Viruses #6

Question 1

1. Give a working definition of a virus and contrast viruses to living cells.

Answer 1

Nucliec acid + protein coat (nucleo capsid)

Question 2

Viral structures:

Answer 2

Icosahedral

Helical

Question 3

3. Describe the typical size of a virus and a viral genome and compare it to a typical bacterium.

Answer 3

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)

Question 4

Influenza virus:

Answer 4

8 segments
ssRNA - sense strand
helical capsid
enveloped

Question 5

6. Describe how the virus that causes the “stomach flu” differ from the one that causes influenza?

Answer 5

Noro virus:
naked
ssRNA + sense
survives well outside
high virulent

Question 6

7. Describe the steps involved in multiplication of a virulent virus, from adsorption to release, and describe variations on these steps shown by different viruses.

Answer 6

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

Question 7

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.

Answer 7

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

Question 8

What difference would you expect to see between an electron microscopy one step growth curve and a plaque assay one step growth curve?

Answer 8

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.

Question 9

9. Describe an assay used to quantify the number of viral particles in a liquid.

Answer 9

Plaque assay: Perform dilution, allow maturation, chloroform lysis (optional), plate on bacterial cells.

Question 10

10. Explain what positive and negative strands of RNA are.

Answer 10

Positive: could be used as an mRNA strand
Negative: is the complimentary strand to an mRNA strand

Question 11

Discuss the problem encountered by a virus that uses RNA as its genetic material, and how +ssRNA and –ssRNA viruses solve these problems.

Answer 11

+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

Question 12

12. Describe a unique characteristic of retroviruses, and name one type of retrovirus.

Answer 12

+ssRNA
goes
+ssRNA –> dsRNA-DNA –> ssDNA –> dsDNA –> insertion into genome

Question 13

13. Describe the three functions of reverse transcriptase, and their role in the retroviral ‘life’ cycle.

Answer 13

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.

Question 14

14. Explain what is meant by the terms “temperate virus”.

Answer 14

Temperate virus: bacteriophage with two lifecycles

• virulent: kills bacteria, normal lytic cycle.
• lysogenic cycle: Non lethal cycle. Grows with cells

Question 15

Lysogenic cycle:

Answer 15

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.

Question 16

15. Describe in detail what causes the shift from a lysogenic cycle to a lytic cycle.

Answer 16

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.

Question 17

16. Describe a situation where a temperate phage causes a human disease.

Answer 17

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.

Question 18

Name two diseases who are lysogenic strains of bacteria (no pathology unless they have a temperate virus within them):

Answer 18

Corynebacterium diphtheria

Clostridium botulinum

Question 19

17. Describe four possible outcomes of infection of animal cells by viruses.

Answer 19

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

Question 20

18. Describe two ways that certain viruses may cause cancer.

Answer 20

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

Question 21

19. 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.

Answer 21

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