Micro exam 1 Flashcards
How do viruses bypass the need for enzymes?
They reproduce via the assembly process
Cytomegalovirus
TQ
Only virus to contain both RNA and DNA
What metabolic features do viruses lack?
ATP, ADP, Free amino acids and sugars
Retroviruses
TQ
- only Diploid virus
Function of Capsid and envelope
Protect the virus core, which contains the genome
-Rigid structure that can withstand harsh environmental conditions
Naked virus = Capsid
Protects nucleic acids, helps introduce viral DNA/RNA
-recognize host cell and bind
Nucleocapsid
Capsid +genome
** 2 different shapes; helical and icosahedral
Mutation. Diversity results from mutations
A change of chemistry of a gene that is perpetuated in subsequent divisions. It is within a SINGLE GENOME
Mutation rates: 1/1 billion in animals, 1/ 1 billion DNA, 1/1000 RNA
***Most mutations occur at the RNA level
Point Mutation = a substitution mutation
Single change in a nucleotide.
Inversion
The switching of 2 adjacent nucleotides
Phenotype vs Genotype
Phenotype: is the inspection of viruses with altered characteristics
Genotype: changes made in the viral genome
Strain
Virus that shows different levels of pathogenicity and infectivity among individuals or geographic locations. The result of minor mutations.
e.g. H1N1 flu virus
Type
Same virus, but responding differently to antibody detection (serotypes)
e.g. adenovirus has 100 serotypes and 51 infect humans
Variant
Virus whose phenotype differs from wild-type (genetic mutation is unknown)
*** Variants arise in the SAME PATIENT
Viral envelope a lipid membrane derived from?
The host cell’s membrane or internal organelle membrane
Naked virus
A virus with only a capsid; no envelope
Functions of a viral envelope
1) host cell recognition
2) binding the host cell post-infection
3) protection of nucleic acids
4) stimulation of immune system to produce Abs (more of an effect than a function)
* **Viral attachment proteins (VAPs) fulfill these roles
characteristics of enveloped viruses
1) they must remain wet and are generally transmitted in fluids
2) Most cannot survive the harsh GI tract
Is Pox viridae helical or icosahedral?
Neither POX in a BOX
Pox virus is a complex oval
What virus carries enzymes with it and what are they?
- Retroviruses carry enzymes, reverse transcriptase
- integrase and protease
- RNA-dependent DNA polymerase
Which Viruses do not have symmetrical geometry?
Pox virus and Reovirus: double protein layer capsid
Influenza has what type of genome?
Double-stranded linear, segmented RNA
Paroviruses contain what type of genome
ssDNA (instead of normal dsDNA)
Reoviruses contain what type of genome?
dsRNA (instead of normal ssRNA)
Silent mutation
No change in phenotype
Missense mutation
A Mutation that results in a noticeable effect on the phenotype
Nonsense mutation
A mutation that results in a stop codon.
-Gene product is truncated or not expressed at all.
What is an “Isolate”?
A virus that has not yet been analzyed
Why do RNA viruses have so many more mutations than DNA viruses?
RNA-dependent RNA polymerases are more prone to mutations than DNA-dependent polymerases because viral RNA polymerase has NO proofreading capabilities.
All RNA viruses encode an RNA-dependent RNA polymerase except which virus?
Retroviruses
What is reverse transcriptase
Contains RNA-dependent DNA polymerase. Has no proofreading mechanism. (incredibly high rate of error)
Why is HIV so hard to treat?
Each retroviral DNA genome is generated by Reverse transcriptase contains at least 1 mutation.
What is recombination
Genetic information exchanged between two distinct genomes
Viruses use a DNA intermediate use what type of intramolecular recombination?
Strand breakage and re-ligation (strand switching)
Copy-choice occurs only in what type of virus?
RNA viruses. A viral polymerase switches template strands during replication.
What has to occur for reassortment to happen?
Coinfection/superinfection.
-Reassortment is the exchange of genetic material between two segmented genomes
Why do we need annual influenza vaccines?
Entire segments of RNA are exchanged routinely between 2 infecting viruses.
Viral replication steps in order
1) Recognition of the host cell -> 2) Attachment -> 3) Penetration -> 4) Uncoating -> 5) Synthesis of nucleic acids (transcription, translation, replication) -> 6) Assembly -> 7) Release from the host cell
Do enveloped or naked viruses usually have a glycoprotein as the viral attachment protein?
