Viruses Flashcards
Virus
An infectious particle consisting of nucleic acids encoded in a protein coat
Why do viruses “straddle” the definition of life?
1) They can’t replicate on their own: Require a host cell to replicate
2) Also can’t perform metabolic processes on their own
Viral Genome Possibilities
1) Single-Stranded RNA
2) Double-Stranded DNA
3) Double-Stranded RNA
4) Single-Stranded DNA
Virus genome can be…
Continuous or segmented: can be one molecule or multiple molecules
Viruses with segmented genome…
tend to have higher rates of mutation as during replication, recombination of the genome can occur
Capsid
The protein shell enclosing a viral genome
What makes up the capsid?
Capsomeres
–> made of a large number of small protein subunits (usually just the same one or few repeating over and over again)
Capsomere
The protein subunits that make up the capsid
Most animal viruses also contain a…
Viral envelope (AKA membranous envelope)
Viral Envelope
Derived from a host cell’s membrane –> Cloaks the viral capsid and gives the virus the cell’s membrane proteins
Bacteriophage
Viruses that infect bacterial cells
First virus found
Tobacco Mosaic Virus
Viruses are “Obligate Intracellular Parasites” which means…
They can only replicate in host cells
–> Lack metabolic enzymes and the equipment needed for making proteins (Ex: they lack ribosomes)
Why are viruses called “selfish genes”?
Because they only exist to replicate themselves and continue on no matter what
Host Range
The range of species/cells/hosts a virus is able to infect
What determines a virus’s host range?
The interaction between viral surface proteins and host cell membrane receptors
–> Interact in a “lock and key” or “handshake” model
Variation of host range
Some species can have:
1) Narrow host range = can only infect one species/cell type
2) Broad host range = can infect multiple species/cell types
Generic reproductive process of viruses (simple DNA virus)
1) Virus enters cells
2) Virus gets uncoated: releases capsid proteins and viral DNA
3) Some of the viral DNA goes to be transcribed and the rest goes to be replicated to produce more viral DNA
4) mRNA produced from transcription of viral DNA then gets translated to produce more capsid proteins
5) Capsid proteins + replicated viral DNA spontaneously self-assemble
6) New virus is formed and leaves the cell in some manner
Reproductive Cycles of Viruses
1) Lytic Cycle
2) Lysogenic Cycle
Lytic Phase
Ends with cell LYSIS (cell dies/is destroyed)
–> Causes massive release of viral progeny which can then go on to infect neighboring cells
Lysogenic Phase
“latent” viral stage; the viral genome integrates into host cell’s genome
–> The genomes coexist with the viral DNA getting replicated alongside the host cell’s DNA
–> Does NOT destroy the host cell
What causes a switch between lytic and lysogenic cycles?
It depends: It’s usually some environmental change (like a certain chemical) that triggers the switch
Lysogenic Phase: Process (in bacteria)
1) Virus infects host cell with genome
2) Viral genome circularizes
3) Viral genes integrate with the host cell’s chromosomes by recombination
4) Cell reproduces normally with viral DNA in it
–> Cell divisions occur = new cells with viral DNA in them
——– Process can end here if the virus never leaves lysogenic phase
5) In some cases, the viral DNA will EXCISE from the host cell genome and enter the LYTIC cycle
Lytic Phase: Process (in bacteria)
1) Virus infects host cell with genome
2) Viral genome circularizes
3) New virus production begins: Production of both capsid proteins (and any other viral proteins) and viral genome
4) Cell lyses and releases a ton of viral progeny
–> cycle begins again in another cell
How is the viral envelope produced?
