L9: Introduction to Virology Flashcards
Are viruses living?
No
Define virus?
Obligate intracellular parasite lacking capacity to make energy or substrates
Can viruses be replicated outside of a cell?
No
Replication occurs how in viruses?
Assembly of parts, NOT binary fission
What is a defective virus?
Genetic mutation that prevents replication without a helper virus?
What is a pseudovirus?
Has host DNA in capsid instead of viral nucleic acid?
Pseudoviruses play a role in what bacterial genetic transfer?
General transduction (Bacteriophage)
What is a viroid?
Single circular RNA without a capsid
What do viroids usually attack?
Plants
90% of viruses are which of the three types?
Defective
What is the origin of viruses?
unknown
How is a virus structured?
Nucleic acid (RNA or DNA) surrounded by a protein coat
Protein coat of viruses is called what?
capsid
Viruses can only be seen under what?
EM
The nucleic acid of viruses can be what?
- DNA (3.2kb to 375kb)
2. RNA (7 to 30kb)
Are viruses haploid or diploid
Haploid except for retroviruses that are diploid
The capsid is made up of what?
Capsomeres making pentamers and hexons
What gives virus its shape?
Capsid (number of hexons and pentamers)
Capsomeres need what to assemble into larger units?
Nothing, self assemble
The capsid can assemble how?
- Around nucleic acid
2. Form empty structure into which the nucleic acid is loaded
Capsids are resistant to what? 4
- drying
- acid
- detergents
- bile
Protein coat + nucleic acid inside = what?
Nucleocapsid
Do all viruses have lipid envelopes?
No, just some
The envelope is acquired how?
Host cell membrane through budding.
Envelopes are susceptible to what?
Organic solvents
Space between capsid and envelope is what?
Tegument
Viral glycoproteins and glycolipids insert into host where? 4
- PM
- NM
- ER
- Golgi
How does the virus localized capsid formation to appropriate location?
Viral matrix proteins associate with glycoproteins
A complete infectious virus particle is called what?
Virion
In naked viruses what is the virion?
Nucleocapsid
In enveloped viruses what is the virion?
Nucleocapsid + envelope
Which is more resitant, enveloped or naked viruses?
Naked viruses (act like endospore)
If a treatment can’t kill an endospore, will it kill a naked virus?
Probably not.
Are naked viruses as resistant as bacterial enzymes?
No, but close.
If you take the envelope away from the enveloped virus, can it still infect?
no
Are viral attachment proteins required for virus attachment
Yes
Naked virus attachment proteins are where?
Where are the enveloped proteins?
In the nucleocapsid
In the membrane
Viruses are grouped in what ways? 5
- nucleic acid type
- envelope presence
- Capsid symmetry
- organism that infect
- replication strategy
What are viruses first classified by?
Then what?
Then what?
DNA vs RNA
Presence of envelope
Nature of the nucleic acid
All negative strand RNA viruses have what feature?
Enveloped
GI viruses are usually naked or enveloped?
Naked
What are 9 steps of virus replication?
- Recognition
- Attachment
- Penetration
- Uncoating
- Transcription
6 Protein synthesis - Replication.
8A: Envelopment
8B: Assembly - Budding and release
9B. Lysis and release
What is the main source of tissue tropism?
Virus recognition proteins for recognizing host cell surface
In order to release capsid, what must enveloped viruses do?
Fuse envelope to membrane
After attachment, what happens to non-enveloped viruses?
Taken into cell by receptor mediated endocytosis or direct penetration through viropexis
Enveloped viruses do what to penetrate?
What is sometimes a requirement for this to happen?
Fuse with host cell membrane.
Correct acidity for the virus (neutral or acidic)
Enveloped viruses can also form what during penetration?
Fuse neighboring cells into multinucleated giant cell
After membrane fusion, what is the state of affairs?
Nucleocapsid is free in cytoplasm
After internatlization where is the nucleocapsid sent?
Site of replication
DNA viruses are sent where in the cell?
Nucleus
RNA viruses are sent where in the cell?
Stay in cytoplasm
One the virus is in the correct location, what happens?
Nucleic acid released from protein coat
Which step is the most complicated?
Genomic replication and expression
Critical step in genomic replication and expression is what?
Making mRNA
With each virus what are the 3 genomic replication questions you must ask yourself?
- What enzyme is needed to replicate the genome?
- What enzyme is needed to express the genome
- Does a eukaryotic cell possess those enzymes?
What are the 2 phases in the replication step for viruses?
- Early gene products made first: Viral polymerases and regulatory molecules
- Late gene products: structural proteins like capsid proteins
What special things must a +ssRNA use to get translated?
Nothing, can be read by host ribosomes
What special things must a -ssRNA use to get translated?
RNA-dependent RNA polymerase must be encoded on the virus in order to convert the negative strand to positive.
Double stranded RNA must use what special things to get translated?
RNA-dependent RNA polymerase must be encoded on the virus.
In order to replicate viral RNA, what is needed?
Negative strand templates
Why do dsRNA need a viral polymerase for replication?
Hydrogen bonding between strands, need something that can separate them.
How is genome replication in dsRNA described?
Completely conservative
Retroviral +ssRNA must carry what for replication?
Reverse transcriptase: RNA dependent DNA polymerase
In retroviral +ssRNA replication, what serves as the primer?
tRNA
What does reverse transcriptase do?
Makes -ssDNA copy of mRNA
The -ssDNA is converted to what in retroviral +ssRNA?
dsDNA in the nucleus.
How is -ssRNA replicated?
Viral polymerase transcribes it to make mRNA, this is then used as template for more negative strand RNA
What increases transcription of viral DNA?
Enhancers
Is there an origin of replication in DNA virus replication?
Yes
What provides the free 3’OH on DNA viruses? (2)
- Inverted repeats
2. Proteins with deoxycytidine monophosphate
Which is faster, viral or host DNA polymerase?
Which has more errors?
Viral
Viral
DNA viruses that use host DNA polymerase can only replicate where?
Actively dividing cells
If a cell is at rest, can it replicate viral DNA?
No
Translation of viral mRNA is done by what?
Host ribosomes
Can viral mRNA’s be modified?
Yes, capped and poly-A-tail
Viruses with a single nucleic acid particle usually encode what?
Polyproteins that are cleaved by proteases
Viral mRNA translation is selected over host mRNA because? (3)
- Viral mRNA outnumbers host mRNA
- Blocking of cellular RNA from nucleus
- Inhibition of macromolecule synthesis and induced degradation of cellular DNA and RNA.
Assembly of viruses begins when?
When viral pieces hit threshold for self assembly
DNA viruses assemble where?
RNA viruses assemble where?
Nucleus
Cytoplasm
Capsid proteins can assemble how? (2)
- around nucleic acid
2. loaded later
Enveloped viruses acquire the lipid membrane when?
What is this process called?
Where can it occur? (3)
Release
Budding
NM, ER, Golgi
Virions leave the cell in what manner? (6)
- lysis
- budding
- exocytosis
- bridge traversal
- Cell fusion
- vertical transmission
what type of virus kills the cell upon release?
Naked