Enveloped viruses. Naked viruses have a surface peptide.
- The host molecules on the host cells are typically glycoproteins or glycolipids.
- glycoproteins make specific receptors, where as carbs are less specific
What is the “tropism” of a particular virus?
The interaction of the viral attachment protein and the cell surface protein (usually a glycoprotein) gives the virus specificity for a certain cell type
e.g. Hepatitis A virus has tropism for hepatocytes
How do naked viruses attach to host cells
Through a direct interaction between viral capsid proteins (ligands) and the host cell receptor
Why is it so hard to cure the common cold?
The rhinovirus ligand lies in a groove on the capsid, which allows it to avoid being recognized by the immune system and bind to host cells
How to enveloped virus attach to host cells?
Through “spikes” that protrude outward from the virion.
What types of penetration can naked cells undergo?
Direct penetration (only genome enters) or endocytosis
What types of penetration can enveloped viruses undergo?
Fusion or endocytosis
What is direct penetration?
is only seen in naked viruses. Only genetic material enters host cell while the capsid remains extracellular.
What is fusion?
Only seen with enveloped viruses. The viral envelope (compromised of host cell membrane) contains many host cell proteins and can “merge” with the host cell, becoming part of the membrane. When this occurs, capsid and genome enter host cell.
What is endocytosis of viruses?
Both enveloped and naked viruses may be endocytosed into the cell. Internalization of clathrin-coated of caveolin-coated pits at the membrane.
What is uncoating, and where do viruses go when they are uncoated?
Uncoating is the removal of the capsid. When this happens:
1) RNA viruses remain in the cytosol
2) DNA viruses may be transported into the nucleus
What are nuclear localization signals?
Genomic material on a DNA virus that allows it to dock at a nuclear pore and pass through the nuclear membrane
Why must DNA viruses enter the nucleus of the host cell
Because they need to utilize the host cell’s DNA polymerase
Which DNA virus does not enter the nucleus and why not?
Poxvirus replicates in the cytoplasm because they are large enough to code their own ezymes.
Which RNA virus enter the nucleus and why?
TQ
Retroviruses must use the host cell DNA as a template.
What type of DNA viral genome can undergo transcription (production of mRNA) immediately?
+/- dsDNA
ssDNA will have to undergo what step before it can be transcribed?
Production of +/- dsDNA intermediate.
Can +ssDNA be transcribed immediately?
No. First -ssDNA must be synthesized, then a +/- dsDNA intermediate must be formed before production of mRNA can begin.
What is the clinical relevance of what type of genome a virus has?
The number of steps a virus has to take to begin transcribing mRNA, in order to translate functional viral proteins, determines how quickly the virus becomes infectious and how much time there is to intervene.
Why are +RNA viruses the most infectious?
The genome itself can be used as mRNA. Shortest incubation time.
What enzyme is capable of making DNA from RNA
Reverse transcriptase
What is the function of replicase?
Replicase copies retroviral -ssDNA to make the +/- dsDNA intermediate
How and where do ssDNA viruses replicate?
In the nucleus through a dsDNA intermediate which functions as a template for synthesis of ssDNA progeny.
Where does poxviridae replicate?
In the cytosol
*definitely a TQ
What is viral replicase?
RNA dependent RNA polymerase required for direct RNA –> RNA replication
How do Retroviruses replicate?
Require an RNA -> DNA step via viral reverse transcriptase (RT);
RNA–> DNA –> RNA
What are inclusion bodies?
Compact masses of viruses that may be present in the nucleus or cytoplasm of the host cell.
How do Icosahedral viruses assemble?
The capsomers spontaneously assemble. The genome includes a “packaging sequence” which is bound by a protein that stuffs the genome into the empty, pre-assembled capsid.
How do helical viruses assemble?
Genomic material includes a “pac site” to which to which the capsomer subunits bind to begin the process of assembly. Once the process has begun, it continues spontaneously until the genome is surrounded.
During what step (not all viruses undergo it) is protease active?
Maturation. Failure of this step in viruses such as HIV, renders the virion non-infectious.
-can block by disrupting the protease thru reverse transcriptase.
What type of release from the host cell can naked viruses undergo?
Cell lysis or exocytosis
What type of release from host cell can enveloped viruses undergo?
Budding or exoctyosis
What is budding?
Assembled virions distend through a host cell membrane, becoming enveloped. The virion incorporates part of the host cell membrane as it exits.
