panel is a blood test that measures antibodies for a group of organisms that cause birth defects. What are they?
Toxoplasma
Other (Syphilis, HBV, coxsackie, EBV, VZV, human parvovirus)
Rubella
Cytomegalovirus
Herpes Simplex
The level of clinical illness usually correlates to what?
The amount of damage done by the virus.
The amount of damage done by a virus depends on its…..
Virulence
The relative capacity of a pathogen to infect and harm a host cell.
Virulence
Localized infections typically _____ at the site of entry.
Shed (Ex. Viral pneumonia is shed by coughing, herpes spreads vial epithelial lesions)
_____is shed in stools and is present before the onset of illness.
Poliovirus
How is HIV primarily shed?
Body fluids
In a generalized disease, the virus spreads through blood and along _____ to the peripheral nerves.
Axons