Through viruses hijacking the endo-membrane system to produce viral glycoproteins that then get secreted onto the plasma membrane
–> Then when new viral progeny bud out of the cell they are enveloped by the membrane containing the viral glycoproteins
Animal Viruses Reproduction Process
1) Viral glycoproteins on virus surface bind to cell membrane receptors
2) Virus enters cell by fusing with PM or endocytosis
3) Enzymes digest the capsid and release the genome
4) Viral DNA goes to replicate + get transcribed
5) mRNA gets translated into capsid proteins and preliminary proteins targeted to the ER
6) Virus uses endo-membrane system to then produce glycoproteins in the ER
7) Viral glycoproteins secreted to the PM surface through transport vesicles that fuse with the PM
8) Capsid proteins and viral genome self-assemble
9) Newly formed viruses bud out of the cell, getting enveloped by PM with the viral glycoproteins on it
Enveloped viruses acquire…
HOST-derived membranes enriched in VIRAL proteins to form their external envelope
3 possible genome functions for single-stranded RNA viruses:
1) Single-stranded RNA that acts as mRNA
2) Single-stranded RNA that acts as a TEMPLATE for mRNA production
3) Single-stranded RNA that acts as a TEMPLATE for DNA production (Retrovirus)
Single-stranded RNA that acts as mRNA
The viral genome is just directly transcribed (since it is acting as an mRNA already)
Single-stranded RNA that acts as TEMPLATE for RNA
RNA genome gets transcribed into a complementary RNA strand (mRNA) that then undergoes translation
–> Requires special enzyme!
Enzyme for RNA(template) —> RNA(synthesis)
RNA Dependent RNA Polymerase (RdRP)
Retrovirus
A “backwards” virus –> Single stranded RNA acts as a TEMPLATE for DNA synthesis
–> Requires a special enzyme!
Enzyme for RNA(template) —> DNA(synthesis)
Reverse Transcriptase
Reverse Transcriptase
Transcribes an RNA template into a DNA strand
What happens to the DNA produced from a retrovirus?
It gets inserted into the host cell’s genome as a “provirus”
Provirus
A virus genome that is integrated into the host cell’s genome
Where is reverse transcriptase kept in the virus?
Packaged with the viral genome within the capsid
Retrovirus reproduction process
1) Virus gets into cell and viral capsid is removed
2) Reverse transcriptase catalyzes the synthesis of a DNA strand from the viral RNA template
–> Once complete, a single DNA strand is produced and the RNA is hydrolyzed (degraded)
3) Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first one
4) The produced double stranded DNA enters the nucleus
5) DNA incorporates into host genome as a provirus
6) Proviral genes are transcribed into:
1. mRNA for translation
2. RNA for the new viral progeny genome
7) Viral proteins produced:
1. capsid proteins
2. Reverse transcriptase
3. glycoproteins
8) Self assembly of the virus
9) Virus buds off and exits cell
3 main sources of viral emergence:
1) Mutations creating new strains
2) Zoonosis mutations
3) Dissemination
Mutations creating new viral strains
The mutation of an existing virus creates new viruses that can spread more easily
–> Can create new viral strains with enough genetic differences from earlier strains that allows for the EVASION OF IMMUNITY (people with acquired immunity from earlier strains are no longer immune to the newer one)
Why do RNA viruses tend to mutate more than others?
Because replication of the viral RNA genome lacks proof reading mechanisms = greater mutation rate
Zoonosis
When a virus undergoes mutation that BROADENS its host range
–> Allows existing viruses to “hop” from one species to another
Dissemination
The spread of a viral disease from a small-isolated population
–> Is more of an issue now that we are able to move more freely around the world = greater spreading
Methods of combatting viral infections
1) Vaccines
2) Anti-viral drugs
Vaccines
Harmless viral variants or derivates that stimulates the immune system to mount defense against the actual harmful pathogen
–> a PREVENTATIVE measure
Vaccines cannot…
treat a viral infection once it has started
Anti-Viral Drugs
Combat viruses by interfering with virus-specific processes: usually by the interference of viral nucleic acid synthesis (stopping viral reproduction)
–> an ACTIVE measure; tries to TREAT an infection that has already begun