What is the difference between budding and exocytosis?
In exocytosis, the virions do NOT acquire a portion of the host cell membrane.
Can an individual enveloped virus undergo both budding and exocytosis?
Yes. An enveloped virus that acquires its envelope from an internal membrane (Herpes,Hep B, Influenza) may then exit the host cell via exocytosis.
- What is a permissive viral infection? 2. What is a non-permissive viral infection?
- A productive infection results
- infection does not result. Virus may or may not enter the host cell.
* need permissive for an infection to occur
What is an abortive infection?
Mature virions are not produced once the virus enters the cell, does not complete all the steps of the replication cycle.
-some viruses require a helper virus to mature.
What are the characteristics of acute/lytic infections?
Rapid onset, a period of disease, followed by clearance of the virus. Often results in the death of the host cell by “lysis”, releasing new virions.
What are the characteristics of persistent infections?
Infection lingers and is not readily cleared by the immune system. In these infections, the host cell survives, harboring the virus.
What are the 3 manifestations of a persistent infection
Chronic, Latent and Transforming
What is a chronic infection?
Can be lifelong. Continuous production and shedding of virions.
e.g. Hep B and C in the liver
What is a latent infection?
Intermittent periods of viral replication with long period of dormancy. Latency may arise out of active suppression of viral replication or due to lack of cellular factors required for replication.
e.g. HSV, VZV
Note: in AIDS or oran transplant pts, reactivation of latent infections can be fatal.
What is a transforming infection?
A virus that causes the cell to lose growth control. May lead to a tumor.
e.g. HTLV
What is the difference between recurrent and latent?
Recurrent is like latent, but the virus clears in between episodes. Due to different Serotypes
What is the Cytopathic effect?
Damage to host cell by the virus.
e.g. changes in shape, size, presence of inclusion bodies, syncytia, lysis
What is an oncogenic virus?
A virus that laters the genetic material of the host cell, leading to cancer. They do this by integrating their dsDNA into the host cell chromosome.
e.g. HTLV I&II, HPV
What are the steps of viral infection, in order?
- Entry into host ->2. Immune evasion -> 3. Entry into cell and primary replication -> 4. Spread within the host -> 5. Cell injury and clinical illness -> 6. Shedding
How do most viruses enter the host?
Through mucosal surfaces (90% of our body is covered with this surface)
What is a localized infection?
Infection at the site where the virus entered the host.
e.g. Rhinitis, rotavirus
What is a generalized infection?
Infectious spread beyond the point of entry. May be specific (hepatitis) or systemic (measles)
What is the most common point of viral entry into a host?
Respiratory tract
-Through exchange of saliva or suspended in aerosol (sneeze)
What type of viruses infect the GI tract and why?
As a rule, naked viruses are GI viruses, because enveloped viruses are disrupted by bile.
How do viruses inhibit antigen presentation by the host cell?
By down-regulating MHC class 1 molecules so the host cannot present viral antigens to T cells
What is antigenic variation?
The process by which viruses change or remove certain surface proteins so that antibodies made during previous infections will no longer recognize the virus
What is molecular mimicry?
The process by which viruses express surface proteins that are similar to host proteins; escaping the immune system by pretending to be “self”.
What are immunologically privileged sites?
Anatomical locations with little immune protection such as the eye or brain.
What is local spread?
The spread of virus on epithelial surfaces. These viruses usually have short incubation periods.
eg. the movement of fluid and mucus in the respiratory trac enables the spread of influenza
What is viremia?
Presence of virus in the bloodstream. It is the most effective vehicle for spread of the virus. May be primary (initial transfer of virus to a specific organ) or secondary (more viruses that spread and can lead to a systemic infection involving multiple organs).
What are the TORCH organisms?
Will be a TQ
TORCH test is a blood test that measures the presences of Abs( in mother or newborn) to these specific organisms
Organisms that cause symptomatic birth defects in newborns.
T= Toxoplasma
O= Other (syphilis HBV,coxsackie virus, EBV,VZV, and human parovirus)
R= Rubella
C= Cytomgalovirus
H= Herpes Simplex Virus
What is Shedding?
Release of virus into the environment.
This is the last step.
Note: in Herpes Virus, the site of entry is also the site of shedding (epithelial lesions)
What is virulence?
The relative capacity of a pathogen to infect and harm a host cell.
*Virulence factors: factors that contribute to the disease process
