Viruses Flashcards
All viruses have?
genome-DNA or RNA, double or single stranded
-capsid: protein coat
What are capsomers?
Individual subunits that comprise the capsid
-self assemble into a capsid
Some Viruses also have
envelope–> lipid bilayer derived from the host cell membrane during viral budding
-spikes–> viral proteins or glycoproteins in the envelope
-enzymes–> for replication or pathogenesis
True or false: virtually all bacteriophages are naked
True
What are the common viral shape
Icosahedron and helical
How can viruses be classified
By host—> bacteriophage vs animal viruses
-By genome type—> single vs double stranded vs negative strand RNA
-enveloped vs naked
What is the first step of Viral Replication
Adsorption–> ligands on viruses bind to receptors on host cells
-ligands may be on spikes or capsid proteins
Can viruses infect any cell?
No they are highly specific
What is the second step of virus replication?
Penetration—> release of the nucleic acid from the capsid
-phage usually inject genome into bacterial cell, leaving capsid outside
During penetration how do animal viruses enter the cell?
Enter cells by fusion or endocytosis
What is the third step of Virus replication?
Synthesis
-dsDNA and ssDNA viruses–> can use host to make new viral genomes
-dsRNA and ssRNA viruses—> need RNA replicase enzyme ro make new viral genomes. RNA replicase enzyme must be carried in the virus in a -strand Virus RNA
-retrovirus—> strand ssRNA genomes uses a dsDNA intermediate
-carries the enzymes reverse transcriptase and integrate
-DNA intermediate integrates into host genome
What are early proteins?
hijack the cell; force the synthesis of new viral genomes and viral coats
-example: viral sigma factor
What are late proteins?
Viral proteins that are part of the mature virion
What is the fourth step of virus replication?
Assembly–> capsids self assemble; genomes packaged inside
-phage assemble in the cytoplasm
-animal viruses may assemble in the nucleus or cytoplasm depending on the virus
What is the final step of virus replication?
Release—> some enveloped viruses are released by budding from the plasma membrane which doesn’t necessarily kill the host
-some types of viruses instead gain their envelope by budding into the ER, ERGIC, or Golgi during assembly nd are released by exocytosis
Virulent phage
can only use lytic replication
Temperate phage
Can use both lytic an lysogenic replication
Lysogen
Bacterial cell containing a prophage
Prophage
The phage genome integrated into the host cell genome
What is the lambda repressor important for?
Is essential for maintaining lysogeny
-is a phage encoded protein that represses the transcription of most phage genes except itself
What is the role of Hfl protease
Can degrade the lambda repressor
High glucose–> low cAMP–> high Hfl
-high glucose lead to increased degradation of lambda repressor by Hfl which favors the lytic cycle
Replication of temperate phage
integration
-circularization(so not degraded)
-site specific recombination(not homologous recombination) between attP and attB catalyzed by phage encoded integrase
Maintenance of lysogeny
-the lambda repressor id essential for maintaining lysogeny
-represses the transcription of most phage genes
Excision:
prophage induction occurs due to environmental conditions that are harmful to bacteria
How can coronavirus enter the cell?
-can enter cells via fusion with the plasma membrane or through endocytosis
-either way spikes of SARS-coV2 binds to ACE2
What are prions?
an infectious proteinaceous particle
-thought to be incorrectly folded proteins that induce other proteins to be misfodled and aggregate
What are characteristics of prions?
- resistant to inactivation by heating to 90 C. A temperature that will inactivate viruses
- not sensitive to radiation which ordinarily damages virus genome
- not destroyed by enzymes that digest RNA or DNA
- sensitive to protein denaturing agent
What is the harmless form of prions?
PrP A which is rich in alpha helices
What is the harmful form of prions
The beta sheet form
Describe the 4 different outcomes of infection of an animal cell by an animal virus. How do these differ from the outcomes of infection of a bacterial cell by phage?
Latency is a characteristic of the herpesviruses, which include Herpes simple viruses, chickenpox virus (varicella), and the virus that causes mononucleosis (Epstein-Barr)
-Latent infection: virus present but not causing harm to cell; later emerges in lytic infection –> long term chronic infection
–the viral genome becomes an episome
–the expression of genes involved in lytic replication is suppressed
–reactivation can occur due to stress or immunosuppression
Describe the steps in the replication of the 2009 A(H1N1) influenza virus. What are the roles of the HA, NA, and PB proteins in the spread of the virus? Why is this virus considered a “quadruple reassortant”?
Roles of proteins in virus
-HA (hemagglutinin): binds to sialic acids on cells of respiratory tract; also plays an important role during virus uncoating in the endosome
-NA (neuraminidase): cleaves the sialic acid residues; role in release/shedding of virion during budding
-PB1 (viral RNA polymerase/RNA replicase): RNA replicase is only present in the virus’s virion; it is an RNA dependent RNA Polymerase that uses the RNA as a template to make DNA.
-the 2009 A(H1N1) is considered a Quadruple Reassortment b/c it has 4 distinct sources of the gene segments in the virus
–Segments: Avianlike, Humanlike, Swinelike, & Eurasian like
Which viruses can rely on the host cell to supply the enzyme that replicates the viral genome? Which viruses cannot rely on the host cell to supply the enzyme that replicates the viral genome? Why?
x-double stranded DNA and single stranded DNA viruses can use the host enzyme s to make new viral genomes b/c the DNA polymerase can use the DNA template to make more DNA
-double stranded RNA and single stranded RNA viruses need RNA replicase enzyme (RNA dependent RNA polymerase) to make new viral genomes. RNA replicase enzyme MUST be carried in the virion in a negative (-) strand RNA virus
Virus vs bacteria
Viruses vs. Bacteria
➢ Bacteria:
-living organism; unicellular; one cell
-larger (1000 nm)
-fission: a form of asexual reproduction
-cell wall: peptidoglycan or lipopolysaccharide
-has ribosomes
-DNA and RNA floating freely in cytoplasm
-“localized” infection
-usually treated w/ antibiotics
➢Viruses:
-not living; no cells
-smaller (20-400 nm)
-reproduction: invades a host cell, taking control and copies the viral DNA/RNA, then destroys the host cell and releases new viruses
-No cell wall; has protein coat (capsid)
-No ribosomes
-DNA or RNA enclosed inside a coat of protein
-“systemic” infection
-antibiotics will not effect the disease
Lysogenic conversion
The prophage alters the genome of the bacterium conferring new properties
what did the the Luria Delbruick fluctuation test show
That mutations occur spontaneously
