Viruses Flashcards
What are the four main characteristics of viruses?
- small in size - average virus size = 100nm
- obligatory intracellular parasites = require living host cell to multiply
- must chemically interact with specific surface cell receptors
- viruses have a host range
What is a host range?
The organisms/hosts a virus has an affinity for - what a virus can infect based on it’s capsid structure
What is tissue tropism?
The affinity of a virus for a certain cell or tissue type
What it the most common disease-causing virus?
Influenza
What is the normal microflora size in humans?
380 trillion viruses
What is the nucleocapsid?
The capsid and nucleic acid of a virus
What is a capsid?
A protective protein coat that surrounds a viruses genetic material
What are capsomeres?
The smaller units that make up the capsid
What are the 4 different shapes a virus can take?
- helical
- polyhedral
- spherical
- complex
What is the nucleic acid? What are the two types of nucleic acids?
The element of the virus that contains the viral genome – can only contain one kind of nucleic acid, either RNA or DNA
What are some characteristics of nucleic acids?
- can be single stranded or double stranded (double has more stability)
- can be linear or circular
How does the genome size differ between DNA and RNA?
DNA has a large genome so it’s less dependent on the host
RNA has a small genome so it’s highly dependent on the host
How does the replication rate vary between DNA and RNA?
DNA has a slow replication rate due to it’s large size
RNA has a fast replication rate due to it’s small size
How does the occurrence of mutations vary between DNA and RNA?
DNA has low occurrence of mutations
RNA has higher occurrence of mutations
**Transcription error affected by rate of replication
What is the envelope of a virus?
A lipid or glycoprotein covering that’s derived from the host cell’s membrane
What are the 3 main proteins that make up the virus envelope and what are they responsible for?
- envelope proteins
- spike proteins
- membrane proteins
** responsible for binding to receptor sites on the host cell
What is virulence?
The ability of a virus to cause harm to the host - affected by variety of factors
How does a virus envelope affect virulence?
- Enveloped viruses are less virulent
- Non-enveloped viruses are more virulent because they result in cell lysis as they exit the host cell
How does a virus envelope affect its resistance to extreme conditions?
*extreme conditions: heat, cold, dry, pH, alcohol/disinfectants
- Enveloped viruses have low resistance = easier to destroy outside of the body
- Non-enveloped viruses have high resistance
How does the virus envelope affect recognition by the immune system?
- Enveloped viruses are harder for the immune system to recognize
- Non-enveloped are easier (think less clothing)
What are some examples of enveloped viruses?
Influenza, RSV, coronaviruses, HIV
What are some examples of non-enveloped viruses?
Norovirus, adenovirus, rhinovirus
*most GI-infecting viruses are non-enveloped (can withstand stomach pH)
What are five factors used to classify viruses?
- Genome type
- Capsid structure
- Presence of envelope
- Type of host it infects
- Mode of action/replication
Why do viruses require a host cell to replicate?
- have no metabolism
- lack enzymes needed for protein synthesis and ATP generation
What are the steps of viral replication?
- Binding and attachment
- Fusion
- Uncoating
- Reverse transciptase
- Integration
- Transcription
- Translation
- Budding
Explain the binding and attachment step of viral replication.
The viruses binding sites attach to the complementary sites on the host cell’s surface
Explain the fusion step of viral replication.
The virus envelope fuses with the host cell’s membrane - allows the viral RNA and enzymes to enter the cytoplasm
Explain the uncoating step of viral replication.
The virus uncoats the capsid and releases the viral genetic information into the host cytoplasm
Explain the reverse transcriptase step of viral replication.
The reverse transcriptase enzyme copies the virus’s single-stranded RNA then copies in reverse to make another mirrored copy = this creates double-stranded DNA
Explain the integration step of viral replication.
The new viral DNA enters the host cell nucleus where the integrase enzyme facilitates integration of the viral DNA into the host chromosome
– viral DNA now a provirus - either remains latent or is transcribed into RNA
Explain the transcription step of viral replication.
Proviral DNA cannot leave the nucleus so it’s transcribed into messenger RNA (mRNA) then transported to the host cell ribosome
Explain the translation step of viral replication.
mRNA is translated into viral proteins using the host cell ribosome
Protease enzyme processes and assembles the new proteins into the new virion
** may remain latent in vacuoles or are released from the cell
Explain the budding step of viral replication.
Virion buds out of the cell using the plasma membrane of the infected cell to create it’s envelope
** virus is now capable of infecting other cells
What are the 3 enzymes involved in viral replication and what are their roles?
- Reverse transcriptase: copies the RNA to create viral DNA
- Integrase: facilitates integration of viral DNA into the host chromosome
- Protease: processes and assembles the newly created viral polypeptides into the new virion
What are the 6 steps of the chain of infection?
- Infectious agent
- Reservoir
- Portal of exit
- Mode of transmisison
- Portal of entry
- Susceptible host
What are the phases of virus disease course?
- Exposure to virus
- Incubation stage
- Prodromal/infectious stage
- Acute stage
- Recovery - convalescence and resolution
Explain the exposure stage of the virus disease course.
Individual is exposed to the virus - enters the body via the respiratory system
Explain the incubation stage of the virus disease course.
Where the virus invades the host cells and begins replicating
- person is asymptomatic
- time period: 1-3 days
What factors influence the time period of the incubation stage?
- Health of the host
- Virulence of the virus
- Size of the innoculum (how much virus enters the body)
Explain the prodromal or infectious period of the virus disease course.
Time where infected person is contagious
Begins ~24 hours prior to symptoms appearing and lasts 3-5 days following onset of symptoms
Explain the acute phase of the influenza disease course.
- respiratory symptoms – runny nose, cough, sore throat
- constitutional symptoms – fever, fatigue, malaise, chills
- complications – secondary infections (sinus + ear infections, pneumonia), death
Explain the convalescence period of the virus disease course.
When the immune system contains the virus and is progressively eliminated from the body – repair of damaged tissues, dissipating symptoms
time period: a few days to 1 week or more
What factor affects the length of the convalescence period?
The health of the individual prior to infection
Explain the resolution phase of the virus disease course.
Total elimination of virus from the body
- no residual symptoms, total return to health
