Viruses #6 Flashcards
- Give a working definition of a virus and contrast viruses to living cells.
Nucliec acid + protein coat (nucleo capsid)
Viral structures:
Icosahedral
Helical
- Describe the typical size of a virus and a viral genome and compare it to a typical bacterium.
30-300 nm ~45 average for virus
1-2 um for bacteria (10-100x as large)
10^4 –> 10^5.5 for viruses
4,000,000 BP for bacteria (10-1000x larger)
Influenza virus:
8 segments
ssRNA - sense strand
helical capsid
enveloped
- Describe how the virus that causes the “stomach flu” differ from the one that causes influenza?
Noro virus: naked ssRNA + sense survives well outside high virulent
- Describe the steps involved in multiplication of a virulent virus, from adsorption to release, and describe variations on these steps shown by different viruses.
1, adsorption: binding to the outside of the cell
2, penetration: entering the cell
3, uncoating: releasing nucleotides
4, replication & synthesis: duh
5, self assembly: coming back together
6, lysis and release: cell lysis and release
Name and describe the different phases of a “growth curve” for a virulent bacterial virus and what technique is needed to measure the length of those phases.
Latent period: The time during which the amount of virus is not increasing
- Adsorption period: Time it takes to get into the cell
- Eclipse period: Time the virus spends in the cell, multiplying and getting ready
Maturation phase: Release of virions and cell lysis
Plaque assay with chloroform lysis: needed to see this curve
What difference would you expect to see between an electron microscopy one step growth curve and a plaque assay one step growth curve?
Electron microscopy: You would see a distinct eclipse phase where you didn’t spot a virus during the latent phase
Plaque assay (without bacterial lysis via chemical like chloroform): You would not see an eclipse phase, you would only see a latent phase where virus levels where held constant.
- Describe an assay used to quantify the number of viral particles in a liquid.
Plaque assay: Perform dilution, allow maturation, chloroform lysis (optional), plate on bacterial cells.
- Explain what positive and negative strands of RNA are.
Positive: could be used as an mRNA strand
Negative: is the complimentary strand to an mRNA strand
Discuss the problem encountered by a virus that uses RNA as its genetic material, and how +ssRNA and –ssRNA viruses solve these problems.
+ssRNA viral replication:
+ssRNA acts as a template for RNA dependent RNA polymerase.
RNA dependent RNA polymerase make -ssRNA from +ssRNA using rolling replication.
Visa versa (+ssRNA is made)
Instructions read, protein capsid made, cell leaves
-ssRNA viral replication:
-ssRNA and RNA dependent RNA polymerase (RDRP) are both injected into the cell.
+ssRNA is made by rolling replication from -ssRNA
Visa versa
Proteins for capsids are made, auto assemble with RDRP and -ssRNA
- Describe a unique characteristic of retroviruses, and name one type of retrovirus.
+ssRNA
goes
+ssRNA –> dsRNA-DNA –> ssDNA –> dsDNA –> insertion into genome
- Describe the three functions of reverse transcriptase, and their role in the retroviral ‘life’ cycle.
1: RNA dependent RNA polymerase
2: RNA nuclease activity: destroys nucleotides.
3: DNA dependent DNA polymerase
Note: is prone to error since it is trying to do too many things.
- Explain what is meant by the terms “temperate virus”.
Temperate virus: bacteriophage with two lifecycles
- virulent: kills bacteria, normal lytic cycle.
- lysogenic cycle: Non lethal cycle. Grows with cells
Lysogenic cycle:
Entering host DNA:
1: viral DNA plasmid finds sequence of homology in host chromosome or plasmid
2: Integrase causes crossing over event, viral DNA is now part of host DNA
Switching from the lysogenic cycle to virulent cycle:
1: excisionase is encoded for by viral DNA but is being repressed (by a repressor)
2: SOS system: Complex. But the most minor form of SOS response is nucleotide excision repair (exonuclease activity), this cuts out nucleotides and replaces them
3: when exonuclease is activated, it cuts the repressor to excisionase, allowing excisionase gene to be transcribed then translated
4: Excisionase binds with integrase and removes viral plasmid from the bacterial chromosome, it is now virulent.
- Describe in detail what causes the shift from a lysogenic cycle to a lytic cycle.
lysogenic cycle to virulent cycle:
1: excisionase is encoded for by viral DNA but is being repressed (by a repressor)
2: SOS system: Complex. But the most minor form of SOS response is nucleotide excision repair (exonuclease activity), this cuts out nucleotides and replaces them
3: when exonuclease is activated, it cuts the repressor to excisionase, allowing excisionase gene to be transcribed then translated
4: Excisionase binds with integrase and removes viral plasmid from the bacterial chromosome, it is now virulent.
- Describe a situation where a temperate phage causes a human disease.
Lysogenic Strains of bacteria: Temperate virus within them is encoding for the production of the toxin. The bacteria itself is not pathogenic without the temperate virus (in its lysogenic phase)
Note: there are not lysogenic strains of viruses (just temperate viruses), so any lysogenic strain is bacterial.
Name two diseases who are lysogenic strains of bacteria (no pathology unless they have a temperate virus within them):
Corynebacterium diphtheria
Clostridium botulinum
- Describe four possible outcomes of infection of animal cells by viruses.
1: Lytic infection: kills cell
- ebola
2: Persistant infection: cell survives keeps producing virus
- HIV
- generally envoloped viruses (so lysis is not necessary)
3: Latent infection: Virus integrates into DNA, no infection until it goes virulent and lyses the cell
- Herpes - Shingles
4: Transformation to Cancerous Cell: when the virus inserts itself into the animals DNA it alters gene expression in a way that causes cancer (alters cell cycle regulation)
- epstein barr
- human papaloma virus
- Describe two ways that certain viruses may cause cancer.
proto oncogene: A gene which if mutated could cause cancer, it would require a mutation in only one of the oncogene alleles.
- Protein kinase: if a protein kinase is mutated and becomes overproduced, it could change regulation and cause cancer
Tumor Suppressor Gene: Gene which produces a product which regulates and stops the cell cycle. If both alleles of a tumor suppressors are knocked out cancer can occur
- Some viruses produce proteins which bind tumor suppressor genes, inducing cancerous growth to maximize viral production
- Watch the video: “HIV Life Cycle & Drugs to Treat It” (in Canvas under Additional Cool
Links). Describe four classes of drugs that interfere with the action of HIV and how they work.
1: Fussion inhibitors: blocks entry into the host cell
2: nucleoside inhibitors, non-nucleoside reverse transcriptase inhibitors: blocks reversetranscriptase from working
3: integrase inhibitors: blocks integrase
4: protease inhibitors: blocks protease
